empy
Summary
A templating system for Python.
Overview
EmPy is a system for embedding Python expressions and statements in template text; it
takes an EmPy source file, processes it, and produces output. This is accomplished via
expansions, which are special signals to the EmPy system and are set off by a special
prefix (by default the at sign, @). EmPy can expand arbitrary Python expressions and
statements in this way, as well as a variety of special forms. Textual data not explicitly
delimited in this way is sent unaffected to the output, allowing Python to be used in
effect as a markup language. Also supported are "hook" callbacks, recording and playback
via diversions, and dynamic, chainable filters. The system is highly configurable via
command line options and embedded commands.
Expressions are embedded in text with the @(...) notation; variations include conditional
expressions with @(...?...:...) and the ability to handle thrown exceptions with
@(...$...). As a shortcut, simple variables and expressions can be abbreviated as
@variable, @object.attribute, @function(arguments), @sequence [2][index]
, and combinations. Full-fledged statements are embedded with @{...}. Forms of
conditional, repeated, and recallable expansion are available via @[...]. A @ followed by
a whitespace character (including a newline) expands to nothing, allowing string
concatenations and line continuations. Comments are indicated with @# and consume the rest
of the line, up to and including the trailing newline. @% indicate "significators," which
are special forms of variable assignment intended to specify per-file identification
information in a format which is easy to parse externally. Escape sequences analogous to
those in C can be specified with @\..., and finally a @@ sequence expands to a single
literal at sign.
Getting the software
The current version of empy is 2.3.
The latest version of the software is available in a tarball here: [3]
http://www.alcyone.com/pyos/empy/empy-latest.tar.gz.
The official URL for this Web site is [4]http://www.alcyone.com/pyos/empy/.
Requirements
EmPy should work with any version of Python from 1.5.x onward. It has been tested with all
major versions of CPython from 1.5 up, and Jython from 2.0 up. The included test script is
intended to run on UNIX-like systems with a Bourne shell.
License
This code is released under the [5]GPL.
Mailing lists
There are two EmPy related mailing lists available. The first is a receive-only, very low
volume list for important announcements (including releases). To subscribe, send an email
to [6]empy-announce-list-subscribe@alcyone.com.
The second is a general discussion list for topics related to EmPy, and is open for
everyone to contribute; announcements related to EmPy will also be made on this list. The
author of EmPy (and any future developers) will also be on the list, so it can be used not
only to discuss EmPy features with other users, but also to ask questions of the
author(s). To subscribe, send an email to [7]empy-list-subscribe@alcyone.com.
Basics
EmPy is intended for embedding Python code in otherwise unprocessed text. Source files are
processed, and the results are written to an output file. Normal text is sent to the
output unchanged, but markups are processed, expanded to their results, and then written
to the output file as strings (that is, with the str function, not repr). The act of
processing EmPy source and handling markups is called "expansion."
Code that is processed is executed exactly as if it were entered into the Python
interpreter; that is, it is executed with the equivalent of eval (for expressions) and
exec (for statements). For instance, inside an expression, abc represents the name abc,
not the string "abc", just as it would in normal Python code.
By default the embedding token prefix is the at sign (@), which appears neither in valid
Python code nor commonly in English text; it can be overridden with the -p option (or with
the empy.setPrefix function). The token prefix indicates to the EmPy interpreter that a
special sequence follows and should be processed rather than sent to the output untouched
(to indicate a literal at sign, it can be doubled as in @@).
When the interpreter starts processing its target file, no modules are imported by
default, save the empy pseudomodule (see below), which is placed in the globals; the empy
pseuodmodule is associated with a particular interpreter; it is important that it not be
removed from that interpreter's globals, nor that it be shared with other interpreters
running concurrently. The globals are not cleared or reset in any way. It is perfectly
legal to set variables or explicitly import modules and then use them in later markups,
e.g., @{import time} ... @time.time(). Scoping rules are as in normal Python, although all
defined variables and objects are taken to be in the global namespace.
Activities you would like to be done before any processing of the main EmPy file can be
specified with the -I, -D, -E, -F, and -P options. -I imports modules, -D executes a
Python variable assignment, -E executes an arbitrary Python (not EmPy) statement, -F
executes a Python (not EmPy) file, and -P processes an EmPy (not Python) file. These
operations are done in the order they appear on the command line; any number of each
(including, of course, zero) can be used.
Expansions
The following markups are supported. For concreteness below, @ is taken for the sake of
argument to be the prefix character, although this can be changed.
@# COMMENT NEWLINE
A comment. Comments, including the trailing newline, are stripped out completely.
Comments should only be present outside of expansions. The comment itself is not
processed in any way: It is completely discarded. This allows @# comments to be
used to disable markups. Note: As special support for "bangpaths" in UNIX like
operating systems, if the first line of a file (or indeed any context) begins with
#!, and the interpreter has a processBangpaths option set to true (default), it is
treated as a @# comment. A #! sequence appearing anywhere else will be handled
literally and unaltered in the expansion. Example:
@# This line is a comment.
@# This will NOT be expanded: @x.
@ WHITESPACE
A @ followed by one whitespace character (a space, horizontal tab, vertical tab,
carriage return, or newline) is expanded to nothing; it serves as a way to
explicitly separate two elements which might otherwise be interpreted as being the
same symbol (such as @name@ s to mean '@(name)s'; see below). Also, since a newline
qualifies as whitespace here, the lone @ at the end of a line represents a line
continuation, similar to the backslash in other languages. Coupled with statement
expansion below, spurious newlines can be eliminated in statement expansions by use
of the @{...}@ construct. Example:
This will appear as one word: salt@ water.
This is a line continuation; @
this text will appear on the same line.
@\ ESCAPE_CODE
An escape code. Escape codes in EmPy are similar to C-style escape codes, although
they all begin with the prefix character. Valid escape codes include:
@\0
NUL, null
@\a
BEL, bell
@\b
BS, backspace
@\d
three-digital decimal code DDD
@\e
ESC, escape
@\f
FF, form feed
@\h
DEL, delete
@\n
LF, linefeed character, newline
@\oOOO
three-digit octal code OOO
@\qQQQQ
four-digit quaternary code QQQQ
@\r
CR, carriage return
@\s
SP, space
@\t
HT, horizontal tab
@\v
VT, vertical tab
@\xHH
two-digit hexadecimal code HH
@\z
EOT, end of transmission
@^X
the control character ^X
Unlike in C-style escape codes, escape codes taking some number of digits afterward
always take the same number to prevent ambiguities. Furthermore, unknown escape
codes are treated as parse errors to discourage potential subtle mistakes. Unlike
in C, to represent an octal value, one must use @\o.... Example:
This embeds a newline.@\nThis is on the following line.
This beeps!@\a
There is a tab here:@\tSee?
This is the character with octal code 141: @\o141.
@@
A literal at sign (@). To embed two adjacent at signs, use @@@@, and so on. Any
literal at sign that you wish to appear in your text must be written this way, so
that it will not be processed by the system. Note: If a prefix other than @ has
been chosen via the command line option, one expresses that literal prefix by
doubling it, not by appending a @. Example:
The prefix character is @@.
To get the expansion of x you would write @@x.
@), @], @}
These expand to literal close parentheses, close brackets, and close braces,
respectively; these are included for completeness and explicitness only. Example:
This is a close parenthesis: @).
@( EXPRESSION )
Evaluate an expression, and replace the tokens with the string (via a call to str)
representation evaluation of that expression. Whitespace immediately inside the
parentheses is ignored; @( expression ) is equivalent to @(expression). If the
expression evaluates to None, nothing is expanded in its place; this allows
function calls that depend on side effects (such as printing) to be called as
expressions. (If you really do want a None to appear in the output, then use the
Python string "None".) Example:
2 + 2 is @(2 + 2).
4 squared is @(4**2).
The value of the variable x is @(x).
This will be blank: @(None).
@( TEST ? THEN (: ELSE)_opt ($ CATCH)_opt )
A special form of expression evaluation representing conditional and protected
evaluation. Evaluate the "test" expression; if it evaluates to true (in the
Pythonic sense), then evaluate the "then" section as an expression and expand with
the str of that result. If false, then the "else" section is evaluated and
similarly expanded. The "else" section is optional and, if omitted, is equivalent
to None (that is, no expansion will take place).
If the "catch" section is present, then if any of the prior expressions raises an
exception when evaluated, the expansion will be substituted with the evaluation of
the catch expression. (If the "catch" expression itself raises, then that exception
will be propagated normally.) The catch section is optional and, if omitted, is
equivalent to None (that is, no expansion will take place). An exception (cough) to
this is if one of these first expressions raises a SyntaxError; in that case the
protected evaluation lets the error through without evaluating the "catch"
expression. The intent of this construct is to catch runtime errors, and if there
is actually a syntax error in the "try" code, that is a problem that should
probably be diagnosed rather than hidden. Example:
What is x? x is @(x ? "true" : "false").
Pluralization: How many words? @x word@(x != 1 ? 's').
The value of foo is @(foo $ "undefined").
The square root of -1 is @(math.sqrt(-1) $ "not real").
@ SIMPLE_EXPRESSION
As a shortcut for the @(...) notation, the parentheses can be omitted if it is
followed by a "simple expression." A simple expression consists of a name followed
by a series of function applications, array subscriptions, or attribute
resolutions, with no intervening whitespace. For example:
+ a name, possibly with qualifying attributes (e.g., @value, @os.environ).
+ a straightforward function call (e.g., @min(2, 3), @time.ctime()), with no space
between the function name and the open parenthesis.
+ an array subscription (e.g., '@array[8][index]', '@os.environ[9][name]', with no
space between the name and the open bracket.
+ any combination of the above (e.g., '@function(args).attr[10][sub].other[11][i]
(foo)').
In essence, simple expressions are expressions that can be written ambiguously from
text, without intervening space. Note that trailing dots are not considered part of
the expansion (e.g., @x. is equivalent to @(x)., not @(x.), which would be illegal
anyway). Also, whitespace is allowed within parentheses or brackets since it is
unambiguous , but not between identifiers and parentheses, brackets, or dots.
Explicit @(...) notation can be used instead of the abbreviation when concatenation
is what one really wants (e.g., @(word)s for simple pluralization of the contents
of the variable word). As above, if the expression evaluates to the None object,
nothing is expanded. Example:
The value of x is @x.
The ith value of a is @a[i].
The result of calling f with q is @f(q).
The attribute a of x is @x.a.
The current time is @time.ctime(time.time()).
The current year is @time.localtime(time.time())[0].
These are the same: @min(2,3) and @min(2, 3).
But these are not the same: @min(2, 3) vs. @min (2, 3).
The plural of @name is @(name)s, or @name@ s.
@` EXPRESSION `
Evaluate a expression, and replace the tokens with the repr (instead of the str
which is the default) of the evaluation of that expression. This expansion is
primarily intended for debugging and is unlikely to be useful in actual practice.
That is, a @`...` is identical to @(repr(...)). Example:
The repr of the value of x is @`x`.
This print the Python repr of a module: @`time`.
This actually does print None: @`None`.
@: EXPRESSION : DUMMY :
Evaluate an expression and then expand to a @:, the original expression, a :, the
evaluation of the expression, and then a :. The current contents of the dummy area
are ignored in the new expansion. In this sense it is self-evaluating; the syntax
is available for use in situations where the same text will be sent through the
EmPy processor multiple times. Example:
This construct allows self-evaluation:
@:2 + 2:this will get replaced with 4:
@[ noop : IGNORED ]
The material contained within the substitution is completely ignored. The
substiution does not expand to anything, and indeed expansion contained within the
ignored block are not expanded. This is included simply for completeness, and can
served as a block comment. Example:
@[noop:
All this stuff would appear here
if it weren't for the noop.
@{
while 1:
print "Testing"
}@
]
@[ if EXPRESSION : CODE ]
Evaluate the Python test expression; if it evaluates to true, then expand the
following code through the EmPy system (which can contain markups), otherwise,
expand to nothing. Example:
@[if x > 0:@x is positive.]
@# If you want to embed unbalanced right brackets:
@[if showPrompt:@\x5dINIT HELLO]
@[ while EXPRESSION : CODE ]
Evaluate the Python expression; if it evaluates to true, then expand the code and
repeat; otherwise stop expanding. Example:
@[while i < 10:@ i is @i.@\n]
@[ for NAME in EXPRESSION : CODE ]
Evaluate the Python expression and treat it as a sequence; iterate over the
sequence, assigning each element to the provided name in the globals, and expanding
the given code each time. Example:
@[for i in range(5):@ The cube of @i is @(i**3).@\n]
@[ macro SIGNATURE : CODE ]
Define a "macro," which is a function-like object that causes an expansion whenever
it is called. The signature defines the name of the function and its parameter
list, if any -- just like normal Python functions, macro signatures can include
optional arguments, keyword arguments, etc. When defined, calling the macro results
in the given code to be expanded, with the function arguments involved as the
locals dictionary in the expansion. Additionally, the doc string of the function
object that is created corresponds to the expansion. Example:
@[macro f(n):@ @[for i in range(n):@ @i**2 is @(i**2)@\n]]
@{ STATEMENTS }
Execute a (potentially compound) statement; statements have no return value, so the
expansion is not replaced with anything. Multiple statements can either be
separated on different lines, or with semicolons; indentation is significant, just
as in normal Python code. Statements, however, can have side effects, including
printing; output to sys.stdout (explicitly or via a print statement) is collected
by the interpreter and sent to the output. The usual Python indentation rules must
be followed, although if the statement consists of only one statement, leading and
trailing whitespace is ignored (e.g., @{ print time.time() } is equivalent to
@{print time.time()}). Example:
@{x = 123}
@{a = 1; b = 2}
@{print time.time()}
@# Note that extra newlines will appear above because of the
@# newlines trailing the close braces. To suppress them
@# use a @ before the newline:
@{
for i in range(10):
print "i is %d" % i
}@
@{print "Welcome to EmPy."}@
@% KEY (WHITESPACE VALUE)_opt NEWLINE
Declare a significator. Significators consume the whole line (including the
trailing newline), and consist of a key string containing no whitespace, and than
optional value prefixed by whitespace. The key may not start with or contain
internal whitespace, but the value may; preceding or following whitespace in the
value is stripped. Significators are totally optional, and are intended to be used
for easy external (that is, outside of EmPy) identification when used in large
scale environments with many EmPy files to be processed. The purpose of
significators is to provide identification information about each file in a
special, easy-to-parse form so that external programs can process the significators
and build databases, independently of EmPy. Inside of EmPy, when a significator is
encountered, its key, value pair is translated into a simple assignment of the form
__KEY__ = VALUE , where "__KEY__" is the key string with two underscores on either
side and "VALUE" is a Python expression. Example:
@%title "Nobody knows the trouble I've seen"
@%keywords ['nobody', 'knows', 'trouble', 'seen']
@%copyright [2000, 2001, 2002]
Substitutions
Supported are conditional and repeated substitutions, which involve testing or iterating
over Python expressions and then possibly expanding EmPy code. These different from normal
Python if, for, and while statements since the result is an EmPy expansion, rather than
the execution of a Python statement; the EmPy expansion may, of course, contain further
expansions. This is useful for in-place conditional or repeated expansion of similar text;
as with all expansions, markups contained within the EmPy code are processed. The simplest
form would consist something like:
@[if x != 0:x is @x]
This will expand x is @x if x is greater than zero. Note that all characters, including
whitespace and newlines, after the colon and before the close bracket are considered part
of the code to be expanded; to put a space in there for readability, you can use the
prefix and a whitespace character:
@[if x != 0:@ x is @x]
Iteration via while is also possible:
@{i = 0}@[while i < 10:@ i is @i@\n@{i = i + 1}]
This is a rather contrived example which iterates i from 0 to 9 and then prints "i is
(value)" for each iteration.
A more practical example can be demonstrated with the for notation:
<table>@[for x in elements:@ <tr><td>@x</td></tr>]</table>
This EmPy fragment would format the contents of elements into an HTML table, with one
element per row.
The macro substitution doesn't get replaced with anything, but instead defines a "macro,"
or recallable expansion, which looks and behaves like a function. When called, it expands
its contents. The arguments to the function -- which can be defined with optional,
remaining, and keyword arguments, just like any Python function -- can be referenced in
the expansion as local variables. For concreteness, the doc string of the macro function
is the original expansion. An macro substitution of the form @[macro SIGNATURE:CODE] is
equivalent to the following Python code:
def SIGNATURE:
repr(CODE) # so it is a doc string
empy.string(repr(CODE), '<macro>', locals())
This can be used to defer the expansion of something to a later time:
@[macro header(title='None'):<head><title>@title</title></head>]
Note that all text up to the trailing bracket is considered part of the EmPy code to be
expanded. If one wishes a stray trailing brackets to appear in the code, one can use an
escape code to indicate it, such as @\x5d. Matching open and close bracket pairs do not
need to be escaped, for either bracket pairs in an expansion or even for further
substitutions:
@[if something:@ This is an unbalanced close bracket: @\x5d]
@[if something:@ This is a balanced bracket pair: [word]]
@[if something:@ @[if somethingElse:@ This is nested.]]
Significators
Significators are intended to represent special assignment in a form that is easy to
externally parse. For instance, if one has a system that contains many EmPy files, each of
which has its own title, one could use a title significator in each file and use a simple
regular expression to find this significator in each file and organize a database of the
EmPy files to be built. This is an easier proposition than, for instance, attempting to
grep for a normal Python assignment (inside a @{...} expansion) of the desired variable.
Significators look like the following:
@%KEY VALUE
including the trailing newline, where "key" is a name and "value" is a Python expression,
and are separated by any whitespace. This is equivalent to the following Python code:
__KEY__ = VALUE
That is to say, a significator key translates to a Python variable consisting of that key
surrounded by double underscores on either side. The value may contain spaces, but the key
may not. So:
@%title "All Roads Lead to Rome"
translates to the Python code:
__title__ = "All Roads Lead to Rome"
but obviously in a way that easier to detect externally than if this Python code were to
appear somewhere in an expansion. Since significator keys are surrounded by double
underscores, significator keys can be any sequence of alphanumeric and underscore
characters; choosing 123 is perfectly valid for a significator (although straight), since
it maps to the name __123__ which is a legal Python identifier.
Note the value can be any Python expression. The value can be omitted; if missing, it is
treated as None.
Significators are completely optional; it is totally legal for a EmPy file or files to be
processed without containing any significators.
A regular expression string designed to match significators (with the default prefix) is
available as empy.SIGNIFICATOR_RE_STRING, and also is a toplevel definition in the em
module itself.
Diversions
EmPy supports an extended form of m4-style diversions, which are a mechanism for deferring
and recalling output on demand. Multiple "streams" of output can be diverted and
undiverted in this manner. A diversion is identified with a name, which is any immutable
object such an integer or string. When recalled, diverted code is not resent through the
EmPy interpreter (although a filter could be set up to do this).
By default, no diversions take place. When no diversion is in effect, processing output
goes directly to the specified output file. This state can be explicitly requested at any
time by calling the empy.stopDiverting function. It is always legal to call this function.
When diverted, however, output goes to a deferred location which can then be recalled
later. Output is diverted with the empy.startDiversion function, which takes an argument
that is the name of the diversion. If there is no diversion by that name, a new diversion
is created and output will be sent to that diversion; if the diversion already exists,
output will be appended to that preexisting diversion.
Output send to diversions can be recalled in two ways. The first is through the
empy.playDiversion function, which takes the name of the diversion as an argument. This
recalls the named diversion, sends it to the output, and then erases that diversion. A
variant of this behavior is the empy.replayDiversion, which recalls the named diversion
but does not eliminate it afterwards; empy.replayDiversion can be repeatedly called with
the same diversion name, and will replay that diversion repeatedly. empy.createDiversion
create a diversion without actually diverting to it, for cases where you want to make sure
a diversion exists but do not yet want to send anything to it.
The diversion object itself can be retrieved with empy.retrieveDiversion. Diversions act
as writable file-objects, supporting the usual write, writelines, flush, and close
methods. The data that has been diverted to them can be retrieved in one of two ways;
either through the asString method, which returns the entire contents of the diversion as
a single strong, or through the asFile method, which returns the contents of the diversion
as a readable (not writable) file-like object.
Diversions can also be explicitly deleted without recalling them with the
empy.purgeDiversion function, which takes the desired diversion name as an argument.
Additionally there are three functions which will apply the above operations to all
existing diversions: empy.playAllDiversions, empy.replayAllDiversions, and
empy.purgeAllDiversions. All three will do the equivalent of a empy.stopDiverting call
before they do their thing.
The name of the current diversion can be requested with the empy.getCurrentDiversion
function; also, the names of all existing diversions (in sorted order) can be retrieved
with empy.getAllDiversions.
When all processing is finished, the equivalent of a call to empy.playAllDiversions is
done.
Filters
EmPy also supports dynamic filters. Filters are put in place right "before" the final
output file, and so are only invoked after all other processing has taken place (including
interpreting and diverting). Filters take input, remap it, and then send it to the output.
The current filter can be retrieved with the empy.getFilter function. The filter can be
cleared (reset to no filter) with empy.resetFilter and a special "null filter" which does
not send any output at all can be installed with empy.nullFilter. A custom filter can be
set with the empy.setFilter function; for convenience, specialized forms of filters
preexist and can be accessed with shortcuts for the empy.setFilter argument:
* None is a special filter meaning "no filter"; when installed, no filtering whatsoever
will take place. empy.setFilter(None) is equivalent to empy.resetFilter().
* 0 (or any other numeric constant equal to zero) is another special filter that
represents the null filter; when installed, no output will ever be sent to the
filter's sink.
* A filter specified as a function (or lambda) is expected to take one string argument
and return one string argument; this filter will execute the function on any input and
use the return value as output.
* A filter that is a string is a 256-character table is substituted with the result of a
call to string.translate using that table.
* A filter can be an instance of a subclass of empy.Filter. This is the most general
form of filter. (In actuality, it can be any object that exhibits a Filter interface,
which would include the normal file-like write, flush, and close methods, as well as
next, attach, and detach methods for filter-specific behavior.)
* Finally, the argument to empy.setFilter can be a Python list consisting of one or more
of the above objects. In that case, those filters are chained together in the order
they appear in the list. An empty list is the equivalent of 'None'; all filters will
be uninstalled.
Filters are, at their core, simply file-like objects (minimally supporting write, flush,
and close methods that behave in the usual way) which, after performing whatever
processing they need to do, send their work to the next file-like object or filter in
line, called that filter's "sink." That is to say, filters can be "chained" together; the
action of each filter takes place in sequence, with the output of one filter being the
input of the next. Additionally, filters support a _flush method (note the leading
underscore) which will always flush the filter's underlying sink; this method should be
not overridden.
Filters also support three additional methods, not part of the traditional file interface:
attach, which takes as an argument a file-like object (perhaps another filter) and sets
that as the filter's "sink" -- that is, the next filter/file-like object in line. detach
(which takes no arguments) is another method which flushes the filter and removes its
sink, leaving it isolated. Finally, next is an accessor method which returns the filter's
sink -- or None, if the filter does not yet have a sink attached.
To create your own filter, you can create an object which supports the above described
interface, or simply derive from the empy.Filter class and override its write and possibly
flush methods. You can chain filters together by passing them as elements in a list to the
empy.setFilter function, or you can chain them together manually with the attach method:
firstFilter.attach(secondFilter)
empy.setFilter(firstFilter)
or just let EmPy do the chaining for you:
empy.setFilter([firstFilter, secondFilter])
In either case, EmPy will walk the filter chain and find the end and then hook that into
the appropriate interpreter stream; you need not do this manually.
Subclasses of empy.Filter are already provided with the above null, function, and string
functionality described above; they are NullFilter, FunctionFilter, and StringFilter,
respectively. In addition, a filter which supports buffering, BufferedFilter, is provided.
Several variants are included: SizeBufferedFilter, a filter which buffers into fixed-sized
chunks, LineBufferedFilter, a filter which buffers by lines, and MaximallyBufferedFilter,
a filter which completely buffers its input.
Hooks
The EmPy system also allows for the usage of "hooks," which are callbacks that can be
registered with an interpreter to get information on the current state of activity and act
upon it.
Hooks are associated with names, which are merely strings; these strings represent a state
of the interpreter. Any number of hooks can be associated with a given name, and are
registered with the empy.addHook function call. Hooks are callable objects which take two
arguments: first, a reference to the interpreter that is running; and second, a dictionary
that contains contextual information about the point at which the hook is invoked; the
contents of this dictionary are dependent on the hook name.
Hooks can perform any reasonable action, with one caveat: When hooks are invoked,
sys.stdout may not be properly wrapped and so should be considered unusable. If one wishes
to really write to the actually stdout stream (not the interpreter), use
sys.__stdout__.write. If one wishes to send output to the interpreter, then use
interpreter.write. Neither references to sys.stdout nor print statements should ever
appear in a hook.
The hooks associated with a given name can be retrieved with empy.getHooks. All hooks
associated with a name can be cleared with empy.clearHooks, and all hooks associated with
all names can be cleared with empy.clearAllHooks. A hook added with empy.addHook can be
removed with empy.removeHook. Finally, hooks can be manually invoked via empy.invokeHook.
The following hooks are supported; also listed in curly braces are the keys contained in
the dictionary argument:
at_shutdown
The interpreter is shutting down.
at_handle {meta}
An exception is being handled; meta is the exception (an instance of MetaError).
Note that this hook is invoked when the exception is handled by the EmPy system,
not when it is thrown.
before_include {name, file}
An empy.include call is about to be processed; name is the context name of the
inclusion and file is the actual file object associated with the include.
after_include
An empy.include was just completed.
before_expand {string, locals}
An empy.expand call is about to be processed. string is the actual data that is
about to be processed; locals is the locals dictionary or None.
after_expand
An empy.expand was just completed.
at_quote {string}
An empy.quote call is about to be processed; string is the string to be quoted.
at_escape {string}
An empy.escape call is about to be processed; string is the string to be escaped.
before_file {name, file}
A file object is just about to be processed. name is the context name associated
with the object and file is the file object itself.
after_file
A file object has just finished processing.
before_string {name, string}
A standalone string is just about to be processed. name is the context name
associated with it and string is the string itself.
after_string
A standalone string has just finished being processed.
at_parse {scanner}
A parsing pass is just about to be performed. scanner is the scanner associated
with the parsing pass.
before_evaluate {expression, locals}
A Python expression is just about to be evaluated. expression is the (string)
expression, and locals is the locals dictionary or None.
after_evaluate
A Python expression was just evaluated.
before_execute {statements, locals}
A chunk of Python statements is just about to be evaluated. statements is the
(string) statement block, and locals is the locals dictionary or None.
before_single {source, locals}
A single interactive source code fragment (just as in the Python interpreter) is
about to be executed via Interpreter.single. source is the code (expression or
statement) to execute, and locals is the locals directory or None.
after_single
A single has just taken place.
before_substitute {substitution}
A @[...] substitution is just about to be done. substitution is the substitution
string itself.
after_substitute
A substitution just took place.
before_significate {key, value}
A significator is just about to be processed; key is the key and value is the
value.
after_significate
A significator was just processed.
As a practical example, this sample Python code would print a pound sign followed by the
name of every file that is included with 'empy.include':
def includeHook(interpreter, keywords):
interpreter.write("# %s\n" % keywords['name'])
empy.addHook('before_include', includeHook)
Note that this snippet properly uses a call to interpreter.write instead of executing a
print statement.
Data flow
input -> interpreter -> diversions -> filters -> output
Here, in summary, is how data flows through a working EmPy system:
1. Input comes from a source, such an .em file on the command line, or via an
empy.include statement.
2. The interpreter processes this material as it comes in, expanding token sequences as
it goes.
3. After interpretation, data is then sent through the diversion layer, which may allow
it directly through (if no diversion is in progress) or defer it temporarily.
Diversions that are recalled initiate from this point.
4. Any filters in place are then used to filter the data and produce filtered data as
output.
5. Finally, any material surviving this far is sent to the output stream. That stream is
stdout by default, but can be changed with the -o or -a options, or may be fully
buffered with the -B option (that is, the output file would not even be opened until
the entire system is finished).
Pseudomodule contents
The empy pseudomodule (available only in an operating EmPy system) contains the following
functions and objects (and their signatures, with a suffixed opt indicating an optional
argument):
First, basic identification:
VERSION
A constant variable which contains a string representation of the EmPy version.
SIGNIFICATOR_RE_STRING
A constant variable representing a regular expression string that can be used to
find significators in EmPy code.
interpreter
The instance of the interpreter that is currently being used to perform execution.
argv
A list consisting of the name of the primary EmPy script and its command line
arguments, in analogue to the sys.argv list.
args
A list of the command line arguments following the primary EmPy script; this is
equivalent to empy.argv[1:].
identify() -> string, integer
Retrieve identification information about the current parsing context. Returns a
2-tuple consisting of a filename and a line number; if the file is something other
than from a physical file (e.g., an explicit expansion with empy.expand, a
file-like object within Python, or via the -E or -F command line options), a string
representation is presented surrounded by angle brackets. Note that the context
only applies to the EmPy context, not the Python context.
setName(name)
Manually set the name of the current context.
setLine(line)
Manually set the line number of the current context; line must be a numeric value.
Note that afterward the line number will increment by one for each newline that is
encountered, as before.
atExit(callable)
Register a callable object (or function) taking no arguments which will be called
at the end of a normal shutdown. Callable objects registered in this way are called
in the reverse order in which they are added, so the first callable registered with
empy.atExit is the last one to be called. Note that although the functionality is
related to hooks, empy.atExit does no work via the hook mechanism, and you are
guaranteed that the interpreter and stdout will be in a consistent state when the
callable is invoked.
Globals manipulation:
getGlobals()
Retrieve the globals dictionary for this interpreter. Unlike when calling globals()
in Python, this dictionary can be manipulated and you can expect changes you make
to it to be reflected in the interpreter that holds it.
setGlobals(globals)
Reseat the globals dictionary associated with this interpreter to the provided
mapping type.
updateGlobals(globals)
Merge the given dictionary into this interpreter's globals.
clearGlobals(globals_opt)
Clear out the globals (restoring, of course, the empy pseudomodule). Optionally,
instead of starting with a refresh dictionary, use the dictionary provided.
Filter classes:
Filter
The base Filter class which can be derived from to make custom filters.
NullFilter
A null filter; all data sent to the filter is discarded.
FunctionFilter
A filter which uses a function taking a string and returning another to perform the
filtering.
StringFilter
A filter which uses a 256-character string table to map any incoming character.
BufferedFilter
A filter which does not modify its input, but instead holds it until it is told to
flush (via the filter's flush method). This also serves as the base class for the
other buffered filters below.
SizeBufferedFilter
A filter which buffers into fixed-size chunks, with the possible exception of the
last chunk. The buffer size is indicated as the sole argument to the constructor.
LineBufferedFilter
A filter which buffers into lines, with the possible exception of the last line
(which may not end with a newline).
MaximallyBufferedFilter
A filter which does not flush any of its contents until it is closed. Note that
since this filter ignores calls to its flush method, this means that installing
this filter and then replacing it with another can result in loss of data.
The following functions allow direct execution; optional locals arguments, if specified,
are treated as the locals dictionary in evaluation and execution:
evaluate(expression, locals_opt)
Evaluate the given expression.
execute(statements, locals_opt)
Execute the given statement(s).
single(source, locals_opt)
Interpret the "single" source code, just as the Python interactive interpreter
would.
substitute(substitution, locals_opt)
Perform the given substitution.
significate(key, value_opt)
Do a manual signification. If value is not specified, it is treated as None.
The following functions relate to source manipulation:
include(file_or_filename, locals_opt)
Include another EmPy file, by processing it in place. The argument can either be a
filename (which is then opened with open in text mode) or a file object, which is
used as is. Once the included file is processed, processing of the current file
continues. Includes can be nested. The call also takes an optional locals
dictionary which will be passed into the evaluation function.
expand(string, locals_opt) -> string
Explicitly invoke the EmPy parsing system to process the given string and return
its expansion. This allows multiple levels of expansion, e.g., @(empy.expand("@(2 +
2)")). The call also takes an optional locals dictionary which will be passed into
the evaluation function. This is necessary when text is being expanded inside a
function definition and it is desired that the function arguments (or just plain
local variables) are available to be referenced within the expansion.
quote(string) -> string
The inverse process of empy.expand, this will take a string and return a new string
that, when expanded, would expand to the original string. In practice, this means
that appearances of the prefix character are doubled, except when they appear
inside a string literal.
escape(string, more_opt) -> string
Given a string, quote the nonprintable characters contained within it with EmPy
escapes. The optional more argument specifies additional characters that should be
escaped.
flush()
Do an explicit flush on the underlying stream.
string(string, name_opt, locals_opt)
Explicitly process a string-like object. This differs from empy.expand in that the
string is directly processed into the EmPy system, rather than being evaluated in
an isolated context and then returned as a string.
Changing the behavior of the pseudomodule itself:
flatten(keys_opt)
Perform the equivalent of from empy import ... in code (which is not directly
possible because empy is a pseudomodule). If keys is omitted, it is taken as being
everything in the empy pseudomodule. Each of the elements of this pseudomodule is
flattened into the globals namespace; after a call to empy.flatten, they can be
referred to simple as globals, e.g., @divert(3) instead of @empy.divert(3). If any
preexisting variables are bound to these names, they are silently overridden. Doing
this is tantamount to declaring an from ... import ... which is often considered
bad form in Python.
Prefix-related functions:
getPrefix() -> char
Return the current prefix.
setPrefix(char)
Set a new prefix. Immediately after this call finishes, the prefix will be changed.
Changing the prefix affects only the current interpreter; any other created
interpreters are unaffected.
Diversions:
stopDiverting()
Any diversions that are currently taking place are stopped; thereafter, output will
go directly to the output file as normal. It is never illegal to call this
function.
createDiversion(name)
Create a diversion, but do not begin diverting to it. This is the equivalent of
starting a diversion and then immediately stopping diversion; it is used in cases
where you want to make sure that a diversion will exist for future replaying but
may be empty.
startDiversion(name)
Start diverting to the specified diversion name. If such a diversion does not
already exist, it is created; if it does, then additional material will be appended
to the preexisting diversions.
playDiversion(name)
Recall the specified diversion and then purge it. The provided diversion name must
exist.
replayDiversion(name)
Recall the specified diversion without purging it. The provided diversion name must
exist.
purgeDiversion(name)
Purge the specified diversion without recalling it. The provided diversion name
must exist.
playAllDiversions()
Play (and purge) all existing diversions in the sorted order of their names. This
call does an implicit empy.stopDiverting before executing.
replayAllDiversions()
Replay (without purging) all existing diversions in the sorted order of their
names. This call does an implicit empy.stopDiverting before executing.
purgeAllDiversions()
Purge all existing diversions without recalling them. This call does an implicit
empy.stopDiverting before executing.
getCurrentDiversion() -> diversion
Return the name of the current diversion.
getAllDiversions() -> sequence
Return a sorted list of all existing diversions.
Filters:
getFilter() -> filter
Retrieve the current filter. None indicates no filter is installed.
resetFilter()
Reset the filter so that no filtering is done.
nullFilter()
Install a special null filter, one which consumes all text and never sends any text
to the output.
setFilter(filter)
Install a new filter. A filter is None or an empty sequence representing no filter,
or 0 for a null filter, a function for a function filter, a string for a string
filter, or an instance of empy.Filter. If filter is a list of the above things,
they will be chained together manually; if it is only one, it will be presumed to
be solitary or to have already been manually chained together. See the "Filters"
section for more information.
Hooks:
enableHooks()
Enable invocation of hooks. By default hooks are enabled.
disableHooks()
Disable invocation of hooks. Hooks can still be added, removed, and queried, but
invocation of hooks will not occur (even explicit invocation with empy.invokeHook).
areHooksEnabled()
Return whether or not hooks are presently enabled.
getHooks(name)
Get a list of the hooks associated with this name.
clearHooks(name)
Clear all hooks associated with this name.
clearAllHooks(name)
Clear all hooks associated with this name.
addHook(name, hook, prepend_opt)
Add this hook to the hooks associated with this name. By default, the hook is
appended to the end of the existing hooks, if any; if the optional insert argument
is present and true, it will be prepended to the list instead.
removeHook(name, hook)
Remove this hook from the hooks associated with this name.
invokeHook(name_, ...)
Manually invoke all the hooks associated with this name. The remaining arguments
are treated as keyword arguments and the resulting dictionary is passed in as the
second argument to the hooks.
Invocation
Basic invocation involves running the interpreter on an EmPy file and some optional
arguments. If no file are specified, or the file is named -, EmPy takes its input from
stdin. One can suppress option evaluation (to, say, specify a file that begins with a
dash) by using the canonical -- option.
-a/--append (filename)
Open the specified file for append instead of using stdout.
-f/--flatten
Before processing, move the contents of the empy pseudomodule into the globals,
just as if empy.flatten() were executed immediately after starting the interpreter.
That is, e.g., empy.include can be referred to simply as include when this flag is
specified on the command line. This can also be specified through the existence of
the EMPY_FLATTEN environment variable.
-h/--help
Print usage and exit.
-H/--extended-help
Print extended usage and exit. Extended usage includes a rundown of all the legal
expansions, escape sequences, pseudomodule contents, used hooks, and supported
environment variables.
-i/--interactive
After the main EmPy file has been processed, the state of the interpreter is left
intact and further processing is done from stdin. This is analogous to the Python
interpreter's -i option, which allows interactive inspection of the state of the
system after a main module is executed. This behaves as expected when the main file
is stdin itself. This can also be specified through the existence of the
EMPY_INTERACTIVE environment variable.
-k/--suppress-errors
Normally when an error is encountered, information about its location is printed
and the EmPy interpreter exits. With this option, when an error is encountered
(except for keyboard interrupts), processing stops and the interpreter enters
interactive mode, so the state of affairs can be assessed. This is also helpful,
for instance, when experimenting with EmPy in an interactive manner. -k implies -i.
-o/--output (filename)
Open the specified file for output instead of using stdout. If a file with that
name already exists it is overwritten.
-p/--prefix (prefix)
Change the prefix used to detect expansions. The argument is the one-character
string that will be used as the prefix. Note that whatever it is changed to, the
way to represent the prefix literally is to double it, so if $ is the prefix, a
literal dollar sign is represented with $$. Note that if the prefix is changed to
one of the secondary characters (those that immediately follow the prefix to
indicate the type of action EmPy should take), it will not be possible to represent
literal prefix characters by doubling them (e.g., if the prefix were unadvisedly
changed to # then ## would already have to represent a comment, so ## could not
represent a literal #). This can also be specified through the EMPY_PREFIX
environment variable.
-r/--raw-errors
Normally, EmPy catches Python exceptions and prints them alongside an error
notation indicating the EmPy context in which it occurred. This option causes EmPy
to display the full Python traceback; this is sometimes helpful for debugging. This
can also be specified through the existence of the EMPY_RAW_ERRORS environment
variable.
-B/--buffered-output
Fully buffer processing output, including the file open itself. This is helpful
when, should an error occur, you wish that no output file be generated at all (for
instance, when using EmPy in conjunction with make). When specified, either the -o
or -a options must be specified (and the -B option must precede them; full
buffering does not work with stdout. This can also be specified through the
existence of the EMPY_BUFFERED_OUTPUT environment variable.
-D/--define (assignment)
Execute a Python assignment of the form variable = expression. If only a variable
name is provided (i.e., the statement does not contain an = sign), then it is taken
as being assigned to None. The -D option is simply a specialized -E option that
special cases the lack of an assignment operator. Multiple -D options can be
specified.
-E/--execute (statement)
Execute the Python (not EmPy) statement before processing any files. Multiple -E
options can be specified.
-F/--execute-file (filename)
Execute the Python (not EmPy) file before processing any files. This is equivalent
to -E execfile("filename") but provides a more readable context. Multiple -F
options can be specified.
-I/--import (module)
Imports the specified module name before processing any files. Multiple modules can
be specified by separating them by commas, or by specifying multiple -I options.
-P/--preprocess (filename)
Process the EmPy file before processing the primary EmPy file on the command line.
-V/--version
Print version and exit.
Environment variables
EmPy also supports a few environment variables to predefine certain behaviors. The
settings chosen by environment variables can be overridden via command line arguments. The
following environment variables have meaning to EmPy:
EMPY_OPTIONS
If present, the contents of this environment variable will be treated as options,
just as if they were entered on the command line, before the actual command line
arguments are processed. Note that these arguments are not processed by the shell,
so quoting, filename globbing, and the like, will not work.
EMPY_PREFIX
If present, the value of this environment variable represents the prefix that will
be used; this is equivalent to the -p command line option.
EMPY_FLATTEN
If defined, this is equivalent to including -f on the command line.
EMPY_RAW_ERRORS
If defined, this is equivalent to including -r on the command line.
EMPY_INTERACTIVE
If defined, this is equivalent to including -i on the command line.
EMPY_BUFFERED_OUTPUT
If defined, this is equivalent to including -B on the command line.
Examples and testing EmPy
See the sample EmPy file sample.em which is included with the distribution. Run EmPy on it
by typing something like (presuming a UNIX-like operating system):
./em.py sample.em
and compare the results and the sample source file side by side. The sample content is
intended to be self-documenting.
The file sample.bench is the benchmark output of the sample. Running the EmPy interpreter
on the provided sample.em file should produce precisely the same results. You can run the
provided test script to see if your EmPy environment is behaving as expected:
./test.sh
By default this will test with the first Python interpreter available in the path; if you
want to test with another interpreter, you can provide it as the first argument on the
command line, e.g.:
./test.sh python2.1
./test.sh /usr/bin/python1.5
./test.sh jython
Embedding EmPy
Embedding EmPy into your application is quite simple. The relative complexity of the
em.invoke function is due to handling every possible combination of options (via the
command line and environment variables). An EmPy interpreter can be created with as code
as simple as:
import em
interpreter = em.Interpreter()
# The following prints the results to stdout:
interpreter.string("@{x = 123}@x\n")
# This expands to the same thing, but puts the results as a
# string in the variable result:
result = interpreter.expand("@{x = 123}@x\n")
# Process an actual file (and output to stdout):
interpreter.file('/path/to/some/file')
When you are finished with your interpreter, it is important to call its shutdown method:
interpreter.shutdown()
This will ensure that the interpreter cleans up all its overhead, entries in the
sys.stdout proxy, and so forth. It is usually advisable that this be used in a
try...finally clause:
interpreter = em.Interpreter(...)
try:
...
finally:
interpreter.shutdown()
The em.Interpreter constructor takes the following arguments; all are optional:
output
The output file which the interpreter will be sending all its processed data to.
This need only be a file-like object; it need not be an actual file. If omitted,
sys.__stdout__ is used.
argv
An argument list analogous to sys.argv, consisting of the script name and zero or
more arguments. These are available to executing interpreters via empy.argv and
empy.args. If omitted, a non-descript script name is used with no arguments.
prefix
The single character prefix. Defaults to @.
options
A dictionary of options that can override the default behavior of the interpreter.
The names of the options are constant names ending in _OPT and their defaults are
given in Interpreter.DEFAULT_OPTIONS.
globals
By default, interpreters begin with a pristine dictionary of globals (except, of
course, for the empy pseudomodule). Specifying this argument will allow the globals
to start with more.
Many things can be done with EmPy interpreters; for the full developer documentation, see
the generated documentation for the em module.
Interpreter options
The following options (passed in as part of the options dictionary to the Interpreter
constructor) have the following meanings. The defaults are shown below and are also
indicated in an Interpreter.DEFAULT_OPTIONS dictionary.
BANGPATH_OPT
Should a bangpath (#!) as the first line of an EmPy file be treated as if it were
an EmPy comment? Note that #! sequences starting lines or appearing anywhere else
in the file are untouched regardless of the value of this option. Default: true.
BUFFERED_OPT
Should an abort method be called upon failure? This relates to the fully-buffered
option, where all output can be buffered including the file open; this option only
relates to the interpreter's behavior after that proxy file object has been
created. Default: false.
RAW_OPT
Should errors be displayed as raw Python errors (that is, the exception is allowed
to propagate through to the toplevel so that the user gets a standard Python
traceback)? Default: false.
EXIT_OPT
Upon an error, should execution continue (although the interpreter stacks will be
purged)? Note that even in the event this is set, the interpreter will halt upon
receiving a KeyboardInterrupt. Default: true.
FLATTEN_OPT
Upon initial startup, should the empy pseudomodule namespace be flattened, i.e.,
should empy.flatten be called? Note this option only has an effect when the
interpreter is first created; thereafter it is ignored. Default: false.
Known issues and caveats
* EmPy was primarily intended for static processing of documents, rather than dynamic
use, and hence speed of processing was not a major consideration in its design.
* EmPy is not threadsafe.
* Expressions (@(...)) are intended primarily for their return value; statements
(@{...}) are intended primarily for their side effects, including of course printing.
If an expression is expanded that as a side effect prints something, then the printing
side effects will appear in the output before the expansion of the expression value.
* Due to Python's curious handling of the print keyword -- particularly the form with a
trailing comma to suppress the final newline -- mixing statement expansions using
prints inline with unexpanded text will often result in surprising behavior, such as
extraneous (sometimes even deferred!) spaces. This is a Python "feature," and occurs
in non-EmPy applications as well; for finer control over output formatting, use
sys.stdout.write or empy.interpreter.write (these will do the same thing) directly.
* To function properly, EmPy must override sys.stdout with a proxy file object, so that
it can capture output of side effects and support diversions for each interpreter
instance. It is important that code executed in an environment not rebind sys.stdout,
although it is perfectly legal to invoke it explicitly (e.g., @sys.stdout.write("Hello
world\n")). If one really needs to access the "true" stdout, then use sys.__stdout__
instead (which should also not be rebound). EmPy uses the standard Python error
handlers when exceptions are raised in EmPy code, which print to sys.stderr.
* The empy "module" exposed through the EmPy interface (e.g., @empy) is an artificial
module. It cannot be imported with the import statement (and shouldn't -- it is an
artifact of the EmPy processing system and does not correspond to any accessible .py
file).
* For an EmPy statement expansion all alone on a line, e.g., @{a = 1}, note that this
will expand to a blank line due to the newline following the closing curly brace. To
suppress this blank line, use the symmetric convention @{a = 1}@.
* When using EmPy with make, note that partial output may be created before an error
occurs; this is a standard caveat when using make. To avoid this, write to a temporary
file and move when complete, delete the file in case of an error, use the -B option to
fully buffer output (including the open), or (with GNU make) define a .DELETE_ON_ERROR
target.
* empy.identify tracks the context of executed EmPy code, not Python code. This means
that blocks of code delimited with @{ and } will identify themselves as appearing on
the line at which the } appears, and that pure Python code executed via the -D, -E and
-F command line arguments will show up as all taking place on line 1. If you're
tracking errors and want more information about the location of the errors from the
Python code, use the -r command line option, which will provide you with the full
Python traceback.
Wish list
Here are some random ideas for future revisions of EmPy. If any of these are of particular
interest to you, your input would be appreciated.
* Some real-world examples should really be included for demonstrating the power and
expressiveness of EmPy first-hand.
* A "trivial" mode, where all the EmPy system does is scan for simple tokens replace
them with evaluations/executions, rather than having to do the contextual scanning it
does now. This has the down side of being much less configurable and powerful but the
upside of being extremely efficient. Perhaps this need not be a separate mode, but an
additional prefix something of the form @<(...)>, @<{...}>, and possibly @<[12][...]>?
Setting the trivial mode might simply disallow other expansions.
* A "debug" mode, where EmPy prints the contents of everything it's about to evaluate
(probably to stderr) before it does?
* The ability to funnel all code through a configurable RExec for user-controlled
security control. This would probably involve abstracting the execution functionality
outside of the interpreter.
* Optimized handling of processing would be nice for the possibility of an Apache module
devoted to EmPy processing.
* An EmPy emacs mode.
* An "unbuffered" option which would lose contextual information like line numbers, but
could potentially be more efficient at processing large files.
* An optimization of offloading diversions to files when they become truly huge.
* Unicode support, particularly for filters. (This may be problematic given Python 1.5.2
support.)
* Support for mapping filters (specified by dictionaries).
* Support for some sort of batch processing, where several EmPy files can be listed at
once and all of them evaluated with the same initial (presumably expensive)
environment.
* A more elaborate interactive mode, perhaps with a prompt and readline support.
* A toplevel run function, which invoke delegates to, that accepts arguments similar to
the command line as keyword arguments. Perhaps also a simplified wrapper just for
doing basic processing, e.g., interpreter.simple?
* A tool to collect significator information from a hierarchy of .em files and put them
in a database form available for individual scripts would be extremely useful.
* A StructuredText and/or reStructuredText filter would be quite useful, as would
SGML/HTML/XML, s-expression, Python, etc. auto-indenter filters.
* A caching system that stores off the compilations of repeated evaluations and
executions so that in a persistent environment the same code does not have to be
repeatedly evaluated/executed. This would probably be a necessity in an Apache
module-based solution.
* An option to change the format of the standard EmPy messages in a traceback.
* An "binary" option to have EmPy process incoming data in chunks, rather than by lines,
for handling of non-textual data or data which may not contain predictably short
lines.
* Support for some manner of implicitly processed /etc/empyrc and/or ~/.empyrc file, and
of course an option to inhibit its processing. This can already be accomplished via an
explicit EMPY_OPTIONS, but still ...
* More uniform handling of the preprocessing directives (-I, -D, -E, -F, and -P),
probably mapping directly to methods in the Interpreter class.
* distutils support.
Author's notes
I originally conceived EmPy as a replacement for my [13]Web templating system which uses
[14]m4 (a general macroprocessing system for UNIX).
Most of my Web sites include a variety of m4 files, some of which are dynamically
generated from databases, which are then scanned by a cataloging tool to organize them
hierarchically (so that, say, a particular m4 file can understand where it is in the
hierarchy, or what the titles of files related to it are without duplicating information);
the results of the catalog are then written in database form as an m4 file (which every
other m4 file implicitly includes), and then GNU make converts each m4 to an HTML file by
processing it.
As the Web sites got more complicated, the use of m4 (which I had originally enjoyed for
the challenge and abstractness) really started to become an impediment to serious work;
while I am very knowledgeable about m4 -- having used it for for so many years -- getting
even simple things done with it is awkward and difficult. Worse yet, as I started to use
Python more and more over the years, the cataloging programs which scanned the m4 and
built m4 databases were migrated to Python and made almost trivial, but writing out huge
awkward tables of m4 definitions simply to make them accessible in other m4 scripts
started to become almost farcical -- especially when coupled with the difficulty in
getting simple things done in m4.
It occurred to me what I really wanted was an all-Python solution. But replacing what used
to be the m4 files with standalone Python programs would result in somewhat awkward
programs normally consisting mostly of unprocessed text punctuated by small portions where
variables and small amounts of code need to be substituted. Thus the idea was a sort of
inverse of a Python interpreter: a program that normally would just pass text through
unmolested, but when it found a special signifier would execute Python code in a
persistent environment. After considering between choices of signifiers, I settled on @
and EmPy was born.
As I developed the tool, I realized it could have general appeal, even to those with
widely varying problems to solve, provided the core tool they needed was an interpreter
that could embed Python code inside templated text. As I continue to use the tool, I have
been adding features as unintrusively as possible as I see areas that can be improved.
A design goal of EmPy is that its feature set should work on several levels; at each
level, if the user does not wish or need to use features from another level, they are
under no obligation to do so. If you have no need of substitutions, for instance, you are
under no obligation to use them. If significators will not help you organize a set of EmPy
scripts globally, then you need not use them. New features that are being added are
whenever possible transparently backward compatible; if you do not need them, their
introduction should not affect you in any way. The use of unknown prefix sequences results
in errors, guaranteeing that they are reserved for future use.
Release history
* 2.3; 2003 Feb 20. Proper and full support for concurrent and recursive interpreters;
protection from closing the true stdout file object; detect edge cases of interpreter
globals or sys.stdout proxy collisions; add globals manipulation functions
empy.getGlobals, empy.setGlobals, and empy.updateGlobals which properly preserve the
empy pseudomodule; separate usage info out into easily accessible lists for easier
presentation; have -h option show simple usage and -H show extened usage; add NullFile
utility class.
* 2.2.6; 2003 Jan 30. Fix a bug in the Filter.detach method (which would not normally be
called anyway).
* 2.2.5; 2003 Jan 9. Strip carriage returns out of executed code blocks for DOS/Windows
compatibility.
* 2.2.4; 2002 Dec 23. Abstract Filter interface to use methods only; add @[noop: ...]
substitution for completeness and block commenting.
* 2.2.3; 2002 Dec 16. Support compatibility with Jython by working around a minor
difference between CPython and Jython in string splitting.
* 2.2.2; 2002 Dec 14. Include better docstrings for pseudomodule functions; segue to a
dictionary-based options system for interpreters; add empy.clearAllHooks and
'empy.clearGlobals'; include a short documentation section on embedding interpreters;
fix a bug in significator regular expression.
* 2.2.1; 2002 Nov 30. Tweak test script to avoid writing unnecessary temporary file; add
Interpreter.single method; expose evaluate, execute, substitute, and single methods to
the pseudomodule; add (rather obvious) EMPY_OPTIONS environment variable support; add
empy.enableHooks and 'empy.disableHooks'; include optimization to transparently
disable hooks until they are actually used.
* 2.2; 2002 Nov 21. Switched to -V option for version information; empy.createDiversion
for creating initially empty diversion; direct access to diversion objects with
'empy.retrieveDiversion'; environment variable support; removed --raw long argument
(use --raw-errors instead); added quaternary escape code (well, why not).
* 2.1; 2002 Oct 18. empy.atExit registry separate from hooks to allow for normal
interpreter support; include a benchmark sample and test.sh verification script;
expose empy.string directly; -D option for explicit defines on command line; remove
ill-conceived support for @else: separator in @[if ...] substitution; handle nested
substitutions properly; @[macro ...] substitution for creating recallable expansions.
* 2.0.1; 2002 Oct 8. Fix missing usage information; fix after_evaluate hook not getting
called; add empy.atExit call to register values.
* 2.0; 2002 Sep 30. Parsing system completely revamped and simplified, eliminating a
whole class of context-related bugs; builtin support for buffered filters; support for
registering hooks; support for command line arguments; interactive mode with -i;
significator value extended to be any valid Python expression.
* 1.5.1; 2002 Sep 24. Allow @] to represent unbalanced close brackets in @[...] markups
[now defunct; use escape codes instead].
* 1.5; 2002 Sep 18. Escape codes (@\...); conditional and repeated expansion
substitutions via @[if E:...], @[for X in E:...], and @[while E:...] notations; fix a
few bugs involving files which do not end in newlines.
* 1.4; 2002 Sep 7. Fix bug with triple quotes; collapse conditional and protected
expression syntaxes into the single generalized @(...) notation; empy.setName and
empy.setLine functions; true support for multiple concurrent interpreters with
improved sys.stdout proxy; proper support for empy.expand to return a string evaluated
in a subinterpreter as intended; merged Context and Parser classes together, and
separated out Scanner functionality.
* 1.3; 2002 Aug 24. Pseudomodule as true instance; move toward more verbose (and clear)
pseudomodule functions; fleshed out diversion model; filters; conditional expressions;
protected expressions; preprocessing with -P (in preparation for possible support for
command line arguments).
* 1.2; 2002 Aug 16. Treat bangpaths as comments; empy.quote for the opposite process of
'empy.expand'; significators (@%... sequences); -I option; -f option; much improved
documentation.
* 1.1.5; 2002 Aug 15. Add a separate invoke function that can be called multiple times
with arguments to simulate multiple runs.
* 1.1.4; 2002 Aug 12. Handle strings thrown as exceptions properly; use getopt to
process command line arguments; cleanup file buffering with AbstractFile; very slight
documentation and code cleanup.
* 1.1.3; 2002 Aug 9. Support for changing the prefix from within the empy pseudomodule.
* 1.1.2; 2002 Aug 5. Renamed buffering option to -B, added -F option for interpreting
Python files from the command line, fixed improper handling of exceptions from command
line options (-E, -F).
* 1.1.1; 2002 Aug 4. Typo bugfixes; documentation clarification.
* 1.1; 2002 Aug 4. Added option for fully buffering output (including file opens),
executing commands through the command line; some documentation errors fixed.
* 1.0; 2002 Jul 23. Renamed project to EmPy. Documentation and sample tweaks; added
empy.flatten. Added -a option.
* 0.3; 2002 Apr 14. Extended "simple expression" syntax, interpreter abstraction, proper
context handling, better error handling, explicit file inclusion, extended samples.
* 0.2; 2002 Apr 13. Bugfixes, support non-expansion of Nones, allow choice of alternate
prefix.
* 0.1.1; 2002 Apr 12. Bugfixes, support for Python 1.5.x, add -r option.
* 0.1; 2002 Apr 12. Initial early access release.
Author
This module was written by [15]Erik Max Francis. If you use this software, have
suggestions for future releases, or bug reports, [16]I'd love to hear about it.
Even if you try out EmPy for a project and find it unsuitable, I'd like to know what
stumbling blocks you ran into so they can potentially be addressed in a future version.
Version
Version 2.3 $Date$ $Author$
Modules and Packages
[17]em
A system for processing Python as markup embedded in text.
_____________________________________________________________________________________
[18]Table of Contents
This document was automatically generated on Thu Feb 20 03:56:27 2003 by [19]HappyDoc
version 2.0.1
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