Summary

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 [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 2.1.

The latest version of the software is available in a tarball here: http://www.alcyone.com/pyos/empy/empy-latest.tar.gz.

The official URL for this Web site is http://www.alcyone.com/pyos/empy/.

Requirements

empy should work with any version of Python from 1.5.x onward.

License

This code is released under the GPL.

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".

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 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 #!, 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

@\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[index]', '@os.environ[name]', with no space between the name and the open bracket.

• any combination of the above (e.g., '@function(args).attr[sub].other[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:

@[ 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.

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. The only difference is that these functions will all 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.

• 0 (the integer constant 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.

• 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 in Python that, after performing whatever processing they need to do, send their work to the next filter in line, or to the final output, should there be no more filters. 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.

To create your own filter, you can derive from the empy.Filter class and override its write method; it should write to the next filter in the chain by accessing the file-like object attribute sink. You can also override its flush and close methods, if need be; by default these simply flush and close the filter's sink, respectively. 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])

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. 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_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.

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 relate to source manipulation:

include(file_or_filename, locals_opt, bangpaths_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; in addition, the optional Boolean argument indicates whether a bangpath (#!) as the first characters of a file will be treated as an empy comment (if true) or left intact (if false).

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.

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.

flush()

Do an explicit flush on the underlying stream.

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.

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.

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:

getHooks(name)

Get a list of the hooks associated with this name.

clearHooks(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.

-h/--help

Print usage and exit.

-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.

-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 #).

-r/--raw/--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.

-v/--version

Print version and exit.

-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; fully buffering does not work with stdout.

-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.

Examples

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 diff to verify that your interpreter 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:

        ./test.sh /usr/bin/python1.5

Known issues and caveats

• empy is 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"; for finer control over output formatting -- as is normal with python -- 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.

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.

• A transparent "pseudo"-sys presented to empy programs might be warranted so that sys.stdout need not be overridden at the top level. This may not be very feasible without enforcing restricted contexts, which is not always desirable.

• It would be good for statement expansions to align total indentations (i.e., the entire snippet of code can be indented by the same amount simply as a visual aid) of code as a convenience.

• The ability to funnel all code through a configurable RExec for user-controlled security control.

• Optimized handling of processing would be nice for the possibility of an Apache module devoted to empy processing.

• Some manner of "freezing" and "restoring," similar to m4's functionality, probably involving pickling and unpickling the globals, or at least some method of resetting them. Since this is so easy in Python itself, it's probably best left to the user.

• 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.

• Various optimizations such as 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 "trivial" mode, where all the empy system does is scan for @...@ tokens and replace them with evaluations/executions. This has the down side of being much less configurable but the upside of being extremely efficient.

• A more elaborate interactive mode, perhaps with a prompt and readlines support.

Author's notes

I originally conceived empy as a replacement for my Web templating system which uses 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 be come 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, usually 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.

Release history

• 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.

• 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 options 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 Erik Max Francis. If you use this software, have suggestions for future releases, or bug reports, I'd love to hear about it.

Version

Version 2.1 $Date: 2002/10/18 $ $Author: max $