357 lines
15 KiB
ReStructuredText
357 lines
15 KiB
ReStructuredText
:mod:`parser` --- Access Python parse trees
|
|
===========================================
|
|
|
|
.. module:: parser
|
|
:synopsis: Access parse trees for Python source code.
|
|
|
|
.. moduleauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
|
|
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
|
|
|
|
.. Copyright 1995 Virginia Polytechnic Institute and State University and Fred
|
|
L. Drake, Jr. This copyright notice must be distributed on all copies, but
|
|
this document otherwise may be distributed as part of the Python
|
|
distribution. No fee may be charged for this document in any representation,
|
|
either on paper or electronically. This restriction does not affect other
|
|
elements in a distributed package in any way.
|
|
|
|
.. index:: single: parsing; Python source code
|
|
|
|
--------------
|
|
|
|
The :mod:`parser` module provides an interface to Python's internal parser and
|
|
byte-code compiler. The primary purpose for this interface is to allow Python
|
|
code to edit the parse tree of a Python expression and create executable code
|
|
from this. This is better than trying to parse and modify an arbitrary Python
|
|
code fragment as a string because parsing is performed in a manner identical to
|
|
the code forming the application. It is also faster.
|
|
|
|
.. note::
|
|
|
|
From Python 2.5 onward, it's much more convenient to cut in at the Abstract
|
|
Syntax Tree (AST) generation and compilation stage, using the :mod:`ast`
|
|
module.
|
|
|
|
There are a few things to note about this module which are important to making
|
|
use of the data structures created. This is not a tutorial on editing the parse
|
|
trees for Python code, but some examples of using the :mod:`parser` module are
|
|
presented.
|
|
|
|
Most importantly, a good understanding of the Python grammar processed by the
|
|
internal parser is required. For full information on the language syntax, refer
|
|
to :ref:`reference-index`. The parser
|
|
itself is created from a grammar specification defined in the file
|
|
:file:`Grammar/Grammar` in the standard Python distribution. The parse trees
|
|
stored in the ST objects created by this module are the actual output from the
|
|
internal parser when created by the :func:`expr` or :func:`suite` functions,
|
|
described below. The ST objects created by :func:`sequence2st` faithfully
|
|
simulate those structures. Be aware that the values of the sequences which are
|
|
considered "correct" will vary from one version of Python to another as the
|
|
formal grammar for the language is revised. However, transporting code from one
|
|
Python version to another as source text will always allow correct parse trees
|
|
to be created in the target version, with the only restriction being that
|
|
migrating to an older version of the interpreter will not support more recent
|
|
language constructs. The parse trees are not typically compatible from one
|
|
version to another, whereas source code has always been forward-compatible.
|
|
|
|
Each element of the sequences returned by :func:`st2list` or :func:`st2tuple`
|
|
has a simple form. Sequences representing non-terminal elements in the grammar
|
|
always have a length greater than one. The first element is an integer which
|
|
identifies a production in the grammar. These integers are given symbolic names
|
|
in the C header file :file:`Include/graminit.h` and the Python module
|
|
:mod:`symbol`. Each additional element of the sequence represents a component
|
|
of the production as recognized in the input string: these are always sequences
|
|
which have the same form as the parent. An important aspect of this structure
|
|
which should be noted is that keywords used to identify the parent node type,
|
|
such as the keyword :keyword:`if` in an :const:`if_stmt`, are included in the
|
|
node tree without any special treatment. For example, the :keyword:`if` keyword
|
|
is represented by the tuple ``(1, 'if')``, where ``1`` is the numeric value
|
|
associated with all :const:`NAME` tokens, including variable and function names
|
|
defined by the user. In an alternate form returned when line number information
|
|
is requested, the same token might be represented as ``(1, 'if', 12)``, where
|
|
the ``12`` represents the line number at which the terminal symbol was found.
|
|
|
|
Terminal elements are represented in much the same way, but without any child
|
|
elements and the addition of the source text which was identified. The example
|
|
of the :keyword:`if` keyword above is representative. The various types of
|
|
terminal symbols are defined in the C header file :file:`Include/token.h` and
|
|
the Python module :mod:`token`.
|
|
|
|
The ST objects are not required to support the functionality of this module,
|
|
but are provided for three purposes: to allow an application to amortize the
|
|
cost of processing complex parse trees, to provide a parse tree representation
|
|
which conserves memory space when compared to the Python list or tuple
|
|
representation, and to ease the creation of additional modules in C which
|
|
manipulate parse trees. A simple "wrapper" class may be created in Python to
|
|
hide the use of ST objects.
|
|
|
|
The :mod:`parser` module defines functions for a few distinct purposes. The
|
|
most important purposes are to create ST objects and to convert ST objects to
|
|
other representations such as parse trees and compiled code objects, but there
|
|
are also functions which serve to query the type of parse tree represented by an
|
|
ST object.
|
|
|
|
|
|
.. seealso::
|
|
|
|
Module :mod:`symbol`
|
|
Useful constants representing internal nodes of the parse tree.
|
|
|
|
Module :mod:`token`
|
|
Useful constants representing leaf nodes of the parse tree and functions for
|
|
testing node values.
|
|
|
|
|
|
.. _creating-sts:
|
|
|
|
Creating ST Objects
|
|
-------------------
|
|
|
|
ST objects may be created from source code or from a parse tree. When creating
|
|
an ST object from source, different functions are used to create the ``'eval'``
|
|
and ``'exec'`` forms.
|
|
|
|
|
|
.. function:: expr(source)
|
|
|
|
The :func:`expr` function parses the parameter *source* as if it were an input
|
|
to ``compile(source, 'file.py', 'eval')``. If the parse succeeds, an ST object
|
|
is created to hold the internal parse tree representation, otherwise an
|
|
appropriate exception is raised.
|
|
|
|
|
|
.. function:: suite(source)
|
|
|
|
The :func:`suite` function parses the parameter *source* as if it were an input
|
|
to ``compile(source, 'file.py', 'exec')``. If the parse succeeds, an ST object
|
|
is created to hold the internal parse tree representation, otherwise an
|
|
appropriate exception is raised.
|
|
|
|
|
|
.. function:: sequence2st(sequence)
|
|
|
|
This function accepts a parse tree represented as a sequence and builds an
|
|
internal representation if possible. If it can validate that the tree conforms
|
|
to the Python grammar and all nodes are valid node types in the host version of
|
|
Python, an ST object is created from the internal representation and returned
|
|
to the called. If there is a problem creating the internal representation, or
|
|
if the tree cannot be validated, a :exc:`ParserError` exception is raised. An
|
|
ST object created this way should not be assumed to compile correctly; normal
|
|
exceptions raised by compilation may still be initiated when the ST object is
|
|
passed to :func:`compilest`. This may indicate problems not related to syntax
|
|
(such as a :exc:`MemoryError` exception), but may also be due to constructs such
|
|
as the result of parsing ``del f(0)``, which escapes the Python parser but is
|
|
checked by the bytecode compiler.
|
|
|
|
Sequences representing terminal tokens may be represented as either two-element
|
|
lists of the form ``(1, 'name')`` or as three-element lists of the form ``(1,
|
|
'name', 56)``. If the third element is present, it is assumed to be a valid
|
|
line number. The line number may be specified for any subset of the terminal
|
|
symbols in the input tree.
|
|
|
|
|
|
.. function:: tuple2st(sequence)
|
|
|
|
This is the same function as :func:`sequence2st`. This entry point is
|
|
maintained for backward compatibility.
|
|
|
|
|
|
.. _converting-sts:
|
|
|
|
Converting ST Objects
|
|
---------------------
|
|
|
|
ST objects, regardless of the input used to create them, may be converted to
|
|
parse trees represented as list- or tuple- trees, or may be compiled into
|
|
executable code objects. Parse trees may be extracted with or without line
|
|
numbering information.
|
|
|
|
|
|
.. function:: st2list(st, line_info=False, col_info=False)
|
|
|
|
This function accepts an ST object from the caller in *st* and returns a
|
|
Python list representing the equivalent parse tree. The resulting list
|
|
representation can be used for inspection or the creation of a new parse tree in
|
|
list form. This function does not fail so long as memory is available to build
|
|
the list representation. If the parse tree will only be used for inspection,
|
|
:func:`st2tuple` should be used instead to reduce memory consumption and
|
|
fragmentation. When the list representation is required, this function is
|
|
significantly faster than retrieving a tuple representation and converting that
|
|
to nested lists.
|
|
|
|
If *line_info* is true, line number information will be included for all
|
|
terminal tokens as a third element of the list representing the token. Note
|
|
that the line number provided specifies the line on which the token *ends*.
|
|
This information is omitted if the flag is false or omitted.
|
|
|
|
|
|
.. function:: st2tuple(st, line_info=False, col_info=False)
|
|
|
|
This function accepts an ST object from the caller in *st* and returns a
|
|
Python tuple representing the equivalent parse tree. Other than returning a
|
|
tuple instead of a list, this function is identical to :func:`st2list`.
|
|
|
|
If *line_info* is true, line number information will be included for all
|
|
terminal tokens as a third element of the list representing the token. This
|
|
information is omitted if the flag is false or omitted.
|
|
|
|
|
|
.. function:: compilest(st, filename='<syntax-tree>')
|
|
|
|
.. index::
|
|
builtin: exec
|
|
builtin: eval
|
|
|
|
The Python byte compiler can be invoked on an ST object to produce code objects
|
|
which can be used as part of a call to the built-in :func:`exec` or :func:`eval`
|
|
functions. This function provides the interface to the compiler, passing the
|
|
internal parse tree from *st* to the parser, using the source file name
|
|
specified by the *filename* parameter. The default value supplied for *filename*
|
|
indicates that the source was an ST object.
|
|
|
|
Compiling an ST object may result in exceptions related to compilation; an
|
|
example would be a :exc:`SyntaxError` caused by the parse tree for ``del f(0)``:
|
|
this statement is considered legal within the formal grammar for Python but is
|
|
not a legal language construct. The :exc:`SyntaxError` raised for this
|
|
condition is actually generated by the Python byte-compiler normally, which is
|
|
why it can be raised at this point by the :mod:`parser` module. Most causes of
|
|
compilation failure can be diagnosed programmatically by inspection of the parse
|
|
tree.
|
|
|
|
|
|
.. _querying-sts:
|
|
|
|
Queries on ST Objects
|
|
---------------------
|
|
|
|
Two functions are provided which allow an application to determine if an ST was
|
|
created as an expression or a suite. Neither of these functions can be used to
|
|
determine if an ST was created from source code via :func:`expr` or
|
|
:func:`suite` or from a parse tree via :func:`sequence2st`.
|
|
|
|
|
|
.. function:: isexpr(st)
|
|
|
|
.. index:: builtin: compile
|
|
|
|
When *st* represents an ``'eval'`` form, this function returns true, otherwise
|
|
it returns false. This is useful, since code objects normally cannot be queried
|
|
for this information using existing built-in functions. Note that the code
|
|
objects created by :func:`compilest` cannot be queried like this either, and
|
|
are identical to those created by the built-in :func:`compile` function.
|
|
|
|
|
|
.. function:: issuite(st)
|
|
|
|
This function mirrors :func:`isexpr` in that it reports whether an ST object
|
|
represents an ``'exec'`` form, commonly known as a "suite." It is not safe to
|
|
assume that this function is equivalent to ``not isexpr(st)``, as additional
|
|
syntactic fragments may be supported in the future.
|
|
|
|
|
|
.. _st-errors:
|
|
|
|
Exceptions and Error Handling
|
|
-----------------------------
|
|
|
|
The parser module defines a single exception, but may also pass other built-in
|
|
exceptions from other portions of the Python runtime environment. See each
|
|
function for information about the exceptions it can raise.
|
|
|
|
|
|
.. exception:: ParserError
|
|
|
|
Exception raised when a failure occurs within the parser module. This is
|
|
generally produced for validation failures rather than the built-in
|
|
:exc:`SyntaxError` raised during normal parsing. The exception argument is
|
|
either a string describing the reason of the failure or a tuple containing a
|
|
sequence causing the failure from a parse tree passed to :func:`sequence2st`
|
|
and an explanatory string. Calls to :func:`sequence2st` need to be able to
|
|
handle either type of exception, while calls to other functions in the module
|
|
will only need to be aware of the simple string values.
|
|
|
|
Note that the functions :func:`compilest`, :func:`expr`, and :func:`suite` may
|
|
raise exceptions which are normally raised by the parsing and compilation
|
|
process. These include the built in exceptions :exc:`MemoryError`,
|
|
:exc:`OverflowError`, :exc:`SyntaxError`, and :exc:`SystemError`. In these
|
|
cases, these exceptions carry all the meaning normally associated with them.
|
|
Refer to the descriptions of each function for detailed information.
|
|
|
|
|
|
.. _st-objects:
|
|
|
|
ST Objects
|
|
----------
|
|
|
|
Ordered and equality comparisons are supported between ST objects. Pickling of
|
|
ST objects (using the :mod:`pickle` module) is also supported.
|
|
|
|
|
|
.. data:: STType
|
|
|
|
The type of the objects returned by :func:`expr`, :func:`suite` and
|
|
:func:`sequence2st`.
|
|
|
|
ST objects have the following methods:
|
|
|
|
|
|
.. method:: ST.compile(filename='<syntax-tree>')
|
|
|
|
Same as ``compilest(st, filename)``.
|
|
|
|
|
|
.. method:: ST.isexpr()
|
|
|
|
Same as ``isexpr(st)``.
|
|
|
|
|
|
.. method:: ST.issuite()
|
|
|
|
Same as ``issuite(st)``.
|
|
|
|
|
|
.. method:: ST.tolist(line_info=False, col_info=False)
|
|
|
|
Same as ``st2list(st, line_info, col_info)``.
|
|
|
|
|
|
.. method:: ST.totuple(line_info=False, col_info=False)
|
|
|
|
Same as ``st2tuple(st, line_info, col_info)``.
|
|
|
|
|
|
Example: Emulation of :func:`compile`
|
|
-------------------------------------
|
|
|
|
While many useful operations may take place between parsing and bytecode
|
|
generation, the simplest operation is to do nothing. For this purpose, using
|
|
the :mod:`parser` module to produce an intermediate data structure is equivalent
|
|
to the code ::
|
|
|
|
>>> code = compile('a + 5', 'file.py', 'eval')
|
|
>>> a = 5
|
|
>>> eval(code)
|
|
10
|
|
|
|
The equivalent operation using the :mod:`parser` module is somewhat longer, and
|
|
allows the intermediate internal parse tree to be retained as an ST object::
|
|
|
|
>>> import parser
|
|
>>> st = parser.expr('a + 5')
|
|
>>> code = st.compile('file.py')
|
|
>>> a = 5
|
|
>>> eval(code)
|
|
10
|
|
|
|
An application which needs both ST and code objects can package this code into
|
|
readily available functions::
|
|
|
|
import parser
|
|
|
|
def load_suite(source_string):
|
|
st = parser.suite(source_string)
|
|
return st, st.compile()
|
|
|
|
def load_expression(source_string):
|
|
st = parser.expr(source_string)
|
|
return st, st.compile()
|