460 lines
20 KiB
TeX
460 lines
20 KiB
TeX
% libparser.tex
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%
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% Introductory documentation for the new parser built-in module.
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%
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% Copyright 1995 Virginia Polytechnic Institute and State University
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% and Fred L. Drake, Jr. This copyright notice must be distributed on
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% all copies, but this document otherwise may be distributed as part
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% of the Python distribution. No fee may be charged for this document
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% in any representation, either on paper or electronically. This
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% restriction does not affect other elements in a distributed package
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% in any way.
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%
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\section{Built-in Module \sectcode{parser}}
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\bimodindex{parser}
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The \code{parser} module provides an interface to Python's internal
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parser and byte-code compiler. The primary purpose for this interface
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is to allow Python code to edit the parse tree of a Python expression
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and create executable code from this. This can be better than trying
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to parse and modify an arbitrary Python code fragment as a string, and
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ensures that parsing is performed in a manner identical to the code
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forming the application. It's also faster.
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There are a few things to note about this module which are important
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to making use of the data structures created. This is not a tutorial
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on editing the parse trees for Python code.
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Most importantly, a good understanding of the Python grammar processed
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by the internal parser is required. For full information on the
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language syntax, refer to the Language Reference. The parser itself
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is created from a grammar specification defined in the file
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\code{Grammar/Grammar} in the standard Python distribution. The parse
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trees stored in the ``AST objects'' created by this module are the
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actual output from the internal parser when created by the
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\code{expr()} or \code{suite()} functions, described below. The AST
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objects created by \code{sequence2ast()} faithfully simulate those
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structures. Be aware that the values of the sequences which are
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considered ``correct'' will vary from one version of Python to another
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as the formal grammar for the language is revised. However,
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transporting code from one Python version to another as source text
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will always allow correct parse trees to be created in the target
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version, with the only restriction being that migrating to an older
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version of the interpreter will not support more recent language
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constructs. The parse trees are not typically compatible from one
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version to another, whereas source code has always been
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forward-compatible.
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Each element of the sequences returned by \code{ast2list} or
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\code{ast2tuple()} has a simple form. Sequences representing
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non-terminal elements in the grammar always have a length greater than
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one. The first element is an integer which identifies a production in
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the grammar. These integers are given symbolic names in the C header
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file \code{Include/graminit.h} and the Python module
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\code{Lib/symbol.py}. Each additional element of the sequence represents
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a component of the production as recognized in the input string: these
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are always sequences which have the same form as the parent. An
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important aspect of this structure which should be noted is that
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keywords used to identify the parent node type, such as the keyword
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\code{if} in an \emph{if\_stmt}, are included in the node tree without
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any special treatment. For example, the \code{if} keyword is
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represented by the tuple \code{(1, 'if')}, where \code{1} is the
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numeric value associated with all \code{NAME} elements, including
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variable and function names defined by the user. In an alternate form
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returned when line number information is requested, the same token
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might be represented as \code{(1, 'if', 12)}, where the \code{12}
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represents the line number at which the terminal symbol was found.
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Terminal elements are represented in much the same way, but without
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any child elements and the addition of the source text which was
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identified. The example of the \code{if} keyword above is
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representative. The various types of terminal symbols are defined in
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the C header file \code{Include/token.h} and the Python module
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\code{Lib/token.py}.
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The AST objects are not actually required to support the functionality
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of this module, but are provided for three purposes: to allow an
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application to amortize the cost of processing complex parse trees, to
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provide a parse tree representation which conserves memory space when
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compared to the Python list or tuple representation, and to ease the
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creation of additional modules in C which manipulate parse trees. A
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simple ``wrapper'' module may be created in Python to hide the use of
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AST objects.
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The \code{parser} module defines the following functions:
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\renewcommand{\indexsubitem}{(in module parser)}
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\begin{funcdesc}{ast2list}{ast\optional{\, line\_info\code{ = 0}}}
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This function accepts an AST object from the caller in
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\code{\var{ast}} and returns a Python list representing the
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equivelent parse tree. The resulting list representation can be used
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for inspection or the creation of a new parse tree in list form.
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This function does not fail so long as memory is available to build
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the list representation. If a parse tree will only be used for
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inspection, \code{ast2tuple()} should be used instead to reduce memory
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consumption and fragmentation. When modifications are to be made to
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the parse tree, this function is significantly faster than retrieving
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a tuple representation and converting that to nested lists.
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If the \code{line\_info} flag is given true value, line number
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information will be included for all terminal tokens as a third
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element of the list representing the token. This information is
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omitted if the flag is false or omitted.
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\end{funcdesc}
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\begin{funcdesc}{ast2tuple}{ast\optional{\, line\_info\code{ = 0}}}
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This function accepts an AST object from the caller in
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\code{\var{ast}} and returns a Python tuple representing the
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equivelent parse tree. Other than returning a tuple instead of a
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list, this function is identical to \code{ast2list()}.
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If the \code{line\_info} flag is given true value, line number
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information will be included for all terminal tokens as a third
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element of the list representing the token. This information is
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omitted if the flag is false or omitted.
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\end{funcdesc}
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\begin{funcdesc}{compileast}{ast\optional{\, filename\code{ = '<ast>'}}}
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The Python byte compiler can be invoked on an AST object to produce
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code objects which can be used as part of an \code{exec} statement or
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a call to the built-in \code{eval()} function. This function provides
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the interface to the compiler, passing the internal parse tree from
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\code{\var{ast}} to the parser, using the source file name specified
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by the \code{\var{filename}} parameter. The default value supplied
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for \code{\var{filename}} indicates that the source was an AST object.
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Compiling an AST object may result in exceptions related to
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compilation; an example would be a \code{SyntaxError} caused by the
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parse tree for \code{del f(0)}; this statement is considered legal
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within the formal grammar for Python but is not a legal language
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construct. The \code{SyntaxError} raised for this condition is
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actually generated by the Python byte-compiler normally, which is why
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it can be raised at this point by the \code{parser} module. Most
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causes of compilation failure can be diagnosed programmatically by
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inspection of the parse tree.
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\end{funcdesc}
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\begin{funcdesc}{expr}{string}
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The \code{expr()} function parses the parameter \code{\var{string}}
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as if it were an input to \code{compile(\var{string}, 'eval')}. If
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the parse succeeds, an AST object is created to hold the internal
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parse tree representation, otherwise an appropriate exception is
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thrown.
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\end{funcdesc}
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\begin{funcdesc}{isexpr}{ast}
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When \code{\var{ast}} represents an \code{'eval'} form, this function
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returns a true value (\code{1}), otherwise it returns false
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(\code{0}). This is useful, since code objects normally cannot be
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queried for this information using existing built-in functions. Note
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that the code objects created by \code{compileast()} cannot be queried
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like this either, and are identical to those created by the built-in
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\code{compile()} function.
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\end{funcdesc}
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\begin{funcdesc}{issuite}{ast}
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This function mirrors \code{isexpr()} in that it reports whether an
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AST object represents a suite of statements. It is not safe to assume
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that this function is equivelent to \code{not isexpr(\var{ast})}, as
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additional syntactic fragments may be supported in the future.
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\end{funcdesc}
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\begin{funcdesc}{suite}{string}
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The \code{suite()} function parses the parameter \code{\var{string}}
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as if it were an input to \code{compile(\var{string}, 'exec')}. If
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the parse succeeds, an AST object is created to hold the internal
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parse tree representation, otherwise an appropriate exception is
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thrown.
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\end{funcdesc}
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\begin{funcdesc}{sequence2ast}{sequence}
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This function accepts a parse tree represented as a sequence and
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builds an internal representation if possible. If it can validate
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that the tree conforms to the Python grammar and all nodes are valid
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node types in the host version of Python, an AST object is created
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from the internal representation and returned to the called. If there
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is a problem creating the internal representation, or if the tree
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cannot be validated, a \code{ParserError} exception is thrown. An AST
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object created this way should not be assumed to compile correctly;
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normal exceptions thrown by compilation may still be initiated when
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the AST object is passed to \code{compileast()}. This will normally
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indicate problems not related to syntax (such as a \code{MemoryError}
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exception), but may also be due to constructs such as the result of
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parsing \code{del f(0)}, which escapes the Python parser but is
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checked by the bytecode compiler.
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Sequences representing terminal tokens may be represented as either
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two-element lists of the form \code{(1, 'name')} or as three-element
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lists of the form \code{(1, 'name', 56)}. If the third element is
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present, it is assumed to be a valid line number. The line number
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may be specified for any subset of the terminal symbols in the input
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tree.
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\end{funcdesc}
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\begin{funcdesc}{tuple2ast}{sequence}
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This is the same function as \code{sequence2ast}. This entry point is
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maintained for backward compatibility.
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\end{funcdesc}
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\subsection{Exceptions and Error Handling}
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The parser module defines a single exception, but may also pass other
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built-in exceptions from other portions of the Python runtime
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environment. See each function for information about the exceptions
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it can raise.
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\begin{excdesc}{ParserError}
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Exception raised when a failure occurs within the parser module. This
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is generally produced for validation failures rather than the built in
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\code{SyntaxError} thrown during normal parsing.
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The exception argument is either a string describing the reason of the
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failure or a tuple containing a sequence causing the failure from a parse
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tree passed to \code{sequence2ast()} and an explanatory string. Calls to
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\code{sequence2ast()} need to be able to handle either type of exception,
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while calls to other functions in the module will only need to be
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aware of the simple string values.
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\end{excdesc}
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Note that the functions \code{compileast()}, \code{expr()}, and
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\code{suite()} may throw exceptions which are normally thrown by the
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parsing and compilation process. These include the built in
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exceptions \code{MemoryError}, \code{OverflowError},
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\code{SyntaxError}, and \code{SystemError}. In these cases, these
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exceptions carry all the meaning normally associated with them. Refer
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to the descriptions of each function for detailed information.
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\subsection{AST Objects}
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AST objects (returned by \code{expr()}, \code{suite()}, and
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\code{tuple2ast()}, described above) have no methods of their own.
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Some of the functions defined which accept an AST object as their
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first argument may change to object methods in the future.
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Ordered and equality comparisons are supported between AST objects.
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\subsection{Example}
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The parser modules allows operations to be performed on the parse tree
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of Python source code before the bytecode is generated, and provides
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for inspection of the parse tree for information gathering purposes as
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well. While many useful operations may take place between parsing and
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bytecode generation, the simplest operation is to do nothing. For
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this purpose, using the \code{parser} module to produce an
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intermediate data structure is equivelent to the code
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\begin{verbatim}
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>>> code = compile('a + 5', 'eval')
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>>> a = 5
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>>> eval(code)
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10
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\end{verbatim}
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The equivelent operation using the \code{parser} module is somewhat
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longer, and allows the intermediate internal parse tree to be retained
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as an AST object:
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\begin{verbatim}
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>>> import parser
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>>> ast = parser.expr('a + 5')
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>>> code = parser.compileast(ast)
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>>> a = 5
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>>> eval(code)
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10
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\end{verbatim}
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Some applications can benfit from access to the parse tree itself, and
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can take advantage of the intermediate data structure provided by the
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\code{parser} module. The remainder of this section of examples will
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demonstrate how the intermediate data structure can provide access to
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module documentation defined in docstrings without requiring that the
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code being examined be imported into a running interpreter. This can
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be very useful for performing analyses of untrusted code.
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Generally, the example will demonstrate how the parse tree may be
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traversed to distill interesting information. Two functions and a set
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of classes is developed which provide programmatic access to high
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level function and class definitions provided by a module. The
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classes extract information from the parse tree and provide access to
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the information at a useful semantic level, one function provides a
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simple low-level pattern matching capability, and the other function
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defines a high-level interface to the classes by handling file
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operations on behalf of the caller. All source files mentioned here
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which are not part of the Python installation are located in the
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\file{Demo/parser} directory of the distribution.
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To construct the upper-level extraction methods, we need to know what
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the parse tree structure looks like and how much of it we actually
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need to be concerned about. Python uses a moderately deep parse tree,
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so there are a large number of intermediate nodes. It is important to
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read and understand the formal grammar used by Python. This is
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specified in the file \file{Grammar/Grammar} in the distribution.
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Consider the simplest case of interest when searching for docstrings:
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a module consisting of a docstring and nothing else:
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\begin{verbatim}
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"""Some documentation.
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"""
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\end{verbatim}
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Using the interpreter to take a look at the parse tree, we find a
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bewildering mass of numbers and parentheses, with the documentation
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buried deep in the nested tuples:
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\begin{verbatim}
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>>> import parser
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>>> import pprint
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>>> ast = parser.suite(open('docstring.py').read())
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>>> tup = parser.ast2tuple(ast)
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>>> pprint.pprint(tup)
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(257,
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(264,
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(265,
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(266,
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(267,
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(307,
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(287,
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(288,
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(289,
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(290,
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(292,
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(293,
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(294,
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(295,
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(296,
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(297,
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(298,
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(299,
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(300, (3, '"""Some documentation.\012"""'))))))))))))))))),
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(4, ''))),
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(4, ''),
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(0, ''))
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\end{verbatim}
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The numbers at the first element of each node in the tree are the node
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types; they map directly to terminal and non-terminal symbols in the
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grammar. Unfortunately, they are represented as integers in the
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internal representation, and the Python structures generated do not
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change that. However, the \code{symbol} and \code{token} modules
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provide symbolic names for the node types and dictionaries which map
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from the integers to the symbolic names for the node types.
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In the output presented above, the outermost tuple contains four
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elements: the integer \code{257} and three additional tuples. Node
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type \code{257} has the symbolic name \code{file_input}. Each of
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these inner tuples contains an integer as the first element; these
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integers, \code{264}, \code{4}, and \code{0}, represent the node types
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\code{stmt}, \code{NEWLINE}, and \code{ENDMARKER}, respectively.
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Note that these values may change depending on the version of Python
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you are using; consult \file{symbol.py} and \file{token.py} for
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details of the mapping. It should be fairly clear that the outermost
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node is related primarily to the input source rather than the contents
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of the file, and may be disregarded for the moment. The \code{stmt}
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node is much more interesting. In particular, all docstrings are
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found in subtrees which are formed exactly as this node is formed,
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with the only difference being the string itself. The association
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between the docstring in a similar tree and the defined entity (class,
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function, or module) which it describes is given by the position of
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the docstring subtree within the tree defining the described
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structure.
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By replacing the actual docstring with something to signify a variable
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component of the tree, we allow a simple pattern matching approach may
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be taken to checking any given subtree for equivelence to the general
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pattern for docstrings. Since the example demonstrates information
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extraction, we can safely require that the tree be in tuple form
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rather than list form, allowing a simple variable representation to be
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\code{['variable\_name']}. A simple recursive function can implement
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the pattern matching, returning a boolean and a dictionary of variable
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name to value mappings.
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\begin{verbatim}
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from types import ListType, TupleType
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def match(pattern, data, vars=None):
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if vars is None:
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vars = {}
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if type(pattern) is ListType:
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vars[pattern[0]] = data
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return 1, vars
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if type(pattern) is not TupleType:
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return (pattern == data), vars
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if len(data) != len(pattern):
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return 0, vars
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for pattern, data in map(None, pattern, data):
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same, vars = match(pattern, data, vars)
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if not same:
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break
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return same, vars
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\end{verbatim}
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Using this simple recursive pattern matching function and the symbolic
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node types, the pattern for the candidate docstring subtrees becomes:
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\begin{verbatim}
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>>> DOCSTRING_STMT_PATTERN = (
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... symbol.stmt,
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... (symbol.simple_stmt,
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... (symbol.small_stmt,
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... (symbol.expr_stmt,
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... (symbol.testlist,
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... (symbol.test,
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... (symbol.and_test,
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... (symbol.not_test,
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... (symbol.comparison,
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... (symbol.expr,
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... (symbol.xor_expr,
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... (symbol.and_expr,
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... (symbol.shift_expr,
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... (symbol.arith_expr,
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... (symbol.term,
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... (symbol.factor,
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... (symbol.power,
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... (symbol.atom,
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... (token.STRING, ['docstring'])
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... )))))))))))))))),
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... (token.NEWLINE, '')
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... ))
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\end{verbatim}
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Using the \code{match()} function with this pattern, extracting the
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module docstring from the parse tree created previously is easy:
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\begin{verbatim}
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>>> found, vars = match(DOCSTRING_STMT_PATTERN, tup[1])
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>>> found
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1
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>>> vars
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{'docstring': '"""Some documentation.\012"""'}
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\end{verbatim}
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Once specific data can be extracted from a location where it is
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expected, the question of where information can be expected
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needs to be answered. When dealing with docstrings, the answer is
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fairly simple: the docstring is the first \code{stmt} node in a code
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block (\code{file_input} or \code{suite} node types). A module
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consists of a single \code{file_input} node, and class and function
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definitions each contain exactly one \code{suite} node. Classes and
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functions are readily identified as subtrees of code block nodes which
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start with \code{(stmt, (compound_stmt, (classdef, ...} or
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\code{(stmt, (compound_stmt, (funcdef, ...}. Note that these subtrees
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cannot be matched by \code{match()} since it does not support multiple
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sibling nodes to match without regard to number. A more elaborate
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matching function could be used to overcome this limitation, but this
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is sufficient for the example.
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%%
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%% end of file
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