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Add some index entries; switch to logical markup.

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Fred Drake 1998-02-09 20:52:48 +00:00
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Doc/lib/libparser.tex
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@ -12,8 +12,9 @@
\section{Built-in Module \sectcode{parser}}
\label{module-parser}
\bimodindex{parser}
\index{parsing!Python source code}
The \code{parser} module provides an interface to Python's internal
The \module{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
@ -24,17 +25,17 @@ forming the application. It is also faster.
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 \code{parser} module are presented.
the \module{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 the Language Reference. The parser itself
is created from a grammar specification defined in the file
language syntax, refer to the \emph{Python Language Reference}. 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 ``AST objects'' created by this module are the
actual output from the internal parser when created by the
\code{expr()} or \code{suite()} functions, described below. The AST
objects created by \code{sequence2ast()} faithfully simulate those
\function{expr()} or \function{suite()} functions, described below. The AST
objects created by \function{sequence2ast()} 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,
@ -46,19 +47,19 @@ 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 \code{ast2list} or
\code{ast2tuple()} has a simple form. Sequences representing
Each element of the sequences returned by \function{ast2list()} or
\function{ast2tuple()} 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
\code{symbol}. Each additional element of the sequence represents
\module{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
\code{if} in an \code{if_stmt}, are included in the node tree without
any special treatment. For example, the \code{if} keyword is
\keyword{if} in an \constant{if_stmt}, are included in the node tree without
any special treatment. For example, the \keyword{if} keyword is
represented by the tuple \code{(1, 'if')}, where \code{1} is the
numeric value associated with all \code{NAME} tokens, including
variable and function names defined by the user. In an alternate form
@ -68,10 +69,10 @@ 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 \code{if} keyword above is
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
\code{token}.
\module{token}.
The AST objects are not required to support the functionality of this
module, but are provided for three purposes: to allow an application
@ -80,10 +81,9 @@ 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 AST
objects; the \code{AST} library module provides a variety of such
classes.
objects.
The \code{parser} module defines functions for a few distinct
The \module{parser} module defines functions for a few distinct
purposes. The most important purposes are to create AST objects and
to convert AST objects to other representations such as parse trees
and compiled code objects, but there are also functions which serve to
@ -99,16 +99,16 @@ When creating an AST object from source, different functions are used
to create the \code{'eval'} and \code{'exec'} forms.
\begin{funcdesc}{expr}{string}
The \code{expr()} function parses the parameter \code{\var{string}}
as if it were an input to \code{compile(\var{string}, 'eval')}. If
The \function{expr()} function parses the parameter \code{\var{string}}
as if it were an input to \samp{compile(\var{string}, 'eval')}. If
the parse succeeds, an AST object is created to hold the internal
parse tree representation, otherwise an appropriate exception is
thrown.
\end{funcdesc}
\begin{funcdesc}{suite}{string}
The \code{suite()} function parses the parameter \code{\var{string}}
as if it were an input to \code{compile(\var{string}, 'exec')}. If
The \function{suite()} function parses the parameter \code{\var{string}}
as if it were an input to \samp{compile(\var{string}, 'exec')}. If
the parse succeeds, an AST object is created to hold the internal
parse tree representation, otherwise an appropriate exception is
thrown.
@ -121,11 +121,11 @@ that the tree conforms to the Python grammar and all nodes are valid
node types in the host version of Python, an AST 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 \code{ParserError} exception is thrown. An AST
cannot be validated, a \exception{ParserError} exception is thrown. An AST
object created this way should not be assumed to compile correctly;
normal exceptions thrown by compilation may still be initiated when
the AST object is passed to \code{compileast()}. This may indicate
problems not related to syntax (such as a \code{MemoryError}
the AST object is passed to \function{compileast()}. This may indicate
problems not related to syntax (such as a \exception{MemoryError}
exception), but may also be due to constructs such as the result of
parsing \code{del f(0)}, which escapes the Python parser but is
checked by the bytecode compiler.
@ -139,7 +139,7 @@ tree.
\end{funcdesc}
\begin{funcdesc}{tuple2ast}{sequence}
This is the same function as \code{sequence2ast()}. This entry point
This is the same function as \function{sequence2ast()}. This entry point
is maintained for backward compatibility.
\end{funcdesc}
@ -158,7 +158,7 @@ equivelent 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, \code{ast2tuple()} should be used instead to reduce memory
inspection, \function{ast2tuple()} 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.
@ -166,7 +166,7 @@ tuple representation and converting that to nested lists.
If \code{\var{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 \emph{ends\/}. This information is
the line on which the token \emph{ends}. This information is
omitted if the flag is false or omitted.
\end{funcdesc}
@ -174,7 +174,7 @@ omitted if the flag is false or omitted.
This function accepts an AST object from the caller in
\code{\var{ast}} and returns a Python tuple representing the
equivelent parse tree. Other than returning a tuple instead of a
list, this function is identical to \code{ast2list()}.
list, this function is identical to \function{ast2list()}.
If \code{\var{line_info}} is true, line number information will be
included for all terminal tokens as a third element of the list
@ -185,19 +185,20 @@ false or omitted.
\begin{funcdesc}{compileast}{ast\optional{\, filename\code{ = '<ast>'}}}
The Python byte compiler can be invoked on an AST object to produce
code objects which can be used as part of an \code{exec} statement or
a call to the built-in \code{eval()} function. This function provides
the interface to the compiler, passing the internal parse tree from
\code{\var{ast}} to the parser, using the source file name specified
by the \code{\var{filename}} parameter. The default value supplied
for \code{\var{filename}} indicates that the source was an AST object.
a call to the built-in \function{eval()}\bifuncindex{eval} function.
This function provides the interface to the compiler, passing the
internal parse tree from \code{\var{ast}} to the parser, using the
source file name specified by the \code{\var{filename}} parameter.
The default value supplied for \code{\var{filename}} indicates that
the source was an AST object.
Compiling an AST object may result in exceptions related to
compilation; an example would be a \code{SyntaxError} caused by the
compilation; an example would be a \exception{SyntaxError} caused by the
parse tree for \code{del f(0)}: this statement is considered legal
within the formal grammar for Python but is not a legal language
construct. The \code{SyntaxError} raised for this condition is
construct. The \exception{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 \code{parser} module. Most
it can be raised at this point by the \module{parser} module. Most
causes of compilation failure can be diagnosed programmatically by
inspection of the parse tree.
\end{funcdesc}
@ -208,25 +209,25 @@ inspection of the parse tree.
Two functions are provided which allow an application to determine if
an AST was create as an expression or a suite. Neither of these
functions can be used to determine if an AST was created from source
code via \code{expr()} or \code{suite()} or from a parse tree via
\code{sequence2ast()}.
code via \function{expr()} or \function{suite()} or from a parse tree
via \function{sequence2ast()}.
\begin{funcdesc}{isexpr}{ast}
When \code{\var{ast}} represents an \code{'eval'} form, this function
returns a true value (\code{1}), otherwise it returns false
(\code{0}). 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 \code{compileast()} cannot be queried
like this either, and are identical to those created by the built-in
\code{compile()} 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
\function{compileast()} cannot be queried like this either, and are
identical to those created by the built-in
\function{compile()}\bifuncindex{compile} function.
\end{funcdesc}
\begin{funcdesc}{issuite}{ast}
This function mirrors \code{isexpr()} in that it reports whether an
This function mirrors \function{isexpr()} in that it reports whether an
AST object represents an \code{'exec'} form, commonly known as a
``suite.'' It is not safe to assume that this function is equivelent
to \code{not isexpr(\var{ast})}, as additional syntactic fragments may
to \samp{not isexpr(\var{ast})}, as additional syntactic fragments may
be supported in the future.
\end{funcdesc}
@ -241,28 +242,28 @@ it can raise.
\begin{excdesc}{ParserError}
Exception raised when a failure occurs within the parser module. This
is generally produced for validation failures rather than the built in
\code{SyntaxError} thrown during normal parsing.
\exception{SyntaxError} thrown 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 \code{sequence2ast()} and an explanatory string. Calls to
\code{sequence2ast()} need to be able to handle either type of exception,
tree passed to \function{sequence2ast()} and an explanatory string. Calls to
\function{sequence2ast()} 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.
\end{excdesc}
Note that the functions \code{compileast()}, \code{expr()}, and
\code{suite()} may throw exceptions which are normally thrown by the
Note that the functions \function{compileast()}, \function{expr()}, and
\function{suite()} may throw exceptions which are normally thrown by the
parsing and compilation process. These include the built in
exceptions \code{MemoryError}, \code{OverflowError},
\code{SyntaxError}, and \code{SystemError}. In these cases, these
exceptions \exception{MemoryError}, \exception{OverflowError},
\exception{SyntaxError}, and \exception{SystemError}. In these cases, these
exceptions carry all the meaning normally associated with them. Refer
to the descriptions of each function for detailed information.
\subsection{AST Objects}
AST objects returned by \code{expr()}, \code{suite()}, and
\code{sequence2ast()} have no methods of their own.
AST objects returned by \function{expr()}, \function{suite()}, and
\function{sequence2ast()} have no methods of their own.
Some of the functions defined which accept an AST object as their
first argument may change to object methods in the future. The type
of these objects is available as \code{ASTType} in the module.
@ -277,14 +278,15 @@ The parser modules allows operations to be performed on the parse tree
of Python source code before the bytecode is generated, and provides
for inspection of the parse tree for information gathering purposes.
Two examples are presented. The simple example demonstrates emulation
of the \code{compile()} built-in function and the complex example
shows the use of a parse tree for information discovery.
of the \function{compile()}\bifuncindex{compile} built-in function and
the complex example shows the use of a parse tree for information
discovery.
\subsubsection{Emulation of \sectcode{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 \code{parser} module to produce an
this purpose, using the \module{parser} module to produce an
intermediate data structure is equivelent to the code
\bcode\begin{verbatim}
@ -294,7 +296,7 @@ intermediate data structure is equivelent to the code
10
\end{verbatim}\ecode
%
The equivelent operation using the \code{parser} module is somewhat
The equivelent operation using the \module{parser} module is somewhat
longer, and allows the intermediate internal parse tree to be retained
as an AST object:
@ -330,7 +332,7 @@ Some applications benefit from direct access to the parse tree. The
remainder of this section demonstrates how the parse tree provides
access to module documentation defined in docstrings without requiring
that the code being examined be loaded into a running interpreter via
\code{import}. This can be very useful for performing analyses of
\keyword{import}. This can be very useful for performing analyses of
untrusted code.
Generally, the example will demonstrate how the parse tree may be
@ -349,7 +351,7 @@ The dynamic nature of Python allows the programmer a great deal of
flexibility, but most modules need only a limited measure of this when
defining classes, functions, and methods. In this example, the only
definitions that will be considered are those which are defined in the
top level of their context, e.g., a function defined by a \code{def}
top level of their context, e.g., a function defined by a \keyword{def}
statement at column zero of a module, but not a function defined
within a branch of an \code{if} ... \code{else} construct, though
there are some good reasons for doing so in some situations. Nesting
@ -408,21 +410,22 @@ The numbers at the first element of each node in the tree are the node
types; they map directly to terminal and non-terminal symbols in the
grammar. Unfortunately, they are represented as integers in the
internal representation, and the Python structures generated do not
change that. However, the \code{symbol} and \code{token} modules
change that. However, the \module{symbol} and \module{token} modules
provide symbolic names for the node types and dictionaries which map
from the integers to the symbolic names for the node types.
In the output presented above, the outermost tuple contains four
elements: the integer \code{257} and three additional tuples. Node
type \code{257} has the symbolic name \code{file_input}. Each of
type \code{257} has the symbolic name \constant{file_input}. Each of
these inner tuples contains an integer as the first element; these
integers, \code{264}, \code{4}, and \code{0}, represent the node types
\code{stmt}, \code{NEWLINE}, and \code{ENDMARKER}, respectively.
\constant{stmt}, \constant{NEWLINE}, and \constant{ENDMARKER},
respectively.
Note that these values may change depending on the version of Python
you are using; consult \file{symbol.py} and \file{token.py} for
details of the mapping. It should be fairly clear that the outermost
node is related primarily to the input source rather than the contents
of the file, and may be disregarded for the moment. The \code{stmt}
of the file, and may be disregarded for the moment. The \constant{stmt}
node is much more interesting. In particular, all docstrings are
found in subtrees which are formed exactly as this node is formed,
with the only difference being the string itself. The association
@ -494,7 +497,7 @@ DOCSTRING_STMT_PATTERN = (
))
\end{verbatim}\ecode
%
Using the \code{match()} function with this pattern, extracting the
Using the \function{match()} function with this pattern, extracting the
module docstring from the parse tree created previously is easy:
\bcode\begin{verbatim}
@ -508,14 +511,14 @@ module docstring from the parse tree created previously is easy:
Once specific data can be extracted from a location where it is
expected, the question of where information can be expected
needs to be answered. When dealing with docstrings, the answer is
fairly simple: the docstring is the first \code{stmt} node in a code
block (\code{file_input} or \code{suite} node types). A module
consists of a single \code{file_input} node, and class and function
definitions each contain exactly one \code{suite} node. Classes and
fairly simple: the docstring is the first \constant{stmt} node in a code
block (\constant{file_input} or \constant{suite} node types). A module
consists of a single \constant{file_input} node, and class and function
definitions each contain exactly one \constant{suite} node. Classes and
functions are readily identified as subtrees of code block nodes which
start with \code{(stmt, (compound_stmt, (classdef, ...} or
\code{(stmt, (compound_stmt, (funcdef, ...}. Note that these subtrees
cannot be matched by \code{match()} since it does not support multiple
cannot be matched by \function{match()} since it does not support multiple
sibling nodes to match without regard to number. A more elaborate
matching function could be used to overcome this limitation, but this
is sufficient for the example.
@ -535,13 +538,13 @@ parse tree which it represents. The \code{ModuleInfo} constructor
accepts an optional \code{\var{name}} parameter since it cannot
otherwise determine the name of the module.
The public classes include \code{ClassInfo}, \code{FunctionInfo},
and \code{ModuleInfo}. All objects provide the
methods \code{get_name()}, \code{get_docstring()},
\code{get_class_names()}, and \code{get_class_info()}. The
\code{ClassInfo} objects support \code{get_method_names()} and
\code{get_method_info()} while the other classes provide
\code{get_function_names()} and \code{get_function_info()}.
The public classes include \class{ClassInfo}, \class{FunctionInfo},
and \class{ModuleInfo}. All objects provide the
methods \method{get_name()}, \method{get_docstring()},
\method{get_class_names()}, and \method{get_class_info()}. The
\class{ClassInfo} objects support \method{get_method_names()} and
\method{get_method_info()} while the other classes provide
\method{get_function_names()} and \method{get_function_info()}.
Within each of the forms of code block that the public classes
represent, most of the required information is in the same form and is
@ -551,20 +554,20 @@ Since the difference in nomenclature reflects a real semantic
distinction from functions defined outside of a class, the
implementation needs to maintain the distinction.
Hence, most of the functionality of the public classes can be
implemented in a common base class, \code{SuiteInfoBase}, with the
implemented in a common base class, \class{SuiteInfoBase}, with the
accessors for function and method information provided elsewhere.
Note that there is only one class which represents function and method
information; this parallels the use of the \code{def} statement to
information; this parallels the use of the \keyword{def} statement to
define both types of elements.
Most of the accessor functions are declared in \code{SuiteInfoBase}
Most of the accessor functions are declared in \class{SuiteInfoBase}
and do not need to be overriden by subclasses. More importantly, the
extraction of most information from a parse tree is handled through a
method called by the \code{SuiteInfoBase} constructor. The example
method called by the \class{SuiteInfoBase} constructor. The example
code for most of the classes is clear when read alongside the formal
grammar, but the method which recursively creates new information
objects requires further examination. Here is the relevant part of
the \code{SuiteInfoBase} definition from \file{example.py}:
the \class{SuiteInfoBase} definition from \file{example.py}:
\bcode\begin{verbatim}
class SuiteInfoBase:
@ -599,13 +602,13 @@ class SuiteInfoBase:
\end{verbatim}\ecode
%
After initializing some internal state, the constructor calls the
\code{_extract_info()} method. This method performs the bulk of the
\method{_extract_info()} method. This method performs the bulk of the
information extraction which takes place in the entire example. The
extraction has two distinct phases: the location of the docstring for
the parse tree passed in, and the discovery of additional definitions
within the code block represented by the parse tree.
The initial \code{if} test determines whether the nested suite is of
The initial \keyword{if} test determines whether the nested suite is of
the ``short form'' or the ``long form.'' The short form is used when
the code block is on the same line as the definition of the code
block, as in
@ -626,23 +629,23 @@ def make_power(exp):
\end{verbatim}\ecode
%
When the short form is used, the code block may contain a docstring as
the first, and possibly only, \code{small_stmt} element. The
the first, and possibly only, \constant{small_stmt} element. The
extraction of such a docstring is slightly different and requires only
a portion of the complete pattern used in the more common case. As
implemented, the docstring will only be found if there is only
one \code{small_stmt} node in the \code{simple_stmt} node. Since most
functions and methods which use the short form do not provide a
docstring, this may be considered sufficient. The extraction of the
docstring proceeds using the \code{match()} function as described
above, and the value of the docstring is stored as an attribute of the
\code{SuiteInfoBase} object.
one \constant{small_stmt} node in the \constant{simple_stmt} node.
Since most functions and methods which use the short form do not
provide a docstring, this may be considered sufficient. The
extraction of the docstring proceeds using the \function{match()} function
as described above, and the value of the docstring is stored as an
attribute of the \class{SuiteInfoBase} object.
After docstring extraction, a simple definition discovery
algorithm operates on the \code{stmt} nodes of the \code{suite} node. The
special case of the short form is not tested; since there are no
\code{stmt} nodes in the short form, the algorithm will silently skip
the single \code{simple_stmt} node and correctly not discover any
nested definitions.
algorithm operates on the \constant{stmt} nodes of the
\constant{suite} node. The special case of the short form is not
tested; since there are no \constant{stmt} nodes in the short form,
the algorithm will silently skip the single \constant{simple_stmt}
node and correctly not discover any nested definitions.
Each statement in the code block is categorized as
a class definition, function or method definition, or
@ -654,7 +657,7 @@ are stored in instance variables and may be retrieved by name using
the appropriate accessor methods.
The public classes provide any accessors required which are more
specific than those provided by the \code{SuiteInfoBase} class, but
specific than those provided by the \class{SuiteInfoBase} class, but
the real extraction algorithm remains common to all forms of code
blocks. A high-level function can be used to extract the complete set
of information from a source file. (See file \file{example.py}.)