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(libparser.tex): Revised parser module documentation; improved logical

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@ -17,20 +17,21 @@
The \code{parser} module provides an interface to Python's internal The \code{parser} module provides an interface to Python's internal
parser and byte-code compiler. The primary purpose for this interface 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 is to allow Python code to edit the parse tree of a Python expression
and create executable code from this. This can be better than trying and create executable code from this. This is better than trying
to parse and modify an arbitrary Python code fragment as a string, and to parse and modify an arbitrary Python code fragment as a string
ensures that parsing is performed in a manner identical to the code because parsing is performed in a manner identical to the code
forming the application. It's also faster. forming the application. It is also faster.
There are a few things to note about this module which are important 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 to making use of the data structures created. This is not a tutorial
on editing the parse trees for Python code. on editing the parse trees for Python code, but some examples of using
the \code{parser} module are presented.
Most importantly, a good understanding of the Python grammar processed Most importantly, a good understanding of the Python grammar processed
by the internal parser is required. For full information on the by the internal parser is required. For full information on the
language syntax, refer to the Language Reference. The parser itself language syntax, refer to the Language Reference. The parser itself
is created from a grammar specification defined in the file is created from a grammar specification defined in the file
\code{Grammar/Grammar} in the standard Python distribution. The parse \file{Grammar/Grammar} in the standard Python distribution. The parse
trees stored in the ``AST objects'' created by this module are the trees stored in the ``AST objects'' created by this module are the
actual output from the internal parser when created by the actual output from the internal parser when created by the
\code{expr()} or \code{suite()} functions, described below. The AST \code{expr()} or \code{suite()} functions, described below. The AST
@ -51,16 +52,16 @@ Each element of the sequences returned by \code{ast2list} or
non-terminal elements in the grammar always have a length greater than non-terminal elements in the grammar always have a length greater than
one. The first element is an integer which identifies a production in one. The first element is an integer which identifies a production in
the grammar. These integers are given symbolic names in the C header the grammar. These integers are given symbolic names in the C header
file \code{Include/graminit.h} and the Python module file \file{Include/graminit.h} and the Python module
\code{Lib/symbol.py}. Each additional element of the sequence represents \file{Lib/symbol.py}. Each additional element of the sequence represents
a component of the production as recognized in the input string: these a component of the production as recognized in the input string: these
are always sequences which have the same form as the parent. An are always sequences which have the same form as the parent. An
important aspect of this structure which should be noted is that important aspect of this structure which should be noted is that
keywords used to identify the parent node type, such as the keyword keywords used to identify the parent node type, such as the keyword
\code{if} in an \emph{if\_stmt}, are included in the node tree without \code{if} in an \code{if_stmt}, are included in the node tree without
any special treatment. For example, the \code{if} keyword is any special treatment. For example, the \code{if} keyword is
represented by the tuple \code{(1, 'if')}, where \code{1} is the represented by the tuple \code{(1, 'if')}, where \code{1} is the
numeric value associated with all \code{NAME} elements, including numeric value associated with all \code{NAME} tokens, including
variable and function names defined by the user. In an alternate form variable and function names defined by the user. In an alternate form
returned when line number information is requested, the same token returned when line number information is requested, the same token
might be represented as \code{(1, 'if', 12)}, where the \code{12} might be represented as \code{(1, 'if', 12)}, where the \code{12}
@ -70,51 +71,115 @@ Terminal elements are represented in much the same way, but without
any child elements and the addition of the source text which was 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 \code{if} keyword above is
representative. The various types of terminal symbols are defined in representative. The various types of terminal symbols are defined in
the C header file \code{Include/token.h} and the Python module the C header file \file{Include/token.h} and the Python module
\code{Lib/token.py}. \file{Lib/token.py}.
The AST objects are not actually required to support the functionality The AST objects are not required to support the functionality of this
of this module, but are provided for three purposes: to allow an module, but are provided for three purposes: to allow an application
application to amortize the cost of processing complex parse trees, to to amortize the cost of processing complex parse trees, to provide a
provide a parse tree representation which conserves memory space when parse tree representation which conserves memory space when compared
compared to the Python list or tuple representation, and to ease the to the Python list or tuple representation, and to ease the creation
creation of additional modules in C which manipulate parse trees. A of additional modules in C which manipulate parse trees. A simple
simple ``wrapper'' module may be created in Python to hide the use of ``wrapper'' class may be created in Python to hide the use of AST
AST objects. objects; the \code{AST} library module provides a variety of such
classes.
The \code{parser} module defines functions for a few distinct
The \code{parser} module defines the following functions: 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
query the type of parse tree represented by an AST object.
\renewcommand{\indexsubitem}{(in module parser)} \renewcommand{\indexsubitem}{(in module parser)}
\begin{funcdesc}{ast2list}{ast\optional{\, line\_info\code{ = 0}}}
\subsection{Creating AST Objects}
AST objects may be created from source code or from a parse tree.
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 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 parse succeeds, an AST object is created to hold the internal
parse tree representation, otherwise an appropriate exception is
thrown.
\end{funcdesc}
\begin{funcdesc}{sequence2ast}{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 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
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}
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.
Sequences representing terminal tokens may be represented as either
two-element lists of the form \code{(1, 'name')} or as three-element
lists of the form \code{(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.
\end{funcdesc}
\begin{funcdesc}{tuple2ast}{sequence}
This is the same function as \code{sequence2ast()}. This entry point
is maintained for backward compatibility.
\end{funcdesc}
\subsection{Converting AST Objects}
AST 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.
\begin{funcdesc}{ast2list}{ast\optional{\, line_info\code{ = 0}}}
This function accepts an AST object from the caller in This function accepts an AST object from the caller in
\code{\var{ast}} and returns a Python list representing the \code{\var{ast}} and returns a Python list representing the
equivelent parse tree. The resulting list representation can be used equivelent parse tree. The resulting list representation can be used
for inspection or the creation of a new parse tree in list form. for inspection or the creation of a new parse tree in list form. This
This function does not fail so long as memory is available to build function does not fail so long as memory is available to build the
the list representation. If a parse tree will only be used for list representation. If the parse tree will only be used for
inspection, \code{ast2tuple()} should be used instead to reduce memory inspection, \code{ast2tuple()} should be used instead to reduce memory
consumption and fragmentation. When modifications are to be made to consumption and fragmentation. When the list representation is
the parse tree, this function is significantly faster than retrieving required, this function is significantly faster than retrieving a
a tuple representation and converting that to nested lists. tuple representation and converting that to nested lists.
If the \code{line\_info} flag is given true value, line number If \code{\var{line_info}} is true, line number information will be
information will be included for all terminal tokens as a third included for all terminal tokens as a third element of the list
element of the list representing the token. This information is representing the token. This information is omitted if the flag is
omitted if the flag is false or omitted. false or omitted.
\end{funcdesc} \end{funcdesc}
\begin{funcdesc}{ast2tuple}{ast\optional{\, line\_info\code{ = 0}}} \begin{funcdesc}{ast2tuple}{ast\optional{\, line_info\code{ = 0}}}
This function accepts an AST object from the caller in This function accepts an AST object from the caller in
\code{\var{ast}} and returns a Python tuple representing the \code{\var{ast}} and returns a Python tuple representing the
equivelent parse tree. Other than returning a tuple instead of a 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 \code{ast2list()}.
If the \code{line\_info} flag is given true value, line number If \code{\var{line_info}} is true, line number information will be
information will be included for all terminal tokens as a third included for all terminal tokens as a third element of the list
element of the list representing the token. This information is representing the token. This information is omitted if the flag is
omitted if the flag is false or omitted. false or omitted.
\end{funcdesc} \end{funcdesc}
\begin{funcdesc}{compileast}{ast\optional{\, filename\code{ = '<ast>'}}} \begin{funcdesc}{compileast}{ast\optional{\, filename\code{ = '<ast>'}}}
@ -128,7 +193,7 @@ for \code{\var{filename}} indicates that the source was an AST object.
Compiling an AST object may result in exceptions related to 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 \code{SyntaxError} caused by the
parse tree for \code{del f(0)}; this statement is considered legal parse tree for \code{del f(0)}: this statement is considered legal
within the formal grammar for Python but is not a legal language within the formal grammar for Python but is not a legal language
construct. The \code{SyntaxError} raised for this condition is construct. The \code{SyntaxError} raised for this condition is
actually generated by the Python byte-compiler normally, which is why actually generated by the Python byte-compiler normally, which is why
@ -138,14 +203,13 @@ inspection of the parse tree.
\end{funcdesc} \end{funcdesc}
\begin{funcdesc}{expr}{string} \subsection{Queries on AST Objects}
The \code{expr()} function parses the parameter \code{\var{string}}
as if it were an input to \code{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}
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()}.
\begin{funcdesc}{isexpr}{ast} \begin{funcdesc}{isexpr}{ast}
When \code{\var{ast}} represents an \code{'eval'} form, this function When \code{\var{ast}} represents an \code{'eval'} form, this function
@ -160,48 +224,10 @@ like this either, and are identical to those created by the built-in
\begin{funcdesc}{issuite}{ast} \begin{funcdesc}{issuite}{ast}
This function mirrors \code{isexpr()} in that it reports whether an This function mirrors \code{isexpr()} in that it reports whether an
AST object represents a suite of statements. It is not safe to assume AST object represents an \code{'exec'} form, commonly known as a
that this function is equivelent to \code{not isexpr(\var{ast})}, as ``suite.'' It is not safe to assume that this function is equivelent
additional syntactic fragments may be supported in the future. to \code{not isexpr(\var{ast})}, as additional syntactic fragments may
\end{funcdesc} be supported in the future.
\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 parse succeeds, an AST object is created to hold the internal
parse tree representation, otherwise an appropriate exception is
thrown.
\end{funcdesc}
\begin{funcdesc}{sequence2ast}{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 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
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 will normally
indicate problems not related to syntax (such as a \code{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.
Sequences representing terminal tokens may be represented as either
two-element lists of the form \code{(1, 'name')} or as three-element
lists of the form \code{(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.
\end{funcdesc}
\begin{funcdesc}{tuple2ast}{sequence}
This is the same function as \code{sequence2ast}. This entry point is
maintained for backward compatibility.
\end{funcdesc} \end{funcdesc}
@ -235,10 +261,11 @@ to the descriptions of each function for detailed information.
\subsection{AST Objects} \subsection{AST Objects}
AST objects (returned by \code{expr()}, \code{suite()}, and AST objects returned by \code{expr()}, \code{suite()}, and
\code{sequence2ast()}, described above) have no methods of their own. \code{sequence2ast()} have no methods of their own.
Some of the functions defined which accept an AST object as their Some of the functions defined which accept an AST object as their
first argument may change to object methods in the future. first argument may change to object methods in the future. The type
of these objects is available as \code{ASTType} in the module.
Ordered and equality comparisons are supported between AST objects. Ordered and equality comparisons are supported between AST objects.
@ -247,12 +274,12 @@ Ordered and equality comparisons are supported between AST objects.
The parser modules allows operations to be performed on the parse tree The parser modules allows operations to be performed on the parse tree
of Python source code before the bytecode is generated, and provides of Python source code before the bytecode is generated, and provides
for inspection of the parse tree for information gathering purposes as for inspection of the parse tree for information gathering purposes.
well. Two examples are presented. The simple example demonstrates Two examples are presented. The simple example demonstrates emulation
emulation of the \code{compile()} built-in function and the complex of the \code{compile()} built-in function and the complex example
example shows the use of a parse tree for information discovery. shows the use of a parse tree for information discovery.
\subsubsection{Emulation of {\tt compile()}} \subsubsection{Emulation of \sectcode{compile()}}
While many useful operations may take place between parsing and While many useful operations may take place between parsing and
bytecode generation, the simplest operation is to do nothing. For bytecode generation, the simplest operation is to do nothing. For
@ -298,17 +325,16 @@ def load_expression(source_string):
\subsubsection{Information Discovery} \subsubsection{Information Discovery}
Some applications can benfit from access to the parse tree itself, and Some applications benefit from direct access to the parse tree. The
can take advantage of the intermediate data structure provided by the remainder of this section demonstrates how the parse tree provides
\code{parser} module. The remainder of this section of examples will access to module documentation defined in docstrings without requiring
demonstrate how the intermediate data structure can provide access to that the code being examined be loaded into a running interpreter via
module documentation defined in docstrings without requiring that the \code{import}. This can be very useful for performing analyses of
code being examined be imported into a running interpreter. This can untrusted code.
be very useful for performing analyses of untrusted code.
Generally, the example will demonstrate how the parse tree may be Generally, the example will demonstrate how the parse tree may be
traversed to distill interesting information. Two functions and a set traversed to distill interesting information. Two functions and a set
of classes is developed which provide programmatic access to high of classes are developed which provide programmatic access to high
level function and class definitions provided by a module. The level function and class definitions provided by a module. The
classes extract information from the parse tree and provide access to classes extract information from the parse tree and provide access to
the information at a useful semantic level, one function provides a the information at a useful semantic level, one function provides a
@ -316,7 +342,7 @@ simple low-level pattern matching capability, and the other function
defines a high-level interface to the classes by handling file defines a high-level interface to the classes by handling file
operations on behalf of the caller. All source files mentioned here operations on behalf of the caller. All source files mentioned here
which are not part of the Python installation are located in the which are not part of the Python installation are located in the
\file{Demo/parser} directory of the distribution. \file{Demo/parser/} directory of the distribution.
The dynamic nature of Python allows the programmer a great deal of The dynamic nature of Python allows the programmer a great deal of
flexibility, but most modules need only a limited measure of this when flexibility, but most modules need only a limited measure of this when
@ -324,13 +350,13 @@ defining classes, functions, and methods. In this example, the only
definitions that will be considered are those which are defined in the 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 \code{def}
statement at column zero of a module, but not a function defined statement at column zero of a module, but not a function defined
within a branch of an \code{if} ... \code{else} construct, thought within a branch of an \code{if} ... \code{else} construct, though
there are some good reasons for doing so in some situations. Nesting there are some good reasons for doing so in some situations. Nesting
of definitions will be handled by the code developed in the example. of definitions will be handled by the code developed in the example.
To construct the upper-level extraction methods, we need to know what To construct the upper-level extraction methods, we need to know what
the parse tree structure looks like and how much of it we actually the parse tree structure looks like and how much of it we actually
need to be concerned about. Python uses a moderately deep parse tree, need to be concerned about. Python uses a moderately deep parse tree
so there are a large number of intermediate nodes. It is important to so there are a large number of intermediate nodes. It is important to
read and understand the formal grammar used by Python. This is read and understand the formal grammar used by Python. This is
specified in the file \file{Grammar/Grammar} in the distribution. specified in the file \file{Grammar/Grammar} in the distribution.
@ -345,7 +371,7 @@ a module consisting of a docstring and nothing else. (See file
Using the interpreter to take a look at the parse tree, we find a Using the interpreter to take a look at the parse tree, we find a
bewildering mass of numbers and parentheses, with the documentation bewildering mass of numbers and parentheses, with the documentation
buried deep in the nested tuples: buried deep in nested tuples.
\begin{verbatim} \begin{verbatim}
>>> import parser >>> import parser
@ -405,12 +431,12 @@ the docstring subtree within the tree defining the described
structure. structure.
By replacing the actual docstring with something to signify a variable By replacing the actual docstring with something to signify a variable
component of the tree, we allow a simple pattern matching approach may component of the tree, we allow a simple pattern matching approach to
be taken to checking any given subtree for equivelence to the general check any given subtree for equivelence to the general pattern for
pattern for docstrings. Since the example demonstrates information docstrings. Since the example demonstrates information extraction, we
extraction, we can safely require that the tree be in tuple form can safely require that the tree be in tuple form rather than list
rather than list form, allowing a simple variable representation to be form, allowing a simple variable representation to be
\code{['variable\_name']}. A simple recursive function can implement \code{['variable_name']}. A simple recursive function can implement
the pattern matching, returning a boolean and a dictionary of variable the pattern matching, returning a boolean and a dictionary of variable
name to value mappings. (See file \file{example.py}.) name to value mappings. (See file \file{example.py}.)
@ -434,7 +460,7 @@ def match(pattern, data, vars=None):
return same, vars return same, vars
\end{verbatim} \end{verbatim}
Using this simple recursive pattern matching function and the symbolic Using this simple representation for syntactic variables and the symbolic
node types, the pattern for the candidate docstring subtrees becomes node types, the pattern for the candidate docstring subtrees becomes
fairly readable. (See file \file{example.py}.) fairly readable. (See file \file{example.py}.)
@ -518,17 +544,17 @@ methods \code{get_name()}, \code{get_docstring()},
Within each of the forms of code block that the public classes 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 represent, most of the required information is in the same form and is
access in the same way, with classes having the distinction that accessed in the same way, with classes having the distinction that
functions defined at the top level are referred to as ``methods.'' functions defined at the top level are referred to as ``methods.''
Since the difference in nomenclature reflects a real semantic Since the difference in nomenclature reflects a real semantic
distinction from functions defined outside of a class, our distinction from functions defined outside of a class, the
implementation needs to maintain the same measure of distinction. implementation needs to maintain the distinction.
Hence, most of the functionality of the public classes can be 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, \code{SuiteInfoBase}, with the
accessors for function and method information provided elsewhere. accessors for function and method information provided elsewhere.
Note that there is only one class which represents function and method Note that there is only one class which represents function and method
information; this mirrors the use of the \code{def} statement to information; this paralels the use of the \code{def} statement to
define both types of functions. define both types of elements.
Most of the accessor functions are declared in \code{SuiteInfoBase} Most of the accessor functions are declared in \code{SuiteInfoBase}
and do not need to be overriden by subclasses. More importantly, the and do not need to be overriden by subclasses. More importantly, the
@ -602,25 +628,25 @@ 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, \code{small_stmt} element. The
extraction of such a docstring is slightly different and requires only extraction of such a docstring is slightly different and requires only
a portion of the complete pattern used in the more common case. As a portion of the complete pattern used in the more common case. As
given in the code, the docstring will only be found if there is only implemented, the docstring will only be found if there is only
one \code{small_stmt} node in the \code{simple_stmt} node. Since most one \code{small_stmt} node in the \code{simple_stmt} node. Since most
functions and methods which use the short form do not provide functions and methods which use the short form do not provide a
docstring, this may be considered sufficient. The extraction of the docstring, this may be considered sufficient. The extraction of the
docstring proceeds using the \code{match()} function as described docstring proceeds using the \code{match()} function as described
above, and the value of the docstring is stored as an attribute of the above, and the value of the docstring is stored as an attribute of the
\code{SuiteInfoBase} object. \code{SuiteInfoBase} object.
After docstring extraction, the operates a simple definition discovery After docstring extraction, a simple definition discovery
algorithm on the \code{stmt} nodes of the \code{suite} node. The 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 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 \code{stmt} nodes in the short form, the algorithm will silently skip
the single \code{simple_stmt} node and correctly not discover any the single \code{simple_stmt} node and correctly not discover any
nested definitions. nested definitions.
Each statement in the code block bing examined is categorized as being Each statement in the code block is categorized as
a class definition, function definition (including methods), or a class definition, function or method definition, or
something else. For the definition statements, the name of the something else. For the definition statements, the name of the
element being defined is extracted and representation object element defined is extracted and a representation object
appropriate to the definition is created with the defining subtree appropriate to the definition is created with the defining subtree
passed as an argument to the constructor. The repesentation objects passed as an argument to the constructor. The repesentation objects
are stored in instance variables and may be retrieved by name using are stored in instance variables and may be retrieved by name using
@ -630,7 +656,7 @@ 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 \code{SuiteInfoBase} class, but
the real extraction algorithm remains common to all forms of code the real extraction algorithm remains common to all forms of code
blocks. A high-level function can be used to extract the complete set blocks. A high-level function can be used to extract the complete set
of information from a source file: of information from a source file. (See file \file{example.py}.)
\begin{verbatim} \begin{verbatim}
def get_docs(fileName): def get_docs(fileName):

282
Doc/libparser.tex
View file

@ -17,20 +17,21 @@
The \code{parser} module provides an interface to Python's internal The \code{parser} module provides an interface to Python's internal
parser and byte-code compiler. The primary purpose for this interface 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 is to allow Python code to edit the parse tree of a Python expression
and create executable code from this. This can be better than trying and create executable code from this. This is better than trying
to parse and modify an arbitrary Python code fragment as a string, and to parse and modify an arbitrary Python code fragment as a string
ensures that parsing is performed in a manner identical to the code because parsing is performed in a manner identical to the code
forming the application. It's also faster. forming the application. It is also faster.
There are a few things to note about this module which are important 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 to making use of the data structures created. This is not a tutorial
on editing the parse trees for Python code. on editing the parse trees for Python code, but some examples of using
the \code{parser} module are presented.
Most importantly, a good understanding of the Python grammar processed Most importantly, a good understanding of the Python grammar processed
by the internal parser is required. For full information on the by the internal parser is required. For full information on the
language syntax, refer to the Language Reference. The parser itself language syntax, refer to the Language Reference. The parser itself
is created from a grammar specification defined in the file is created from a grammar specification defined in the file
\code{Grammar/Grammar} in the standard Python distribution. The parse \file{Grammar/Grammar} in the standard Python distribution. The parse
trees stored in the ``AST objects'' created by this module are the trees stored in the ``AST objects'' created by this module are the
actual output from the internal parser when created by the actual output from the internal parser when created by the
\code{expr()} or \code{suite()} functions, described below. The AST \code{expr()} or \code{suite()} functions, described below. The AST
@ -51,16 +52,16 @@ Each element of the sequences returned by \code{ast2list} or
non-terminal elements in the grammar always have a length greater than non-terminal elements in the grammar always have a length greater than
one. The first element is an integer which identifies a production in one. The first element is an integer which identifies a production in
the grammar. These integers are given symbolic names in the C header the grammar. These integers are given symbolic names in the C header
file \code{Include/graminit.h} and the Python module file \file{Include/graminit.h} and the Python module
\code{Lib/symbol.py}. Each additional element of the sequence represents \file{Lib/symbol.py}. Each additional element of the sequence represents
a component of the production as recognized in the input string: these a component of the production as recognized in the input string: these
are always sequences which have the same form as the parent. An are always sequences which have the same form as the parent. An
important aspect of this structure which should be noted is that important aspect of this structure which should be noted is that
keywords used to identify the parent node type, such as the keyword keywords used to identify the parent node type, such as the keyword
\code{if} in an \emph{if\_stmt}, are included in the node tree without \code{if} in an \code{if_stmt}, are included in the node tree without
any special treatment. For example, the \code{if} keyword is any special treatment. For example, the \code{if} keyword is
represented by the tuple \code{(1, 'if')}, where \code{1} is the represented by the tuple \code{(1, 'if')}, where \code{1} is the
numeric value associated with all \code{NAME} elements, including numeric value associated with all \code{NAME} tokens, including
variable and function names defined by the user. In an alternate form variable and function names defined by the user. In an alternate form
returned when line number information is requested, the same token returned when line number information is requested, the same token
might be represented as \code{(1, 'if', 12)}, where the \code{12} might be represented as \code{(1, 'if', 12)}, where the \code{12}
@ -70,51 +71,115 @@ Terminal elements are represented in much the same way, but without
any child elements and the addition of the source text which was 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 \code{if} keyword above is
representative. The various types of terminal symbols are defined in representative. The various types of terminal symbols are defined in
the C header file \code{Include/token.h} and the Python module the C header file \file{Include/token.h} and the Python module
\code{Lib/token.py}. \file{Lib/token.py}.
The AST objects are not actually required to support the functionality The AST objects are not required to support the functionality of this
of this module, but are provided for three purposes: to allow an module, but are provided for three purposes: to allow an application
application to amortize the cost of processing complex parse trees, to to amortize the cost of processing complex parse trees, to provide a
provide a parse tree representation which conserves memory space when parse tree representation which conserves memory space when compared
compared to the Python list or tuple representation, and to ease the to the Python list or tuple representation, and to ease the creation
creation of additional modules in C which manipulate parse trees. A of additional modules in C which manipulate parse trees. A simple
simple ``wrapper'' module may be created in Python to hide the use of ``wrapper'' class may be created in Python to hide the use of AST
AST objects. objects; the \code{AST} library module provides a variety of such
classes.
The \code{parser} module defines functions for a few distinct
The \code{parser} module defines the following functions: 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
query the type of parse tree represented by an AST object.
\renewcommand{\indexsubitem}{(in module parser)} \renewcommand{\indexsubitem}{(in module parser)}
\begin{funcdesc}{ast2list}{ast\optional{\, line\_info\code{ = 0}}}
\subsection{Creating AST Objects}
AST objects may be created from source code or from a parse tree.
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 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 parse succeeds, an AST object is created to hold the internal
parse tree representation, otherwise an appropriate exception is
thrown.
\end{funcdesc}
\begin{funcdesc}{sequence2ast}{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 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
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}
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.
Sequences representing terminal tokens may be represented as either
two-element lists of the form \code{(1, 'name')} or as three-element
lists of the form \code{(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.
\end{funcdesc}
\begin{funcdesc}{tuple2ast}{sequence}
This is the same function as \code{sequence2ast()}. This entry point
is maintained for backward compatibility.
\end{funcdesc}
\subsection{Converting AST Objects}
AST 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.
\begin{funcdesc}{ast2list}{ast\optional{\, line_info\code{ = 0}}}
This function accepts an AST object from the caller in This function accepts an AST object from the caller in
\code{\var{ast}} and returns a Python list representing the \code{\var{ast}} and returns a Python list representing the
equivelent parse tree. The resulting list representation can be used equivelent parse tree. The resulting list representation can be used
for inspection or the creation of a new parse tree in list form. for inspection or the creation of a new parse tree in list form. This
This function does not fail so long as memory is available to build function does not fail so long as memory is available to build the
the list representation. If a parse tree will only be used for list representation. If the parse tree will only be used for
inspection, \code{ast2tuple()} should be used instead to reduce memory inspection, \code{ast2tuple()} should be used instead to reduce memory
consumption and fragmentation. When modifications are to be made to consumption and fragmentation. When the list representation is
the parse tree, this function is significantly faster than retrieving required, this function is significantly faster than retrieving a
a tuple representation and converting that to nested lists. tuple representation and converting that to nested lists.
If the \code{line\_info} flag is given true value, line number If \code{\var{line_info}} is true, line number information will be
information will be included for all terminal tokens as a third included for all terminal tokens as a third element of the list
element of the list representing the token. This information is representing the token. This information is omitted if the flag is
omitted if the flag is false or omitted. false or omitted.
\end{funcdesc} \end{funcdesc}
\begin{funcdesc}{ast2tuple}{ast\optional{\, line\_info\code{ = 0}}} \begin{funcdesc}{ast2tuple}{ast\optional{\, line_info\code{ = 0}}}
This function accepts an AST object from the caller in This function accepts an AST object from the caller in
\code{\var{ast}} and returns a Python tuple representing the \code{\var{ast}} and returns a Python tuple representing the
equivelent parse tree. Other than returning a tuple instead of a 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 \code{ast2list()}.
If the \code{line\_info} flag is given true value, line number If \code{\var{line_info}} is true, line number information will be
information will be included for all terminal tokens as a third included for all terminal tokens as a third element of the list
element of the list representing the token. This information is representing the token. This information is omitted if the flag is
omitted if the flag is false or omitted. false or omitted.
\end{funcdesc} \end{funcdesc}
\begin{funcdesc}{compileast}{ast\optional{\, filename\code{ = '<ast>'}}} \begin{funcdesc}{compileast}{ast\optional{\, filename\code{ = '<ast>'}}}
@ -128,7 +193,7 @@ for \code{\var{filename}} indicates that the source was an AST object.
Compiling an AST object may result in exceptions related to 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 \code{SyntaxError} caused by the
parse tree for \code{del f(0)}; this statement is considered legal parse tree for \code{del f(0)}: this statement is considered legal
within the formal grammar for Python but is not a legal language within the formal grammar for Python but is not a legal language
construct. The \code{SyntaxError} raised for this condition is construct. The \code{SyntaxError} raised for this condition is
actually generated by the Python byte-compiler normally, which is why actually generated by the Python byte-compiler normally, which is why
@ -138,14 +203,13 @@ inspection of the parse tree.
\end{funcdesc} \end{funcdesc}
\begin{funcdesc}{expr}{string} \subsection{Queries on AST Objects}
The \code{expr()} function parses the parameter \code{\var{string}}
as if it were an input to \code{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}
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()}.
\begin{funcdesc}{isexpr}{ast} \begin{funcdesc}{isexpr}{ast}
When \code{\var{ast}} represents an \code{'eval'} form, this function When \code{\var{ast}} represents an \code{'eval'} form, this function
@ -160,48 +224,10 @@ like this either, and are identical to those created by the built-in
\begin{funcdesc}{issuite}{ast} \begin{funcdesc}{issuite}{ast}
This function mirrors \code{isexpr()} in that it reports whether an This function mirrors \code{isexpr()} in that it reports whether an
AST object represents a suite of statements. It is not safe to assume AST object represents an \code{'exec'} form, commonly known as a
that this function is equivelent to \code{not isexpr(\var{ast})}, as ``suite.'' It is not safe to assume that this function is equivelent
additional syntactic fragments may be supported in the future. to \code{not isexpr(\var{ast})}, as additional syntactic fragments may
\end{funcdesc} be supported in the future.
\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 parse succeeds, an AST object is created to hold the internal
parse tree representation, otherwise an appropriate exception is
thrown.
\end{funcdesc}
\begin{funcdesc}{sequence2ast}{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 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
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 will normally
indicate problems not related to syntax (such as a \code{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.
Sequences representing terminal tokens may be represented as either
two-element lists of the form \code{(1, 'name')} or as three-element
lists of the form \code{(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.
\end{funcdesc}
\begin{funcdesc}{tuple2ast}{sequence}
This is the same function as \code{sequence2ast}. This entry point is
maintained for backward compatibility.
\end{funcdesc} \end{funcdesc}
@ -235,10 +261,11 @@ to the descriptions of each function for detailed information.
\subsection{AST Objects} \subsection{AST Objects}
AST objects (returned by \code{expr()}, \code{suite()}, and AST objects returned by \code{expr()}, \code{suite()}, and
\code{sequence2ast()}, described above) have no methods of their own. \code{sequence2ast()} have no methods of their own.
Some of the functions defined which accept an AST object as their Some of the functions defined which accept an AST object as their
first argument may change to object methods in the future. first argument may change to object methods in the future. The type
of these objects is available as \code{ASTType} in the module.
Ordered and equality comparisons are supported between AST objects. Ordered and equality comparisons are supported between AST objects.
@ -247,12 +274,12 @@ Ordered and equality comparisons are supported between AST objects.
The parser modules allows operations to be performed on the parse tree The parser modules allows operations to be performed on the parse tree
of Python source code before the bytecode is generated, and provides of Python source code before the bytecode is generated, and provides
for inspection of the parse tree for information gathering purposes as for inspection of the parse tree for information gathering purposes.
well. Two examples are presented. The simple example demonstrates Two examples are presented. The simple example demonstrates emulation
emulation of the \code{compile()} built-in function and the complex of the \code{compile()} built-in function and the complex example
example shows the use of a parse tree for information discovery. shows the use of a parse tree for information discovery.
\subsubsection{Emulation of {\tt compile()}} \subsubsection{Emulation of \sectcode{compile()}}
While many useful operations may take place between parsing and While many useful operations may take place between parsing and
bytecode generation, the simplest operation is to do nothing. For bytecode generation, the simplest operation is to do nothing. For
@ -298,17 +325,16 @@ def load_expression(source_string):
\subsubsection{Information Discovery} \subsubsection{Information Discovery}
Some applications can benfit from access to the parse tree itself, and Some applications benefit from direct access to the parse tree. The
can take advantage of the intermediate data structure provided by the remainder of this section demonstrates how the parse tree provides
\code{parser} module. The remainder of this section of examples will access to module documentation defined in docstrings without requiring
demonstrate how the intermediate data structure can provide access to that the code being examined be loaded into a running interpreter via
module documentation defined in docstrings without requiring that the \code{import}. This can be very useful for performing analyses of
code being examined be imported into a running interpreter. This can untrusted code.
be very useful for performing analyses of untrusted code.
Generally, the example will demonstrate how the parse tree may be Generally, the example will demonstrate how the parse tree may be
traversed to distill interesting information. Two functions and a set traversed to distill interesting information. Two functions and a set
of classes is developed which provide programmatic access to high of classes are developed which provide programmatic access to high
level function and class definitions provided by a module. The level function and class definitions provided by a module. The
classes extract information from the parse tree and provide access to classes extract information from the parse tree and provide access to
the information at a useful semantic level, one function provides a the information at a useful semantic level, one function provides a
@ -316,7 +342,7 @@ simple low-level pattern matching capability, and the other function
defines a high-level interface to the classes by handling file defines a high-level interface to the classes by handling file
operations on behalf of the caller. All source files mentioned here operations on behalf of the caller. All source files mentioned here
which are not part of the Python installation are located in the which are not part of the Python installation are located in the
\file{Demo/parser} directory of the distribution. \file{Demo/parser/} directory of the distribution.
The dynamic nature of Python allows the programmer a great deal of The dynamic nature of Python allows the programmer a great deal of
flexibility, but most modules need only a limited measure of this when flexibility, but most modules need only a limited measure of this when
@ -324,13 +350,13 @@ defining classes, functions, and methods. In this example, the only
definitions that will be considered are those which are defined in the 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 \code{def}
statement at column zero of a module, but not a function defined statement at column zero of a module, but not a function defined
within a branch of an \code{if} ... \code{else} construct, thought within a branch of an \code{if} ... \code{else} construct, though
there are some good reasons for doing so in some situations. Nesting there are some good reasons for doing so in some situations. Nesting
of definitions will be handled by the code developed in the example. of definitions will be handled by the code developed in the example.
To construct the upper-level extraction methods, we need to know what To construct the upper-level extraction methods, we need to know what
the parse tree structure looks like and how much of it we actually the parse tree structure looks like and how much of it we actually
need to be concerned about. Python uses a moderately deep parse tree, need to be concerned about. Python uses a moderately deep parse tree
so there are a large number of intermediate nodes. It is important to so there are a large number of intermediate nodes. It is important to
read and understand the formal grammar used by Python. This is read and understand the formal grammar used by Python. This is
specified in the file \file{Grammar/Grammar} in the distribution. specified in the file \file{Grammar/Grammar} in the distribution.
@ -345,7 +371,7 @@ a module consisting of a docstring and nothing else. (See file
Using the interpreter to take a look at the parse tree, we find a Using the interpreter to take a look at the parse tree, we find a
bewildering mass of numbers and parentheses, with the documentation bewildering mass of numbers and parentheses, with the documentation
buried deep in the nested tuples: buried deep in nested tuples.
\begin{verbatim} \begin{verbatim}
>>> import parser >>> import parser
@ -405,12 +431,12 @@ the docstring subtree within the tree defining the described
structure. structure.
By replacing the actual docstring with something to signify a variable By replacing the actual docstring with something to signify a variable
component of the tree, we allow a simple pattern matching approach may component of the tree, we allow a simple pattern matching approach to
be taken to checking any given subtree for equivelence to the general check any given subtree for equivelence to the general pattern for
pattern for docstrings. Since the example demonstrates information docstrings. Since the example demonstrates information extraction, we
extraction, we can safely require that the tree be in tuple form can safely require that the tree be in tuple form rather than list
rather than list form, allowing a simple variable representation to be form, allowing a simple variable representation to be
\code{['variable\_name']}. A simple recursive function can implement \code{['variable_name']}. A simple recursive function can implement
the pattern matching, returning a boolean and a dictionary of variable the pattern matching, returning a boolean and a dictionary of variable
name to value mappings. (See file \file{example.py}.) name to value mappings. (See file \file{example.py}.)
@ -434,7 +460,7 @@ def match(pattern, data, vars=None):
return same, vars return same, vars
\end{verbatim} \end{verbatim}
Using this simple recursive pattern matching function and the symbolic Using this simple representation for syntactic variables and the symbolic
node types, the pattern for the candidate docstring subtrees becomes node types, the pattern for the candidate docstring subtrees becomes
fairly readable. (See file \file{example.py}.) fairly readable. (See file \file{example.py}.)
@ -518,17 +544,17 @@ methods \code{get_name()}, \code{get_docstring()},
Within each of the forms of code block that the public classes 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 represent, most of the required information is in the same form and is
access in the same way, with classes having the distinction that accessed in the same way, with classes having the distinction that
functions defined at the top level are referred to as ``methods.'' functions defined at the top level are referred to as ``methods.''
Since the difference in nomenclature reflects a real semantic Since the difference in nomenclature reflects a real semantic
distinction from functions defined outside of a class, our distinction from functions defined outside of a class, the
implementation needs to maintain the same measure of distinction. implementation needs to maintain the distinction.
Hence, most of the functionality of the public classes can be 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, \code{SuiteInfoBase}, with the
accessors for function and method information provided elsewhere. accessors for function and method information provided elsewhere.
Note that there is only one class which represents function and method Note that there is only one class which represents function and method
information; this mirrors the use of the \code{def} statement to information; this paralels the use of the \code{def} statement to
define both types of functions. define both types of elements.
Most of the accessor functions are declared in \code{SuiteInfoBase} Most of the accessor functions are declared in \code{SuiteInfoBase}
and do not need to be overriden by subclasses. More importantly, the and do not need to be overriden by subclasses. More importantly, the
@ -602,25 +628,25 @@ 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, \code{small_stmt} element. The
extraction of such a docstring is slightly different and requires only extraction of such a docstring is slightly different and requires only
a portion of the complete pattern used in the more common case. As a portion of the complete pattern used in the more common case. As
given in the code, the docstring will only be found if there is only implemented, the docstring will only be found if there is only
one \code{small_stmt} node in the \code{simple_stmt} node. Since most one \code{small_stmt} node in the \code{simple_stmt} node. Since most
functions and methods which use the short form do not provide functions and methods which use the short form do not provide a
docstring, this may be considered sufficient. The extraction of the docstring, this may be considered sufficient. The extraction of the
docstring proceeds using the \code{match()} function as described docstring proceeds using the \code{match()} function as described
above, and the value of the docstring is stored as an attribute of the above, and the value of the docstring is stored as an attribute of the
\code{SuiteInfoBase} object. \code{SuiteInfoBase} object.
After docstring extraction, the operates a simple definition discovery After docstring extraction, a simple definition discovery
algorithm on the \code{stmt} nodes of the \code{suite} node. The 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 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 \code{stmt} nodes in the short form, the algorithm will silently skip
the single \code{simple_stmt} node and correctly not discover any the single \code{simple_stmt} node and correctly not discover any
nested definitions. nested definitions.
Each statement in the code block bing examined is categorized as being Each statement in the code block is categorized as
a class definition, function definition (including methods), or a class definition, function or method definition, or
something else. For the definition statements, the name of the something else. For the definition statements, the name of the
element being defined is extracted and representation object element defined is extracted and a representation object
appropriate to the definition is created with the defining subtree appropriate to the definition is created with the defining subtree
passed as an argument to the constructor. The repesentation objects passed as an argument to the constructor. The repesentation objects
are stored in instance variables and may be retrieved by name using are stored in instance variables and may be retrieved by name using
@ -630,7 +656,7 @@ 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 \code{SuiteInfoBase} class, but
the real extraction algorithm remains common to all forms of code the real extraction algorithm remains common to all forms of code
blocks. A high-level function can be used to extract the complete set blocks. A high-level function can be used to extract the complete set
of information from a source file: of information from a source file. (See file \file{example.py}.)
\begin{verbatim} \begin{verbatim}
def get_docs(fileName): def get_docs(fileName):