303 lines
11 KiB
TeX
303 lines
11 KiB
TeX
% Complete documentation on the extended LaTeX markup used for Python
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% documentation is available in ``Documenting Python'', which is part
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% of the standard documentation for Python. It may be found online
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% at:
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%
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% http://www.python.org/doc/current/doc/doc.html
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\documentclass{howto}
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\title{Python compiler package}
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\author{Jeremy Hylton}
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% Please at least include a long-lived email address;
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% the rest is at your discretion.
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\authoraddress{
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PythonLabs \\
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Zope Corporation \\
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Email: \email{jeremy@zope.com}
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}
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\date{August 15, 2001} % update before release!
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% Use an explicit date so that reformatting
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% doesn't cause a new date to be used. Setting
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% the date to \today can be used during draft
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% stages to make it easier to handle versions.
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\release{2.2} % release version; this is used to define the
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% \version macro
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\makeindex % tell \index to actually write the .idx file
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\makemodindex % If this contains a lot of module sections.
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\begin{document}
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\maketitle
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\begin{abstract}
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\noindent
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The Python compiler package is a tool for analyzing Python source code
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and generating Python bytecode. The compiler contains libraries to
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generate an abstract syntax tree from Python source code and to
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generate Python bytecode from the tree.
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\end{abstract}
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\tableofcontents
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\section{Introduction\label{Introduction}}
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XXX Need basic intro
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XXX what are the major advantages... the abstract syntax is much
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closer to the python source...
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\section{The basic interface}
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\declaremodule{}{compiler}
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The top-level of the package defines four functions.
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\begin{funcdesc}{parse}{buf}
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Returns an abstract syntax tree for the Python source code in \var{buf}.
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The function raises SyntaxError if there is an error in the source
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code. The return value is a \class{compiler.ast.Module} instance that
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contains the tree.
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\end{funcdesc}
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\begin{funcdesc}{parseFile}{path}
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Return an abstract syntax tree for the Python source code in the file
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specified by \var{path}. It is equivalent to
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\code{parse(open(\var{path}).read())}.
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\end{funcdesc}
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\begin{funcdesc}{walk}{ast, visitor\optional{, verbose}}
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Do a pre-order walk over the abstract syntax tree \var{ast}. Call the
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appropriate method on the \var{visitor} instance for each node
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encountered.
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\end{funcdesc}
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\begin{funcdesc}{compile}{path}
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Compile the file \var{path} and generate the corresponding \file{.pyc}
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file.
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\end{funcdesc}
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The \module{compiler} package contains the following modules:
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\refmodule[compiler.ast]{ast}, \module{consts}, \module{future},
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\module{misc}, \module{pyassem}, \module{pycodegen}, \module{symbols},
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\module{transformer}, and \refmodule[compiler.visitor]{visitor}.
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\section{Limitations}
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There are some problems with the error checking of the compiler
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package. The interpreter detects syntax errors in two distinct
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phases. One set of errors is detected by the interpreter's parser,
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the other set by the compiler. The compiler package relies on the
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interpreter's parser, so it get the first phases of error checking for
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free. It implements the second phase itself, and that implement is
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incomplete. For example, the compiler package does not raise an error
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if a name appears more than once in an argument list:
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\code{def f(x, x): ...}
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\section{Python Abstract Syntax}
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The \module{compiler.ast} module defines an abstract syntax for
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Python. In the abstract syntax tree, each node represents a syntactic
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construct. The root of the tree is \class{Module} object.
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The abstract syntax offers a higher level interface to parsed Python
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source code. The \ulink{\module{parser}}
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{http://www.python.org/doc/current/lib/module-parser.html}
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module and the compiler written in C for the Python interpreter use a
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concrete syntax tree. The concrete syntax is tied closely to the
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grammar description used for the Python parser. Instead of a single
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node for a construct, there are often several levels of nested nodes
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that are introduced by Python's precedence rules.
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The abstract syntax tree is created by the
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\module{compiler.transformer} module. The transformer relies on the
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builtin Python parser to generate a concrete syntax tree. It
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generates an abstract syntax tree from the concrete tree.
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The \module{transformer} module was created by Greg
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Stein\index{Stein, Greg} and Bill Tutt\index{Tutt, Bill} for an
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experimental Python-to-C compiler. The current version contains a
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number of modifications and improvements, but the basic form of the
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abstract syntax and of the transformer are due to Stein and Tutt.
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\section{AST Nodes}
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\declaremodule{}{compiler.ast}
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The \module{compiler.ast} module is generated from a text file that
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describes each node type and its elements. Each node type is
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represented as a class that inherits from the abstract base class
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\class{compiler.ast.Node} and defines a set of named attributes for
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child nodes.
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\begin{classdesc}{Node}{}
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The \class{Node} instances are created automatically by the parser
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generator. The recommended interface for specific \class{Node}
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instances is to use the public attributes to access child nodes. A
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public attribute may be bound to a single node or to a sequence of
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nodes, depending on the \class{Node} type. For example, the
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\member{bases} attribute of the \class{Class} node, is bound to a
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list of base class nodes, and the \member{doc} attribute is bound to
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a single node.
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Each \class{Node} instance has a \member{lineno} attribute which may
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be \code{None}. XXX Not sure what the rules are for which nodes
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will have a useful lineno.
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\end{classdesc}
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All \class{Node} objects offer the following methods:
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\begin{methoddesc}{getChildren}{}
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Returns a flattened list of the child nodes and objects in the
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order they occur. Specifically, the order of the nodes is the
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order in which they appear in the Python grammar. Not all of the
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children are \class{Node} instances. The names of functions and
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classes, for example, are plain strings.
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\end{methoddesc}
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\begin{methoddesc}{getChildNodes}{}
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Returns a flattened list of the child nodes in the order they
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occur. This method is like \method{getChildren()}, except that it
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only returns those children that are \class{Node} instances.
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\end{methoddesc}
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Two examples illustrate the general structure of \class{Node}
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classes. The \keyword{while} statement is defined by the following
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grammar production:
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\begin{verbatim}
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while_stmt: "while" expression ":" suite
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["else" ":" suite]
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\end{verbatim}
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The \class{While} node has three attributes: \member{test},
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\member{body}, and \member{else_}. (If the natural name for an
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attribute is also a Python reserved word, it can't be used as an
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attribute name. An underscore is appended to the word to make it a
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legal identifier, hence \member{else_} instead of \keyword{else}.)
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The \keyword{if} statement is more complicated because it can include
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several tests.
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\begin{verbatim}
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if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]
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\end{verbatim}
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The \class{If} node only defines two attributes: \member{tests} and
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\member{else_}. The \member{tests} attribute is a sequence of test
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expression, consequent body pairs. There is one pair for each
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\keyword{if}/\keyword{elif} clause. The first element of the pair is
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the test expression. The second elements is a \class{Stmt} node that
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contains the code to execute if the test is true.
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The \method{getChildren()} method of \class{If} returns a flat list of
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child nodes. If there are three \keyword{if}/\keyword{elif} clauses
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and no \keyword{else} clause, then \method{getChildren()} will return
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a list of six elements: the first test expression, the first
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\class{Stmt}, the second text expression, etc.
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The following table lists each of the \class{Node} subclasses defined
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in \module{compiler.ast} and each of the public attributes available
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on their instances. The values of most of the attributes are
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themselves \class{Node} instances or sequences of instances. When the
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value is something other than an instance, the type is noted in the
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comment. The attributes are listed in the order in which they are
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returned by \method{getChildren()} and \method{getChildNodes()}.
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\input{asttable}
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\section{Assignment nodes}
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There is a collection of nodes used to represent assignments. Each
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assignment statement in the source code becomes a single
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\class{Assign} node in the AST. The \member{nodes} attribute is a
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list that contains a node for each assignment target. This is
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necessary because assignment can be chained, e.g. \code{a = b = 2}.
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Each \class{Node} in the list will be one of the following classes:
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\class{AssAttr}, \class{AssList}, \class{AssName}, or
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\class{AssTuple}.
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XXX Explain what the AssXXX nodes are for. Mention \code{a.b.c = 2}
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as an example. Explain what the flags are for.
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\section{Using Visitors to Walk ASTs}
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\declaremodule{}{compiler.visitor}
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The visitor pattern is ... The \refmodule{compiler} package uses a
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variant on the visitor pattern that takes advantage of Python's
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introspection features to elminiate the need for much of the visitor's
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infrastructure.
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The classes being visited do not need to be programmed to accept
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visitors. The visitor need only define visit methods for classes it
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is specifically interested in; a default visit method can handle the
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rest.
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XXX The magic \method{visit()} method for visitors.
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\begin{funcdesc}{walk}{tree, visitor\optional{, verbose}}
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\end{funcdesc}
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\begin{classdesc}{ASTVisitor}{}
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The \class{ASTVisitor} is responsible for walking over the tree in the
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correct order. A walk begins with a call to \method{preorder()}. For
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each node, it checks the \var{visitor} argument to \method{preorder()}
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for a method named `visitNodeType,' where NodeType is the name of the
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node's class, e.g. for a \class{While} node a \method{visitWhile()}
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would be called. If the method exists, it is called with the node as
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its first argument.
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The visitor method for a particular node type can control how child
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nodes are visited during the walk. The \class{ASTVisitor} modifies
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the visitor argument by adding a visit method to the visitor; this
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method can be used to visit a particular child node. If no visitor is
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found for a particular node type, the \method{default()} method is
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called.
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\end{classdesc}
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\class{ASTVisitor} objects have the following methods:
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XXX describe extra arguments
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\begin{methoddesc}{default}{node\optional{, \moreargs}}
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\end{methoddesc}
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\begin{methoddesc}{dispatch}{node\optional{, \moreargs}}
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\end{methoddesc}
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\begin{methoddesc}{preorder}{tree, visitor}
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\end{methoddesc}
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\section{Bytecode Generation}
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The code generator is a visitor that emits bytecodes. Each visit method
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can call the \method{emit()} method to emit a new bytecode. The basic
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code generator is specialized for modules, classes, and functions. An
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assembler converts that emitted instructions to the low-level bytecode
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format. It handles things like generator of constant lists of code
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objects and calculation of jump offsets.
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\input{compiler.ind} % Index
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\end{document}
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