/Doc/library/compiler.rst

  1
  2.. _compiler:
  3
  4***********************
  5Python compiler package
  6***********************
  7
  8.. deprecated:: 2.6
  9   The :mod:`compiler` package has been removed in Python 3.0.
 10
 11.. sectionauthor:: Jeremy Hylton <jeremy@zope.com>
 12
 13
 14The Python compiler package is a tool for analyzing Python source code and
 15generating Python bytecode.  The compiler contains libraries to generate an
 16abstract syntax tree from Python source code and to generate Python
 17:term:`bytecode` from the tree.
 18
 19The :mod:`compiler` package is a Python source to bytecode translator written in
 20Python.  It uses the built-in parser and standard :mod:`parser` module to
 21generated a concrete syntax tree.  This tree is used to generate an abstract
 22syntax tree (AST) and then Python bytecode.
 23
 24The full functionality of the package duplicates the builtin compiler provided
 25with the Python interpreter.  It is intended to match its behavior almost
 26exactly.  Why implement another compiler that does the same thing?  The package
 27is useful for a variety of purposes.  It can be modified more easily than the
 28builtin compiler.  The AST it generates is useful for analyzing Python source
 29code.
 30
 31This chapter explains how the various components of the :mod:`compiler` package
 32work.  It blends reference material with a tutorial.
 33
 34
 35The basic interface
 36===================
 37
 38.. module:: compiler
 39   :synopsis: Python code compiler written in Python.
 40   :deprecated:
 41
 42
 43The top-level of the package defines four functions.  If you import
 44:mod:`compiler`, you will get these functions and a collection of modules
 45contained in the package.
 46
 47
 48.. function:: parse(buf)
 49
 50   Returns an abstract syntax tree for the Python source code in *buf*. The
 51   function raises :exc:`SyntaxError` if there is an error in the source code.  The
 52   return value is a :class:`compiler.ast.Module` instance that contains the tree.
 53
 54
 55.. function:: parseFile(path)
 56
 57   Return an abstract syntax tree for the Python source code in the file specified
 58   by *path*.  It is equivalent to ``parse(open(path).read())``.
 59
 60
 61.. function:: walk(ast, visitor[, verbose])
 62
 63   Do a pre-order walk over the abstract syntax tree *ast*.  Call the appropriate
 64   method on the *visitor* instance for each node encountered.
 65
 66
 67.. function:: compile(source, filename, mode, flags=None,  dont_inherit=None)
 68
 69   Compile the string *source*, a Python module, statement or expression, into a
 70   code object that can be executed by the exec statement or :func:`eval`. This
 71   function is a replacement for the built-in :func:`compile` function.
 72
 73   The *filename* will be used for run-time error messages.
 74
 75   The *mode* must be 'exec' to compile a module, 'single' to compile a single
 76   (interactive) statement, or 'eval' to compile an expression.
 77
 78   The *flags* and *dont_inherit* arguments affect future-related statements, but
 79   are not supported yet.
 80
 81
 82.. function:: compileFile(source)
 83
 84   Compiles the file *source* and generates a .pyc file.
 85
 86The :mod:`compiler` package contains the following modules: :mod:`ast`,
 87:mod:`consts`, :mod:`future`, :mod:`misc`, :mod:`pyassem`, :mod:`pycodegen`,
 88:mod:`symbols`, :mod:`transformer`, and :mod:`visitor`.
 89
 90
 91Limitations
 92===========
 93
 94There are some problems with the error checking of the compiler package.  The
 95interpreter detects syntax errors in two distinct phases.  One set of errors is
 96detected by the interpreter's parser, the other set by the compiler.  The
 97compiler package relies on the interpreter's parser, so it get the first phases
 98of error checking for free.  It implements the second phase itself, and that
 99implementation is incomplete.  For example, the compiler package does not raise
100an error if a name appears more than once in an argument list:  ``def f(x, x):
101...``
102
103A future version of the compiler should fix these problems.
104
105
106Python Abstract Syntax
107======================
108
109The :mod:`compiler.ast` module defines an abstract syntax for Python.  In the
110abstract syntax tree, each node represents a syntactic construct.  The root of
111the tree is :class:`Module` object.
112
113The abstract syntax offers a higher level interface to parsed Python source
114code.  The :mod:`parser` module and the compiler written in C for the Python
115interpreter use a concrete syntax tree.  The concrete syntax is tied closely to
116the grammar description used for the Python parser.  Instead of a single node
117for a construct, there are often several levels of nested nodes that are
118introduced by Python's precedence rules.
119
120The abstract syntax tree is created by the :mod:`compiler.transformer` module.
121The transformer relies on the builtin Python parser to generate a concrete
122syntax tree.  It generates an abstract syntax tree from the concrete tree.
123
124.. index::
125   single: Stein, Greg
126   single: Tutt, Bill
127
128The :mod:`transformer` module was created by Greg Stein and Bill Tutt for an
129experimental Python-to-C compiler.  The current version contains a number of
130modifications and improvements, but the basic form of the abstract syntax and of
131the transformer are due to Stein and Tutt.
132
133
134AST Nodes
135---------
136
137.. module:: compiler.ast
138
139
140The :mod:`compiler.ast` module is generated from a text file that describes each
141node type and its elements.  Each node type is represented as a class that
142inherits from the abstract base class :class:`compiler.ast.Node` and defines a
143set of named attributes for child nodes.
144
145
146.. class:: Node()
147
148   The :class:`Node` instances are created automatically by the parser generator.
149   The recommended interface for specific :class:`Node` instances is to use the
150   public attributes to access child nodes.  A public attribute may be bound to a
151   single node or to a sequence of nodes, depending on the :class:`Node` type.  For
152   example, the :attr:`bases` attribute of the :class:`Class` node, is bound to a
153   list of base class nodes, and the :attr:`doc` attribute is bound to a single
154   node.
155
156   Each :class:`Node` instance has a :attr:`lineno` attribute which may be
157   ``None``.  XXX Not sure what the rules are for which nodes will have a useful
158   lineno.
159
160   All :class:`Node` objects offer the following methods:
161
162
163   .. method:: getChildren()
164
165      Returns a flattened list of the child nodes and objects in the order they
166      occur.  Specifically, the order of the nodes is the order in which they
167      appear in the Python grammar.  Not all of the children are :class:`Node`
168      instances.  The names of functions and classes, for example, are plain
169      strings.
170
171
172   .. method:: getChildNodes()
173
174      Returns a flattened list of the child nodes in the order they occur.  This
175      method is like :meth:`getChildren`, except that it only returns those
176      children that are :class:`Node` instances.
177
178
179Two examples illustrate the general structure of :class:`Node` classes.  The
180:keyword:`while` statement is defined by the following grammar production::
181
182   while_stmt:     "while" expression ":" suite
183                  ["else" ":" suite]
184
185The :class:`While` node has three attributes: :attr:`test`, :attr:`body`, and
186:attr:`else_`.  (If the natural name for an attribute is also a Python reserved
187word, it can't be used as an attribute name.  An underscore is appended to the
188word to make it a legal identifier, hence :attr:`else_` instead of
189:keyword:`else`.)
190
191The :keyword:`if` statement is more complicated because it can include several
192tests.   ::
193
194   if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]
195
196The :class:`If` node only defines two attributes: :attr:`tests` and
197:attr:`else_`.  The :attr:`tests` attribute is a sequence of test expression,
198consequent body pairs.  There is one pair for each :keyword:`if`/:keyword:`elif`
199clause.  The first element of the pair is the test expression.  The second
200elements is a :class:`Stmt` node that contains the code to execute if the test
201is true.
202
203The :meth:`getChildren` method of :class:`If` returns a flat list of child
204nodes.  If there are three :keyword:`if`/:keyword:`elif` clauses and no
205:keyword:`else` clause, then :meth:`getChildren` will return a list of six
206elements: the first test expression, the first :class:`Stmt`, the second text
207expression, etc.
208
209The following table lists each of the :class:`Node` subclasses defined in
210:mod:`compiler.ast` and each of the public attributes available on their
211instances.  The values of most of the attributes are themselves :class:`Node`
212instances or sequences of instances.  When the value is something other than an
213instance, the type is noted in the comment.  The attributes are listed in the
214order in which they are returned by :meth:`getChildren` and
215:meth:`getChildNodes`.
216
217+-----------------------+--------------------+---------------------------------+
218| Node type             | Attribute          | Value                           |
219+=======================+====================+=================================+
220| :class:`Add`          | :attr:`left`       | left operand                    |
221+-----------------------+--------------------+---------------------------------+
222|                       | :attr:`right`      | right operand                   |
223+-----------------------+--------------------+---------------------------------+
224| :class:`And`          | :attr:`nodes`      | list of operands                |
225+-----------------------+--------------------+---------------------------------+
226| :class:`AssAttr`      |                    | *attribute as target of         |
227|                       |                    | assignment*                     |
228+-----------------------+--------------------+---------------------------------+
229|                       | :attr:`expr`       | expression on the left-hand     |
230|                       |                    | side of the dot                 |
231+-----------------------+--------------------+---------------------------------+
232|                       | :attr:`attrname`   | the attribute name, a string    |
233+-----------------------+--------------------+---------------------------------+
234|                       | :attr:`flags`      | XXX                             |
235+-----------------------+--------------------+---------------------------------+
236| :class:`AssList`      | :attr:`nodes`      | list of list elements being     |
237|                       |                    | assigned to                     |
238+-----------------------+--------------------+---------------------------------+
239| :class:`AssName`      | :attr:`name`       | name being assigned to          |
240+-----------------------+--------------------+---------------------------------+
241|                       | :attr:`flags`      | XXX                             |
242+-----------------------+--------------------+---------------------------------+
243| :class:`AssTuple`     | :attr:`nodes`      | list of tuple elements being    |
244|                       |                    | assigned to                     |
245+-----------------------+--------------------+---------------------------------+
246| :class:`Assert`       | :attr:`test`       | the expression to be tested     |
247+-----------------------+--------------------+---------------------------------+
248|                       | :attr:`fail`       | the value of the                |
249|                       |                    | :exc:`AssertionError`           |
250+-----------------------+--------------------+---------------------------------+
251| :class:`Assign`       | :attr:`nodes`      | a list of assignment targets,   |
252|                       |                    | one per equal sign              |
253+-----------------------+--------------------+---------------------------------+
254|                       | :attr:`expr`       | the value being assigned        |
255+-----------------------+--------------------+---------------------------------+
256| :class:`AugAssign`    | :attr:`node`       |                                 |
257+-----------------------+--------------------+---------------------------------+
258|                       | :attr:`op`         |                                 |
259+-----------------------+--------------------+---------------------------------+
260|                       | :attr:`expr`       |                                 |
261+-----------------------+--------------------+---------------------------------+
262| :class:`Backquote`    | :attr:`expr`       |                                 |
263+-----------------------+--------------------+---------------------------------+
264| :class:`Bitand`       | :attr:`nodes`      |                                 |
265+-----------------------+--------------------+---------------------------------+
266| :class:`Bitor`        | :attr:`nodes`      |                                 |
267+-----------------------+--------------------+---------------------------------+
268| :class:`Bitxor`       | :attr:`nodes`      |                                 |
269+-----------------------+--------------------+---------------------------------+
270| :class:`Break`        |                    |                                 |
271+-----------------------+--------------------+---------------------------------+
272| :class:`CallFunc`     | :attr:`node`       | expression for the callee       |
273+-----------------------+--------------------+---------------------------------+
274|                       | :attr:`args`       | a list of arguments             |
275+-----------------------+--------------------+---------------------------------+
276|                       | :attr:`star_args`  | the extended \*-arg value       |
277+-----------------------+--------------------+---------------------------------+
278|                       | :attr:`dstar_args` | the extended \*\*-arg value     |
279+-----------------------+--------------------+---------------------------------+
280| :class:`Class`        | :attr:`name`       | the name of the class, a string |
281+-----------------------+--------------------+---------------------------------+
282|                       | :attr:`bases`      | a list of base classes          |
283+-----------------------+--------------------+---------------------------------+
284|                       | :attr:`doc`        | doc string, a string or         |
285|                       |                    | ``None``                        |
286+-----------------------+--------------------+---------------------------------+
287|                       | :attr:`code`       | the body of the class statement |
288+-----------------------+--------------------+---------------------------------+
289| :class:`Compare`      | :attr:`expr`       |                                 |
290+-----------------------+--------------------+---------------------------------+
291|                       | :attr:`ops`        |                                 |
292+-----------------------+--------------------+---------------------------------+
293| :class:`Const`        | :attr:`value`      |                                 |
294+-----------------------+--------------------+---------------------------------+
295| :class:`Continue`     |                    |                                 |
296+-----------------------+--------------------+---------------------------------+
297| :class:`Decorators`   | :attr:`nodes`      | List of function decorator      |
298|                       |                    | expressions                     |
299+-----------------------+--------------------+---------------------------------+
300| :class:`Dict`         | :attr:`items`      |                                 |
301+-----------------------+--------------------+---------------------------------+
302| :class:`Discard`      | :attr:`expr`       |                                 |
303+-----------------------+--------------------+---------------------------------+
304| :class:`Div`          | :attr:`left`       |                                 |
305+-----------------------+--------------------+---------------------------------+
306|                       | :attr:`right`      |                                 |
307+-----------------------+--------------------+---------------------------------+
308| :class:`Ellipsis`     |                    |                                 |
309+-----------------------+--------------------+---------------------------------+
310| :class:`Expression`   | :attr:`node`       |                                 |
311+-----------------------+--------------------+---------------------------------+
312| :class:`Exec`         | :attr:`expr`       |                                 |
313+-----------------------+--------------------+---------------------------------+
314|                       | :attr:`locals`     |                                 |
315+-----------------------+--------------------+---------------------------------+
316|                       | :attr:`globals`    |                                 |
317+-----------------------+--------------------+---------------------------------+
318| :class:`FloorDiv`     | :attr:`left`       |                                 |
319+-----------------------+--------------------+---------------------------------+
320|                       | :attr:`right`      |                                 |
321+-----------------------+--------------------+---------------------------------+
322| :class:`For`          | :attr:`assign`     |                                 |
323+-----------------------+--------------------+---------------------------------+
324|                       | :attr:`list`       |                                 |
325+-----------------------+--------------------+---------------------------------+
326|                       | :attr:`body`       |                                 |
327+-----------------------+--------------------+---------------------------------+
328|                       | :attr:`else_`      |                                 |
329+-----------------------+--------------------+---------------------------------+
330| :class:`From`         | :attr:`modname`    |                                 |
331+-----------------------+--------------------+---------------------------------+
332|                       | :attr:`names`      |                                 |
333+-----------------------+--------------------+---------------------------------+
334| :class:`Function`     | :attr:`decorators` | :class:`Decorators` or ``None`` |
335+-----------------------+--------------------+---------------------------------+
336|                       | :attr:`name`       | name used in def, a string      |
337+-----------------------+--------------------+---------------------------------+
338|                       | :attr:`argnames`   | list of argument names, as      |
339|                       |                    | strings                         |
340+-----------------------+--------------------+---------------------------------+
341|                       | :attr:`defaults`   | list of default values          |
342+-----------------------+--------------------+---------------------------------+
343|                       | :attr:`flags`      | xxx                             |
344+-----------------------+--------------------+---------------------------------+
345|                       | :attr:`doc`        | doc string, a string or         |
346|                       |                    | ``None``                        |
347+-----------------------+--------------------+---------------------------------+
348|                       | :attr:`code`       | the body of the function        |
349+-----------------------+--------------------+---------------------------------+
350| :class:`GenExpr`      | :attr:`code`       |                                 |
351+-----------------------+--------------------+---------------------------------+
352| :class:`GenExprFor`   | :attr:`assign`     |                                 |
353+-----------------------+--------------------+---------------------------------+
354|                       | :attr:`iter`       |                                 |
355+-----------------------+--------------------+---------------------------------+
356|                       | :attr:`ifs`        |                                 |
357+-----------------------+--------------------+---------------------------------+
358| :class:`GenExprIf`    | :attr:`test`       |                                 |
359+-----------------------+--------------------+---------------------------------+
360| :class:`GenExprInner` | :attr:`expr`       |                                 |
361+-----------------------+--------------------+---------------------------------+
362|                       | :attr:`quals`      |                                 |
363+-----------------------+--------------------+---------------------------------+
364| :class:`Getattr`      | :attr:`expr`       |                                 |
365+-----------------------+--------------------+---------------------------------+
366|                       | :attr:`attrname`   |                                 |
367+-----------------------+--------------------+---------------------------------+
368| :class:`Global`       | :attr:`names`      |                                 |
369+-----------------------+--------------------+---------------------------------+
370| :class:`If`           | :attr:`tests`      |                                 |
371+-----------------------+--------------------+---------------------------------+
372|                       | :attr:`else_`      |                                 |
373+-----------------------+--------------------+---------------------------------+
374| :class:`Import`       | :attr:`names`      |                                 |
375+-----------------------+--------------------+---------------------------------+
376| :class:`Invert`       | :attr:`expr`       |                                 |
377+-----------------------+--------------------+---------------------------------+
378| :class:`Keyword`      | :attr:`name`       |                                 |
379+-----------------------+--------------------+---------------------------------+
380|                       | :attr:`expr`       |                                 |
381+-----------------------+--------------------+---------------------------------+
382| :class:`Lambda`       | :attr:`argnames`   |                                 |
383+-----------------------+--------------------+---------------------------------+
384|                       | :attr:`defaults`   |                                 |
385+-----------------------+--------------------+---------------------------------+
386|                       | :attr:`flags`      |                                 |
387+-----------------------+--------------------+---------------------------------+
388|                       | :attr:`code`       |                                 |
389+-----------------------+--------------------+---------------------------------+
390| :class:`LeftShift`    | :attr:`left`       |                                 |
391+-----------------------+--------------------+---------------------------------+
392|                       | :attr:`right`      |                                 |
393+-----------------------+--------------------+---------------------------------+
394| :class:`List`         | :attr:`nodes`      |                                 |
395+-----------------------+--------------------+---------------------------------+
396| :class:`ListComp`     | :attr:`expr`       |                                 |
397+-----------------------+--------------------+---------------------------------+
398|                       | :attr:`quals`      |                                 |
399+-----------------------+--------------------+---------------------------------+
400| :class:`ListCompFor`  | :attr:`assign`     |                                 |
401+-----------------------+--------------------+---------------------------------+
402|                       | :attr:`list`       |                                 |
403+-----------------------+--------------------+---------------------------------+
404|                       | :attr:`ifs`        |                                 |
405+-----------------------+--------------------+---------------------------------+
406| :class:`ListCompIf`   | :attr:`test`       |                                 |
407+-----------------------+--------------------+---------------------------------+
408| :class:`Mod`          | :attr:`left`       |                                 |
409+-----------------------+--------------------+---------------------------------+
410|                       | :attr:`right`      |                                 |
411+-----------------------+--------------------+---------------------------------+
412| :class:`Module`       | :attr:`doc`        | doc string, a string or         |
413|                       |                    | ``None``                        |
414+-----------------------+--------------------+---------------------------------+
415|                       | :attr:`node`       | body of the module, a           |
416|                       |                    | :class:`Stmt`                   |
417+-----------------------+--------------------+---------------------------------+
418| :class:`Mul`          | :attr:`left`       |                                 |
419+-----------------------+--------------------+---------------------------------+
420|                       | :attr:`right`      |                                 |
421+-----------------------+--------------------+---------------------------------+
422| :class:`Name`         | :attr:`name`       |                                 |
423+-----------------------+--------------------+---------------------------------+
424| :class:`Not`          | :attr:`expr`       |                                 |
425+-----------------------+--------------------+---------------------------------+
426| :class:`Or`           | :attr:`nodes`      |                                 |
427+-----------------------+--------------------+---------------------------------+
428| :class:`Pass`         |                    |                                 |
429+-----------------------+--------------------+---------------------------------+
430| :class:`Power`        | :attr:`left`       |                                 |
431+-----------------------+--------------------+---------------------------------+
432|                       | :attr:`right`      |                                 |
433+-----------------------+--------------------+---------------------------------+
434| :class:`Print`        | :attr:`nodes`      |                                 |
435+-----------------------+--------------------+---------------------------------+
436|                       | :attr:`dest`       |                                 |
437+-----------------------+--------------------+---------------------------------+
438| :class:`Printnl`      | :attr:`nodes`      |                                 |
439+-----------------------+--------------------+---------------------------------+
440|                       | :attr:`dest`       |                                 |
441+-----------------------+--------------------+---------------------------------+
442| :class:`Raise`        | :attr:`expr1`      |                                 |
443+-----------------------+--------------------+---------------------------------+
444|                       | :attr:`expr2`      |                                 |
445+-----------------------+--------------------+---------------------------------+
446|                       | :attr:`expr3`      |                                 |
447+-----------------------+--------------------+---------------------------------+
448| :class:`Return`       | :attr:`value`      |                                 |
449+-----------------------+--------------------+---------------------------------+
450| :class:`RightShift`   | :attr:`left`       |                                 |
451+-----------------------+--------------------+---------------------------------+
452|                       | :attr:`right`      |                                 |
453+-----------------------+--------------------+---------------------------------+
454| :class:`Slice`        | :attr:`expr`       |                                 |
455+-----------------------+--------------------+---------------------------------+
456|                       | :attr:`flags`      |                                 |
457+-----------------------+--------------------+---------------------------------+
458|                       | :attr:`lower`      |                                 |
459+-----------------------+--------------------+---------------------------------+
460|                       | :attr:`upper`      |                                 |
461+-----------------------+--------------------+---------------------------------+
462| :class:`Sliceobj`     | :attr:`nodes`      | list of statements              |
463+-----------------------+--------------------+---------------------------------+
464| :class:`Stmt`         | :attr:`nodes`      |                                 |
465+-----------------------+--------------------+---------------------------------+
466| :class:`Sub`          | :attr:`left`       |                                 |
467+-----------------------+--------------------+---------------------------------+
468|                       | :attr:`right`      |                                 |
469+-----------------------+--------------------+---------------------------------+
470| :class:`Subscript`    | :attr:`expr`       |                                 |
471+-----------------------+--------------------+---------------------------------+
472|                       | :attr:`flags`      |                                 |
473+-----------------------+--------------------+---------------------------------+
474|                       | :attr:`subs`       |                                 |
475+-----------------------+--------------------+---------------------------------+
476| :class:`TryExcept`    | :attr:`body`       |                                 |
477+-----------------------+--------------------+---------------------------------+
478|                       | :attr:`handlers`   |                                 |
479+-----------------------+--------------------+---------------------------------+
480|                       | :attr:`else_`      |                                 |
481+-----------------------+--------------------+---------------------------------+
482| :class:`TryFinally`   | :attr:`body`       |                                 |
483+-----------------------+--------------------+---------------------------------+
484|                       | :attr:`final`      |                                 |
485+-----------------------+--------------------+---------------------------------+
486| :class:`Tuple`        | :attr:`nodes`      |                                 |
487+-----------------------+--------------------+---------------------------------+
488| :class:`UnaryAdd`     | :attr:`expr`       |                                 |
489+-----------------------+--------------------+---------------------------------+
490| :class:`UnarySub`     | :attr:`expr`       |                                 |
491+-----------------------+--------------------+---------------------------------+
492| :class:`While`        | :attr:`test`       |                                 |
493+-----------------------+--------------------+---------------------------------+
494|                       | :attr:`body`       |                                 |
495+-----------------------+--------------------+---------------------------------+
496|                       | :attr:`else_`      |                                 |
497+-----------------------+--------------------+---------------------------------+
498| :class:`With`         | :attr:`expr`       |                                 |
499+-----------------------+--------------------+---------------------------------+
500|                       | :attr:`vars`       |                                 |
501+-----------------------+--------------------+---------------------------------+
502|                       | :attr:`body`       |                                 |
503+-----------------------+--------------------+---------------------------------+
504| :class:`Yield`        | :attr:`value`      |                                 |
505+-----------------------+--------------------+---------------------------------+
506
507
508Assignment nodes
509----------------
510
511There is a collection of nodes used to represent assignments.  Each assignment
512statement in the source code becomes a single :class:`Assign` node in the AST.
513The :attr:`nodes` attribute is a list that contains a node for each assignment
514target.  This is necessary because assignment can be chained, e.g. ``a = b =
5152``. Each :class:`Node` in the list will be one of the following classes:
516:class:`AssAttr`, :class:`AssList`, :class:`AssName`, or :class:`AssTuple`.
517
518Each target assignment node will describe the kind of object being assigned to:
519:class:`AssName` for a simple name, e.g. ``a = 1``. :class:`AssAttr` for an
520attribute assigned, e.g. ``a.x = 1``. :class:`AssList` and :class:`AssTuple` for
521list and tuple expansion respectively, e.g. ``a, b, c = a_tuple``.
522
523The target assignment nodes also have a :attr:`flags` attribute that indicates
524whether the node is being used for assignment or in a delete statement.  The
525:class:`AssName` is also used to represent a delete statement, e.g. :class:`del
526x`.
527
528When an expression contains several attribute references, an assignment or
529delete statement will contain only one :class:`AssAttr` node -- for the final
530attribute reference.  The other attribute references will be represented as
531:class:`Getattr` nodes in the :attr:`expr` attribute of the :class:`AssAttr`
532instance.
533
534
535Examples
536--------
537
538This section shows several simple examples of ASTs for Python source code.  The
539examples demonstrate how to use the :func:`parse` function, what the repr of an
540AST looks like, and how to access attributes of an AST node.
541
542The first module defines a single function.  Assume it is stored in
543:file:`/tmp/doublelib.py`.  ::
544
545   """This is an example module.
546
547   This is the docstring.
548   """
549
550   def double(x):
551       "Return twice the argument"
552       return x * 2
553
554In the interactive interpreter session below, I have reformatted the long AST
555reprs for readability.  The AST reprs use unqualified class names.  If you want
556to create an instance from a repr, you must import the class names from the
557:mod:`compiler.ast` module. ::
558
559   >>> import compiler
560   >>> mod = compiler.parseFile("/tmp/doublelib.py")
561   >>> mod
562   Module('This is an example module.\n\nThis is the docstring.\n',
563          Stmt([Function(None, 'double', ['x'], [], 0,
564                         'Return twice the argument',
565                         Stmt([Return(Mul((Name('x'), Const(2))))]))]))
566   >>> from compiler.ast import *
567   >>> Module('This is an example module.\n\nThis is the docstring.\n',
568   ...    Stmt([Function(None, 'double', ['x'], [], 0,
569   ...                   'Return twice the argument',
570   ...                   Stmt([Return(Mul((Name('x'), Const(2))))]))]))
571   Module('This is an example module.\n\nThis is the docstring.\n',
572          Stmt([Function(None, 'double', ['x'], [], 0,
573                         'Return twice the argument',
574                         Stmt([Return(Mul((Name('x'), Const(2))))]))]))
575   >>> mod.doc
576   'This is an example module.\n\nThis is the docstring.\n'
577   >>> for node in mod.node.nodes:
578   ...     print node
579   ...
580   Function(None, 'double', ['x'], [], 0, 'Return twice the argument',
581            Stmt([Return(Mul((Name('x'), Const(2))))]))
582   >>> func = mod.node.nodes[0]
583   >>> func.code
584   Stmt([Return(Mul((Name('x'), Const(2))))])
585
586
587Using Visitors to Walk ASTs
588===========================
589
590.. module:: compiler.visitor
591
592
593The visitor pattern is ...  The :mod:`compiler` package uses a variant on the
594visitor pattern that takes advantage of Python's introspection features to
595eliminate the need for much of the visitor's infrastructure.
596
597The classes being visited do not need to be programmed to accept visitors.  The
598visitor need only define visit methods for classes it is specifically interested
599in; a default visit method can handle the rest.
600
601XXX The magic :meth:`visit` method for visitors.
602
603
604.. function:: walk(tree, visitor[, verbose])
605
606
607.. class:: ASTVisitor()
608
609   The :class:`ASTVisitor` is responsible for walking over the tree in the correct
610   order.  A walk begins with a call to :meth:`preorder`.  For each node, it checks
611   the *visitor* argument to :meth:`preorder` for a method named 'visitNodeType,'
612   where NodeType is the name of the node's class, e.g. for a :class:`While` node a
613   :meth:`visitWhile` would be called.  If the method exists, it is called with the
614   node as its first argument.
615
616   The visitor method for a particular node type can control how child nodes are
617   visited during the walk.  The :class:`ASTVisitor` modifies the visitor argument
618   by adding a visit method to the visitor; this method can be used to visit a
619   particular child node.  If no visitor is found for a particular node type, the
620   :meth:`default` method is called.
621
622   :class:`ASTVisitor` objects have the following methods:
623
624   XXX describe extra arguments
625
626
627   .. method:: default(node[, ...])
628
629
630   .. method:: dispatch(node[, ...])
631
632
633   .. method:: preorder(tree, visitor)
634
635
636Bytecode Generation
637===================
638
639The code generator is a visitor that emits bytecodes.  Each visit method can
640call the :meth:`emit` method to emit a new bytecode.  The basic code generator
641is specialized for modules, classes, and functions.  An assembler converts that
642emitted instructions to the low-level bytecode format.  It handles things like
643generation of constant lists of code objects and calculation of jump offsets.
644