Doc/library/compiler.rst

   1
   2 .. _compiler:
   3
   4 ***********************
   5 Python 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
  14 The Python compiler package is a tool for analyzing Python source code and
  15 generating Python bytecode.  The compiler contains libraries to generate an
  16 abstract syntax tree from Python source code and to generate Python
  17 :term:`bytecode` from the tree.
  18
  19 The :mod:`compiler` package is a Python source to bytecode translator written in
  20 Python.  It uses the built-in parser and standard :mod:`parser` module to
  21 generated a concrete syntax tree.  This tree is used to generate an abstract
  22 syntax tree (AST) and then Python bytecode.
  23
  24 The full functionality of the package duplicates the built-in compiler provided
  25 with the Python interpreter.  It is intended to match its behavior almost
  26 exactly.  Why implement another compiler that does the same thing?  The package
  27 is useful for a variety of purposes.  It can be modified more easily than the
  28 built-in compiler.  The AST it generates is useful for analyzing Python source
  29 code.
  30
  31 This chapter explains how the various components of the :mod:`compiler` package
  32 work.  It blends reference material with a tutorial.
  33
  34
  35 The basic interface
  36 ===================
  37
  38 .. module:: compiler
  39    :synopsis: Python code compiler written in Python.
  40    :deprecated:
  41
  42
  43 The top-level of the package defines four functions.  If you import
  44 :mod:`compiler`, you will get these functions and a collection of modules
  45 contained 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
  86 The :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
  91 Limitations
  92 ===========
  93
  94 There are some problems with the error checking of the compiler package.  The
  95 interpreter detects syntax errors in two distinct phases.  One set of errors is
  96 detected by the interpreter's parser, the other set by the compiler.  The
  97 compiler package relies on the interpreter's parser, so it get the first phases
  98 of error checking for free.  It implements the second phase itself, and that
  99 implementation is incomplete.  For example, the compiler package does not raise
 100 an error if a name appears more than once in an argument list:  ``def f(x, x):
 101 ...``
 102
 103 A future version of the compiler should fix these problems.
 104
 105
 106 Python Abstract Syntax
 107 ======================
 108
 109 The :mod:`compiler.ast` module defines an abstract syntax for Python.  In the
 110 abstract syntax tree, each node represents a syntactic construct.  The root of
 111 the tree is :class:`Module` object.
 112
 113 The abstract syntax offers a higher level interface to parsed Python source
 114 code.  The :mod:`parser` module and the compiler written in C for the Python
 115 interpreter use a concrete syntax tree.  The concrete syntax is tied closely to
 116 the grammar description used for the Python parser.  Instead of a single node
 117 for a construct, there are often several levels of nested nodes that are
 118 introduced by Python's precedence rules.
 119
 120 The abstract syntax tree is created by the :mod:`compiler.transformer` module.
 121 The transformer relies on the built-in Python parser to generate a concrete
 122 syntax tree.  It generates an abstract syntax tree from the concrete tree.
 123
 124 .. index::
 125    single: Stein, Greg
 126    single: Tutt, Bill
 127
 128 The :mod:`transformer` module was created by Greg Stein and Bill Tutt for an
 129 experimental Python-to-C compiler.  The current version contains a number of
 130 modifications and improvements, but the basic form of the abstract syntax and of
 131 the transformer are due to Stein and Tutt.
 132
 133
 134 AST Nodes
 135 ---------
 136
 137 .. module:: compiler.ast
 138
 139
 140 The :mod:`compiler.ast` module is generated from a text file that describes each
 141 node type and its elements.  Each node type is represented as a class that
 142 inherits from the abstract base class :class:`compiler.ast.Node` and defines a
 143 set 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
 179 Two 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
 185 The :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
 187 word, it can't be used as an attribute name.  An underscore is appended to the
 188 word to make it a legal identifier, hence :attr:`else_` instead of
 189 :keyword:`else`.)
 190
 191 The :keyword:`if` statement is more complicated because it can include several
 192 tests.   ::
 193
 194    if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]
 195
 196 The :class:`If` node only defines two attributes: :attr:`tests` and
 197 :attr:`else_`.  The :attr:`tests` attribute is a sequence of test expression,
 198 consequent body pairs.  There is one pair for each :keyword:`if`/:keyword:`elif`
 199 clause.  The first element of the pair is the test expression.  The second
 200 elements is a :class:`Stmt` node that contains the code to execute if the test
 201 is true.
 202
 203 The :meth:`getChildren` method of :class:`If` returns a flat list of child
 204 nodes.  If there are three :keyword:`if`/:keyword:`elif` clauses and no
 205 :keyword:`else` clause, then :meth:`getChildren` will return a list of six
 206 elements: the first test expression, the first :class:`Stmt`, the second text
 207 expression, etc.
 208
 209 The following table lists each of the :class:`Node` subclasses defined in
 210 :mod:`compiler.ast` and each of the public attributes available on their
 211 instances.  The values of most of the attributes are themselves :class:`Node`
 212 instances or sequences of instances.  When the value is something other than an
 213 instance, the type is noted in the comment.  The attributes are listed in the
 214 order 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
 508 Assignment nodes
 509 ----------------
 510
 511 There is a collection of nodes used to represent assignments.  Each assignment
 512 statement in the source code becomes a single :class:`Assign` node in the AST.
 513 The :attr:`nodes` attribute is a list that contains a node for each assignment
 514 target.  This is necessary because assignment can be chained, e.g. ``a = b =
 515 2``. Each :class:`Node` in the list will be one of the following classes:
 516 :class:`AssAttr`, :class:`AssList`, :class:`AssName`, or :class:`AssTuple`.
 517
 518 Each 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
 520 attribute assigned, e.g. ``a.x = 1``. :class:`AssList` and :class:`AssTuple` for
 521 list and tuple expansion respectively, e.g. ``a, b, c = a_tuple``.
 522
 523 The target assignment nodes also have a :attr:`flags` attribute that indicates
 524 whether 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
 526 x`.
 527
 528 When an expression contains several attribute references, an assignment or
 529 delete statement will contain only one :class:`AssAttr` node -- for the final
 530 attribute reference.  The other attribute references will be represented as
 531 :class:`Getattr` nodes in the :attr:`expr` attribute of the :class:`AssAttr`
 532 instance.
 533
 534
 535 Examples
 536 --------
 537
 538 This section shows several simple examples of ASTs for Python source code.  The
 539 examples demonstrate how to use the :func:`parse` function, what the repr of an
 540 AST looks like, and how to access attributes of an AST node.
 541
 542 The 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
 554 In the interactive interpreter session below, I have reformatted the long AST
 555 reprs for readability.  The AST reprs use unqualified class names.  If you want
 556 to 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
 587 Using Visitors to Walk ASTs
 588 ===========================
 589
 590 .. module:: compiler.visitor
 591
 592
 593 The visitor pattern is ...  The :mod:`compiler` package uses a variant on the
 594 visitor pattern that takes advantage of Python's introspection features to
 595 eliminate the need for much of the visitor's infrastructure.
 596
 597 The classes being visited do not need to be programmed to accept visitors.  The
 598 visitor need only define visit methods for classes it is specifically interested
 599 in; a default visit method can handle the rest.
 600
 601 XXX 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
 636 Bytecode Generation
 637 ===================
 638
 639 The code generator is a visitor that emits bytecodes.  Each visit method can
 640 call the :meth:`emit` method to emit a new bytecode.  The basic code generator
 641 is specialized for modules, classes, and functions.  An assembler converts that
 642 emitted instructions to the low-level bytecode format.  It handles things like
 643 generation of constant lists of code objects and calculation of jump offsets.
 644