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. 10 11 .. sectionauthor:: Jeremy Hylton <jeremy (a] 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 generate 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:`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("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. 644 645
