31.3.3 Examples

This section shows several simple examples of ASTs for Python source code. The examples demonstrate how to use the parse() function, what the repr of an AST looks like, and how to access attributes of an AST node.

The first module defines a single function. Assume it is stored in /tmp/doublelib.py.

"""This is an example module.

This is the docstring.
"""

def double(x):
    "Return twice the argument"
    return x * 2

In the interactive interpreter session below, I have reformatted the long AST reprs for readability. The AST reprs use unqualified class names. If you want to create an instance from a repr, you must import the class names from the compiler.ast module.

>>> import compiler
>>> mod = compiler.parseFile("/tmp/doublelib.py")
>>> mod
Module('This is an example module.\n\nThis is the docstring.\n', 
       Stmt([Function(None, 'double', ['x'], [], 0,
                      'Return twice the argument', 
                      Stmt([Return(Mul((Name('x'), Const(2))))]))]))
>>> from compiler.ast import *
>>> Module('This is an example module.\n\nThis is the docstring.\n', 
...    Stmt([Function(None, 'double', ['x'], [], 0,
...                   'Return twice the argument', 
...                   Stmt([Return(Mul((Name('x'), Const(2))))]))]))
Module('This is an example module.\n\nThis is the docstring.\n', 
       Stmt([Function(None, 'double', ['x'], [], 0,
                      'Return twice the argument', 
                      Stmt([Return(Mul((Name('x'), Const(2))))]))]))
>>> mod.doc
'This is an example module.\n\nThis is the docstring.\n'
>>> for node in mod.node.nodes:
...     print node
... 
Function(None, 'double', ['x'], [], 0, 'Return twice the argument',
         Stmt([Return(Mul((Name('x'), Const(2))))]))
>>> func = mod.node.nodes[0]
>>> func.code
Stmt([Return(Mul((Name('x'), Const(2))))])