Monthly Archives: March 2012

Something Fishy with Python ASTs

Posted on Monday 19 March 2012 by J. D. Bartlett

$ python
Python 2.7.1+ (r271:86832, Apr 11 2011, 18:05:24) 
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import ast
>>> m = ast.parse('from __future__ import division')
>>> m
<_ast.Module object at 0xb764418c>
>>> compile(m, '<string>', 'exec')
<code object <module> at 0xb7636e78, file "<string>", line 1>

$ python

Python 2.7.1+ (r271:86832, Apr 11 2011, 18:05:24)

[GCC 4.5.2] on linux2

Type "help", "copyright", "credits" or "license" for more information.

>>> import ast

>>> m = ast.parse('from __future__ import division')

>>> m

<_ast.Module object at 0xb764418c>

>>> compile(m, '<string>', 'exec')

Ok, so far so good. Now comes the interesting bit.

>>> import pickle
>>> m2 = pickle.loads(pickle.dumps(m))
>>> m2
<_ast.Module object at 0xb7651c0c>
>>> compile(m2, '<string>', 'exec')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
SyntaxError: from __future__ imports must occur at the beginning of the file

>>> import pickle

>>> m2 = pickle.loads(pickle.dumps(m))

>>> m2

<_ast.Module object at 0xb7651c0c>

>>> compile(m2, '<string>', 'exec')

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

File "<string>", line 1

SyntaxError: from __future__ imports must occur at the beginning of the file

When you parse an AST with a __future__ import, it looks as if it stores some hidden flags that suppress this error. These flags, if they exist, are … Continue reading →

Posted in short | Tagged mystery, parsing, python | 12 Comments

Monthly Archives: March 2012

Something Fishy with Python ASTs

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