[issue14378] __future__ imports fail when compiling from python ast

[J. D. Bartlett]

New submission from J. D. Bartlett :

GOAL
I am trying to compile an AST which contains an ImportFrom node which performs 
a __future__ import. The ImportFrom node in question is the first node within 
the AST's body (as it should be, because __future__ imports must occur at the 
beginning of modules).

ISSUE
Compiling the AST fails with the error "SyntaxError: from __future__ imports 
must occur at the beginning of the file".

ANALYSIS
The future_parse() function in future.c looks for __future__ imports and 
identifies them based on the condition (ds->v.ImportFrom.module == future) 
where future is constructed using PyString_InternFromString("__future__"). That 
is, the module attribute of the ImportFrom node is compared by identity with 
the interned string "__future__".

The AST which I was compiling used the string "__future__" after extracting 
that string from a much longer string of user input, and as a result, the 
string was not interned.

The attached file futureimport.py is a simple script demonstrating this issue. 
I have confirmed that the issue occurs in Python 2.7.2 and 3.2.2.

--
components: Interpreter Core
files: futureimport.py
messages: 156477
nosy: talljosh
priority: normal
severity: normal
status: open
title: __future__ imports fail when compiling from python ast
type: behavior
versions: Python 2.7, Python 3.2
Added file: http://bugs.python.org/file24981/futureimport.py

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[issue14378] __future__ imports fail when compiling from python ast

[J. D. Bartlett]

J. D. Bartlett  added the comment:

Incidentally, the workaround that I'm using for the time being is to run the 
following code before attempting to compile root_node.

for node in ast.walk(root_node):
if isinstance(node, ast.ImportFrom) and node.module == '__future__':
node.module = '__future__'

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