import ast

s = 'from __future__ import division'

# This works fine.
a = ast.parse(s)
c = compile(a, '<ast1>', 'exec')

assert isinstance(a.body[0], ast.ImportFrom)
assert a.body[0].module == '__future__'
assert a.body[0].module is '__future__' # Both are the interned string.

# Now we get the string '__future__' that's not interned.
x = 'x' * 1024 + '__future__'
y = x[-10:]
assert y == '__future__'
assert y is not '__future__'    # Not the interned string.

# Inject this into the AST and watch it fail.
a.body[0].module = y
assert a.body[0].module == '__future__'
c = compile(a, '<ast1>', 'exec')    # This raises SyntaxError but should not