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A local *unexecuted* import appears to be changing the namespace. Attached files are ready to run.
# over.py
SOMETHING = "overridden"
# main.py
OVERRIDE = False
SOMETHING = "original"
def main():
 #global SOMETHING # uncomment and it works
 if OVERRIDE:
 from over import SOMETHING # comment out and it works
 pass
 print SOMETHING # UnboundLocalError: local variable 'SOMETHING' referenced before assignment
The SOMETHING variable has a value from the module global namespace, but it gets lost due to an import that is never executed.
I would think an unexecuted statement shouldn't have any effect on anything.
The second file will have to be submitted in a follow-on, it appears

Add'l over.py file

Not a bug. Basically, import is an explicit assignment statement.

Even an *unexecuted* import assignment statement?

hippmr: the problem is that by importing SOMETHING inside that function you're creating a *local variable* called SOMETHING. If the override isn't executed, and SOMETHING isn't global, then that local variable doesn't exist - which is why you get that error.
So even if the import isn't executed, its existence in the function tells Python that name is local to the function.

>>> OVERRIDE = False
>>> SOMETHING = "original"
>>> 
>>> def main():
... if OVERRIDE:
... SOMETHING = None
... print SOMETHING
... 
>>> main()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 4, in main
UnboundLocalError: local variable 'SOMETHING' referenced before assignment
http://docs.python.org/faq/programming.html#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value