Question

I have a question which seems to be rather fundamental but I can’t seem to find any help on this anywhere.

file_a.py >>

from xyz import XYZ
class A:
    .
    .
    .

file_b.py >>

import file_a
from file_a import A
class B(A):
    def __init__(self):
        A.__init__(self)

    def someMethod(self):
        XYZ.doSomething()

XYZ.doSomething() fails saying NameError: name ‘XYZ’ is not defined
Even standard imports like ‘import sys’ done from file_a does not seem to render it usable in file_b. I assumed that should work. Is my understanding wrong? If yes, then is there a way to have common imports and global variables across files? (If it is of nay help, I’ve been a C++ and java programmer and am now starting to use python. )

Answer 1

Is my understanding wrong?

Yes, because the line from file_a import A import only class A into the namespace of file_b. The namespace of file_a is left alone. If it were not like this, there would be little sense in having both syntax:

import modulename
from modulename import something

as if your thinking was right, then after the second form you would always be able to use modulename.someotherthing.

If yes, then is there a way to have common imports and global variables across files?

Yes, with the star * operator:

from modulename import *

but this brings the issue of namespace pollution, for example from file_a import * will import in file_b also all the imports done in file_a. You will eventually lose control of your imports and this will bite you at some time… trust me on this!

When for some reason from module import * is needed, a workaround to namespace pollution is to define in module the variable __all__, which whitelists what should be imported with the star operator.

HTH!