Python - Importing module gets global variables

Question

I have two Python scripts, one testclass.py:

import numpy
zz = numpy

class Something(object):
    def __init__(self):
        self.xp = zz

and one testscript.py:

from testclass import Something
x = Something()
print(x.xp)

I expected testscript.py to throw an error because I thought that testscript only imports the class Something (with its __init__ method), and not the global variable zz. So, given this bevahiour, my question is, when importing from a module, does Python "run" everything in the module file?

Blender · Accepted Answer · 2017-09-02 01:57:48Z

2

Yes. When you execute:

from testclass import Something

It has the same effect as:

import testclass
Something = testclass.Something

More generally, the Python interpreter can't know beforehand what objects your module exposes (unless you explicitly name them in __all__). For an extreme case, consider the following:

a.py:

import random

if random.random() > 0.5:
    class Foo(object):
        pass
else:
    class Bar(object):
        pass

Running from a import Foo has a 50% chance of failing because the a module object may or may not have a Foo attribute.

answered Sep 2, 2017 at 1:57

Blender

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3 Comments

spectras Over a year ago

I like the idea of defining random stuff in a module deep in your project. Preferably keeping the same name, but a different MRO, would make for fun times.

Zach Gates Over a year ago

Will executing from a import Foo multiple times increase/decrease the chance of failing? Or will the defined class remain the same during the process after the first import?

Blender Over a year ago

@ZachGates: The module object is cached in sys.modules after it's first imported.

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