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?

Answer 1

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.

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