python - Can I use __init__.py to define global variables?

Question

I want to define a constant that should be available in all of the submodules of a package. I've thought that the best place would be in in the __init__.py file of the root package. But I don't know how to do this. Suppose I have a few subpackages and each with several modules. How can I access that variable from these modules?

Of course, if this is totally wrong, and there is a better alternative, I'd like to know it.

Answer 1

You should be able to put them in __init__.py. This is done all the time.

mypackage/__init__.py:

MY_CONSTANT = 42

mypackage/mymodule.py:

from mypackage import MY_CONSTANT
print "my constant is", MY_CONSTANT

Then, import mymodule:

>>> from mypackage import mymodule
my constant is 42

Still, if you do have constants, it would be reasonable (best practices, probably) to put them in a separate module (constants.py, config.py, ...) and then if you want them in the package namespace, import them.

mypackage/__init__.py:

from mypackage.constants import *

Still, this doesn't automatically include the constants in the namespaces of the package modules. Each of the modules in the package will still have to import constants explicitly either from mypackage or from mypackage.constants.