Can I use __init__.py to define global variables?

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_CONSTANTprint "my constant is", MY_CONSTANT

Then, import mymodule:

>>> from mypackage import mymodulemy 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.

You cannot do that. You will have to explicitely import your constants into each individual module's namespace. The best way to achieve this is to define your constants in a "config" module and import it everywhere you require it:

# mypackage/config.pyMY_CONST = 17# mypackage/main.pyfrom mypackage.config import *

You can define global variables from anywhere, but it is a really bad idea. import the __builtin__ module and modify or add attributes to this modules, and suddenly you have new builtin constants or functions. In fact, when my application installs gettext, I get the _() function in all my modules, without importing anything. So this is possible, but of course only for Application-type projects, not for reusable packages or modules.

And I guess no one would recommend this practice anyway. What's wrong with a namespace? Said application has the version module, so that I have "global" variables available like version.VERSION, version.PACKAGE_NAME etc.