Can someone explains __all__ in Python?

Question

I have been using Python more and more, and I keep seeing the variable __all__ set in different __init__.py files. Can someone explain what this does?

Answer 1

The variable __all__ is a list of public objects of that module, as interpreted by import *. This variable overrides the default of hiding everything that begins with an underscore.

In other words, __all__  is a list of strings defining what symbols in a module will be exported when from <module> import * is used on the module.

For example, the below-mentioned code saved as foo.py explicitly exports the symbols bar and baz:

__all__ = ['bar', 'baz'] 

waz = 5 

bar = 10 

def baz():

return 'baz'

So, the two symbols bar and baz can be imported like follows:

from foo import * 

print(bar) 

print(baz)

# The following will produce an exception, as "waz" is not exported by the module 

print(waz)

So, if the above part of the code which contains __all__  is commented out, then this code will be executed to completion, as the default behavior of import * is to import all symbols that do not begin with an underscore, from the given namespace.

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