Can someone explain __all__ in Python?

Linked to, but not explicitly mentioned here, is exactly when __all__ is used. It 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 following code in a foo.py explicitly exports the symbols bar and baz:

__all__ = ['bar', 'baz']

waz = 5
bar = 10
def baz(): return 'baz'

These symbols can then be imported like so:

from foo import *

print(bar)
print(baz)

# The following will trigger an exception, as "waz" is not exported by the module
print(waz)

If the __all__ above is commented out, this code will then execute to completion, as the default behaviour of import * is to import all symbols that do not begin with an underscore, from the given namespace.

Reference: https://docs.python.org/tutorial/modules.html#importing-from-a-package

NOTE: __all__ affects the from <module> import * behavior only. Members that are not mentioned in __all__ are still accessible from outside the module and can be imported with from <module> import <member>.

It's a list of public objects of that module, as interpreted by import *. It overrides the default of hiding everything that begins with an underscore.