<s>[INST] Can someone explain __all__ in Python? 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?
 [/INST] 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/3.5/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>.
 </s>