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?
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.
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
__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)
__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.
__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>.