What is __all__ in Python for?

Question

I’ve seen some code written in Python whose file __init__.py had a variable __all__ with a list assigned.

Example:

__all__ = ['foo', 'bar']

I’ve noticed that in Python, when things start with underline, means it’s some special functionality.

So I’d like to know what this is for __all__?

Answer 1

The __all__ should be a list of strings defining which symbols will be exported from a module when used from <module> import *.

It also serves to facilitate the reading of the code. Anyone reading the source code will easily know which members are publicly exposed to this module.

For example

Module code

__all__ = ['foo', 'bar']

baz = 5
bar = 10
def foo(): 
    return 'qualquer coisa'

In use

from module import *

print foo()
print bar

# A linha abaixo causará uma exception, porque o membro não foi exportado
print baz

^{I put the code on Github for future reference}

If the __all__ from the above example is removed, all members whose name does not start with underline will be imported.

Observing: It is important to note that the members cited in __all__ will only affect the behavior of import *. Therefore, members not named therein remain accessible by "import ". In the example, it would be possible to do from modulo import baz and use this value.