What is __all__ for in Python?

Question:

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

Example:

__all__ = ['foo', 'bar']

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

So I would like to know what this __all__ is for?

Answer:

__all__ must be a list of strings that define which symbols will be exported from a module when using from <module> import * .

It also makes it easier to read the code. Anyone reading the source code will easily know what the publicly exposed members of this module are.

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've put the code on GitHub for future reference}

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

Note : It is important to note that the members quoted in __all__ will only affect the behavior of import * . Therefore, the members not mentioned in it are still accessible by "direct import ". In the example, it would be possible to do from modulo import baz and use this value.