Can someone explain __all__ in Python?

Question

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 d

Answer 1

__all__ is used to document the public API of a Python module. Although it is optional, __all__ should be used.

Here is the relevant excerpt from the Python language reference:

The public names defined by a module are determined by checking the module’s namespace for a variable named __all__; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in __all__ are all considered public and are required to exist. If __all__ is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character ('_'). __all__ should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).

PEP 8 uses similar wording, although it also makes it clear that imported names are not part of the public API when __all__ is absent:

To better support introspection, modules should explicitly declare the names in their public API using the __all__ attribute. Setting __all__ to an empty list indicates that the module has no public API.

[...]

Imported names should always be considered an implementation detail. Other modules must not rely on indirect access to such imported names unless they are an explicitly documented part of the containing module's API, such as os.path or a package's __init__ module that exposes functionality from submodules.

Furthermore, as pointed out in other answers, __all__ is used to enable wildcard importing for packages:

The import statement uses the following convention: if a package’s __init__.py code defines a list named __all__, it is taken to be the list of module names that should be imported when from package import * is encountered.

Answer 2

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.

Answer 3

__all__ affects how from foo import * works.

Code that is inside a module body (but not in the body of a function or class) may use an asterisk (*) in a from statement:

from foo import *

The * requests that all attributes of module foo (except those beginning with underscores) be bound as global variables in the importing module. When foo has an attribute __all__, the attribute's value is the list of the names that are bound by this type of from statement.

If foo is a package and its __init__.py defines a list named __all__, it is taken to be the list of submodule names that should be imported when from foo import * is encountered. If __all__ is not defined, the statement from foo import * imports whatever names are defined in the package. This includes any names defined (and submodules explicitly loaded) by __init__.py.

Note that __all__ doesn't have to be a list. As per the documentation on the import statement, if defined, __all__ must be a sequence of strings which are names defined or imported by the module. So you may as well use a tuple to save some memory and CPU cycles. Just don't forget a comma in case the module defines a single public name:

__all__ = ('some_name',)

See also Why is “import *” bad?

Answer 4

It also changes what pydoc will show:

module1.py

a = "A"
b = "B"
c = "C"

module2.py

__all__ = ['a', 'b']

a = "A"
b = "B"
c = "C"

$ pydoc module1

Help on module module1:

NAME
    module1

FILE
    module1.py

DATA
    a = 'A'
    b = 'B'
    c = 'C'

$ pydoc module2

Help on module module2:

NAME
    module2

FILE
    module2.py

DATA
    __all__ = ['a', 'b']
    a = 'A'
    b = 'B'

I declare __all__ in all my modules, as well as underscore internal details, these really help when using things you've never used before in live interpreter sessions.

Answer 5

From (An Unofficial) Python Reference Wiki:

The public names defined by a module are determined by checking the module's namespace for a variable named __all__; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in __all__ are all considered public and are required to exist. If __all__ is not defined, the set of public names includes all names found in the module's namespace which do not begin with an underscore character ("_"). __all__ should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).