Python all variable

The __all__ variable is a special variable used in Python modules to define which names should be imported when a client imports a module using the wildcard *. By default, if the __all__ variable is not defined in a module, the import * statement imports all names that are defined in the module, except for those that begin with an underscore (_). However, it's considered a best practice to define the __all__ variable in your modules to explicitly specify which names should be imported.

In this tutorial, we will learn how to use the __all__ variable in Python modules.

Creating a Python module:

Let's create a Python module named my_module.py with the following content:

# my_module.py

def public_function():
    print("This is a public function.")

def _private_function():
    print("This is a private function.")

class PublicClass:
    pass

class _PrivateClass:
    pass

CONSTANT = 42

This module contains two functions, two classes, and a constant. By convention, names that begin with an underscore (_) are considered private and should not be used by external clients.

Importing names from the module without __all__:

If the __all__ variable is not defined in the module, the import * statement imports all public names (i.e., those that do not begin with an underscore).

# main.py

from my_module import *

public_function()  # This works
_PublicClass()     # This will raise an error, as _PrivateClass is not imported

Defining the __all__ variable in the module:

To explicitly specify which names should be imported when using the import * statement, you can define the __all__ variable in your module. The __all__ variable should be a list of strings that represent the names of the objects you want to make public.

Update the my_module.py module to include the __all__ variable:

# my_module.py

__all__ = ["public_function", "PublicClass", "CONSTANT"]

def public_function():
    print("This is a public function.")

def _private_function():
    print("This is a private function.")

class PublicClass:
    pass

class _PrivateClass:
    pass

CONSTANT = 42

Now, when a client imports the module using the import * statement, only the names listed in the __all__ variable will be imported.

# main.py

from my_module import *

public_function()  # This works
CONSTANT           # This works
_PublicClass()     # This will raise an error, as _PrivateClass is not imported

In summary, the __all__ variable in Python is used to explicitly define which names should be imported when a client imports a module using the wildcard *. It's a good practice to define the __all__ variable in your modules to provide a clear API for clients and prevent the accidental import of private names.

Python __all__ variable

Python all variable