Understanding the Power of __all__ in Python Modules

When diving into the depths of Python, one might stumble upon a peculiar variable named __all__. This special variable plays a crucial role in how symbols (functions, classes, variables, etc.) are imported from modules. Understanding __all__ can significantly enhance your Python coding skills, especially when dealing with large projects or creating libraries. Let's unravel the mystery of __all__ and see how it can be utilized effectively.

What is __all__?

In Python, __all__ is a list of strings defining what symbols in a module will be exported when from module import * is used on the module. The * here means "everything." However, without __all__, Python will indeed attempt to import everything defined in the module, along with everything it can find from other modules imported in it. This can lead to an overcrowded namespace, potential conflicts, or unwanted behavior.

Why Use __all__?

Using __all__ allows the module's author to explicitly control what gets exported and what stays private. This is particularly useful in large modules where you want to offer a clean, well-defined interface to the users of your module, or when you want to avoid polluting the namespace.

How to Use __all__?

Defining __all__ is straightforward. It's just a list of strings where each string is a name of a symbol in the module.

Consider the following example:

# mymodule.py
__all__ = ['foo', 'Bar']

def foo():
    print("foo function")

def baz():
    print("baz function")

class Bar:

class Baz:

In this example, if someone uses from mymodule import *, they will only have access to foo and Bar. The function baz and the class Baz will not be imported because they are not listed in __all__.

Best Practices

  1. Explicit is better than implicit: It's generally a good idea to define __all__ in your modules to make your API clear and to control what is exported.
  2. Maintainability: Keep __all__ updated as your module grows or changes. This helps in maintaining a clean namespace.
  3. *Use with `from ... import sparingly**: Even thoughallhelps in controlling the namespace pollution, usingfrom ... import *` is still frowned upon in many coding standards. It's usually better to import only what you need.


__all__ is a powerful feature in Python for controlling what symbols are exported from a module. By using __all__, you can make your modules cleaner and more predictable, enhancing both usability and maintainability. Remember, clear and explicit definitions lead to better code, and __all__ is a tool that helps you achieve that in your Python modules.