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Lesson 08 – Understanding __init__.py

Prerequisites: Complete Lesson 07 – Environment Variables and .env Files to understand how configuration data may be loaded as packages initialize.

Python uses the __init__.py file to mark a directory as a package. This file can be empty, but it's often used to control what gets imported when you use from package import * and to run package initialization code.

1. Why __init__.py Exists

Before Python 3.3, a directory without an __init__.py file could not be imported as a module. The presence of this file signaled to the interpreter that the directory should be treated as a package. Even though modern Python no longer requires it for namespace packages, adding __init__.py keeps your package structure explicit and compatible with older tooling.

2. Common Uses

  • Initializing Package State: You can execute setup logic like loading configuration data or setting package-level variables.
  • Controlling Imports: By defining the __all__ list, you specify which modules or attributes get imported when users run from package import *.
  • Aggregating Submodules: Import submodules inside __init__.py so they are available directly from the package, e.g., package.module.
# Inside mypackage/__init__.py
from .utilities import helper
__all__ = ["helper"]

3. Keeping It Lightweight

Avoid heavy computations or side effects inside __init__.py. Doing too much can slow down imports and lead to unexpected behavior. Use it for lightweight setup and leave complex logic in separate modules.

Key Takeaways

  • __init__.py makes a directory a package and can run initialization code.
  • Define __all__ to control wildcard imports and re-export submodules.
  • Keep the file simple to avoid slowing down package imports.

Next Up

Shift from backend structure to user experience in Lesson 09 – AJAX for Dynamic Pages.