In Python, __init__.py is the package’s initialization module. A directory containing it is recognized as a regular package, and the file can be empty. If you are asking what is __init__.py, it is the place where a package can define setup code, metadata, and convenient package-level names.

What does __init__.py do? Python runs its top-level code when the package is imported. It also lets a package expose a smaller, clearer public API instead of making users import every name from an internal module.

How Python __init__.py Defines Package Behavior

A regular package is a directory that normally contains __init__.py. The file tells Python to treat that directory as one package and gives the package a module body that can configure its behavior. It is unrelated to a class’s __init__ method, which initializes an object instance.

For example, this small package tree contains a pricing module and a nested web package:

• shop/
  • __init__.py
  • pricing.py
  • web/
    • __init__.py
    • routes.py

With this structure, users can write import shop, from shop import pricing, or from shop.web.routes import home, assuming the corresponding names exist. An empty initializer is enough when the package only needs this structure and does not need package-level setup.

What Is __init__.py? Package Discovery and Import Timing

When Python evaluates import shop.web.routes, it loads shop/__init__.py first, then shop/web/__init__.py, and finally routes.py. Each module’s top-level statements execute as that module is loaded. Python normally caches the resulting modules in sys.modules, so a second import in the same process does not repeat ordinary initialization.

This timing makes imports inside an initializer useful for setup and composition. For example, shop/__init__.py might contain from .pricing import TaxRule. Importing shop then loads pricing.py and binds TaxRule on the package object. Code elsewhere can use from shop import TaxRule without knowing where the class is implemented.

Initialization code also runs when a user imports a submodule, because Python must initialize each parent package first. An exception raised in __init__.py causes the package import to fail. Importing the package alone does not automatically execute every other module in its directory; those modules run only when imported directly or by the initializer.

What Does __init__.py Do for Re-Exports and __all__?

A re-export imports a name from an internal module and makes it available through the package’s top-level namespace. A concise initializer might contain:

from .pricing import TaxRule
from .pricing import calculate_total

Users can then write from shop import TaxRule, calculate_total. This creates a stable public entry point even if the implementation later moves from pricing.py to another module. Re-export only names that belong to the supported API; importing too much can increase startup time and create circular-import problems.

The special variable __all__ declares names for wildcard imports such as from shop import *. For the example above, an initializer could define __all__ = [“TaxRule”, “calculate_total”]. The listed names should be available in the package namespace, usually through imports in the same file.

__all__ does not prevent explicit imports and is not a security boundary. It documents the intended wildcard surface and controls which names that form imports. An initializer can also define metadata such as __version__, package-level constants, or a small configuration value.

When Is an Empty __init__.py Enough?

Leave the file empty when the package needs only a regular package boundary and direct submodule imports. This choice avoids unnecessary work during every package import and reduces the risk of circular imports or surprising side effects. Add code only when package-level re-exports, metadata, registration, or lightweight setup provides a clear benefit.

Not every Python package needs an __init__.py file. Since Python 3.3, namespace packages can be formed from directories without an initializer. Their portions can be distributed across multiple directories or installed distributions and combined under one package name. Because there is no initializer, there is also no package-specific file in which to run initialization code or define re-exports.

Use a regular package when you want explicit initialization and a controlled top-level API. Use an empty initializer when that boundary is useful but no setup is needed. Avoid network calls, expensive I/O, and unrelated application work in __init__.py; imports should remain predictable and lightweight.