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What is the difference between a module and a package in Python? How does Python's module import system avoid re-executing a module multiple times when imported from different places? What is the difference between an absolute import and a relative import within a package? What does the `if __name__ == '__main__':` idiom accomplish, and why is it important for modules? How does pyproject.toml serve as the modern standard for defining a Python package's build configuration and dependencies? What is __all__ and how does it control `from module import *` behavior? What is the purpose of a `requirements.txt` file, and how does it differ from dependency declarations in pyproject.toml? What does 'editable install' (pip install -e .) mean, and why is it useful during development? How does Python locate modules during import, and what is sys.path's role? What is a namespace package, and how does it differ from a regular package?
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What does the __init__.py file do inside a package directory?

Beginner

Answer

__init__.py marks a directory as a regular Python package and runs when the package is first imported, letting you control what's exposed at the package level (e.g., re-exporting submodule contents), set up package-wide state, or simply exist as an empty file to establish the package structure. Since Python 3.3, packages can technically work without it (namespace packages), but explicit __init__.py remains the common convention.
# mypackage/__init__.py
from .models import User
from .utils import helper

__all__ = ['User', 'helper']

# now: from mypackage import User
Real-world example A library's __init__.py re-exports key classes from internal submodules so users write `from mylib import Client` instead of the longer `from mylib.core.client import Client`.

Common follow-ups: What is __all__ and how does it affect `from package import *`?;What are namespace packages and how do they differ?

Modules & Packaging;Multiple Inheritance & MRO
All 15 Modules & Packaging questions