How do Python modules, packages, and imports fit together?

Explains module identity, package organization, import caching, execution context, circular dependencies, and side-effect boundaries.

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TL;DR

A module is a Python source or extension unit loaded under a module name. A package organizes modules under a package namespace. Importing resolves the requested name using the configured import path, loads and executes the module if needed, stores it in sys.modules, and binds the requested name in the importing scope. Later imports of the same module name normally reuse that loaded module object rather than executing it again.

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A production-ready import model that can diagnose duplicate execution, circular imports, path-dependent behavior, and startup side effects.

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Core answer
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Overview
Imports both locate a module and execute its top-level code, so organization and startup behavior are connected.
A module is a Python source or extension unit loaded under a module name. A package organizes modules under a package namespace. Importing resolves the requested name using the configured import path, loads and executes the module if needed, stores it in sys.modules, and binds the requested name in the importing scope. Later imports of the same module name normally reuse that loaded module object rather than executing it again.
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Absolute imports make package ownership clearer, while explicit relative imports can be useful inside a package. Running a file directly can give it the name __main__, so importing the same file by its package name may create a second module identity and duplicate state. Circular imports fail when modules need names before the other module has finished initialization. Moving shared contracts, reducing top-level work, or changing dependency direction is usually better than scattering delayed imports.

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Details

Assuming import only copies text declarations without executing top-level code.
Running package files directly and depending on local path accidents.
Fixing every circular import with a hidden local import.
Opening external resources during module import.

How do Python modules, packages, and imports fit together?

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