How Python's Import System Works and Why It Matters for Debugging

Short summary

A walkthrough of Python's import system covering module caching in sys.modules, execution order, and why circular imports fail. The key insight is that Python adds a module to sys.modules before executing its code, so a circular import returns a partially initialized module. The article also explains the __name__ == '__main__' pattern and includes a promotional plug for PyCodeIt at the end.

• •Python caches modules in sys.modules before executing their code
• •Circular imports fail because partially initialized modules lack expected names
• •Fix circular imports with lazy imports or import module instead of from module import name

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