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How does Python's module import system avoid re-executing a module multiple times when imported from different places?

Intermediate

Answer

Python caches every imported module in sys.modules keyed by its fully qualified name. When any code does `import foo`, Python checks sys.modules first; if already present, it reuses the cached module object instead of re-running foo.py, ensuring module-level code executes exactly once per process and all importers share the same module object (and its state).
import sys

import json
print('json' in sys.modules)  # True after first import

# Subsequent imports reuse the cached module -- no re-execution
import json as json2
print(json is json2)  # True, same object
Real-world example A configuration module that loads settings once at import time relies on this caching -- every part of the application importing `config` gets the same already-initialized settings object, not a fresh reload.

Common follow-ups: How do you force a module to reload with importlib.reload()?;Why can module-level side effects be risky given this caching?

Modules & Packaging;Memory Management & Garbage Collection
All 15 Modules & Packaging questions