How does Python get the length with len() function?
summary
Shows that len() calls __len__ and enforces an int return, raising TypeError if __len__ returns non-int and OverflowError for absurdly large values. Demonstrates with lists, dicts and custom classes (including one that returns changing or non-int lengths) and confirms behavior by reading the CPython source and design/history FAQ and PEPs. Frames len as an adapter pattern that validates and converts object-provided length semantics, and shows the bool logic uses __bool__ or falls back to __len__ with the same checks.
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chapters (7)
- 0:00Introduction
- 2:55How len() works
- 6:36Adapter pattern and len()
- 11:21Protocols and len()
- 15:58bool() adapter
- 21:04Summary
- 25:07Q&A
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