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Banner: TypeError unhashable type after adding __eq__ to a Python class because __hash__ became None

Programming Languages Bug Report

"TypeError: unhashable type" after adding eq to a Python class: hash became None

Lucas Moreau · Jul 14, 2025 · 7 min read

A small refactor added __eq__ to our Money class so amounts could be compared. Soon a cache decorator and several set operations failed with TypeError: unhashable type: 'Money', because defining __eq__ without __hash__ sets __hash__ to None. How equality and hashing must agree.

Banner: one Python list shared by many function calls, leaking items between unrelated requests

Programming Languages Bug Report

Python mutable default argument leaked checkout warnings between customers

Daniel Tran · Oct 29, 2024 · 9 min read

Customers started seeing checkout warnings that belonged to other people's orders. The cause was a list used as a default argument, created once per worker process and appended to on every request. Here is how we traced it and made it impossible to reintroduce.