BytesWarning can be raised during compilation. $ ./python -Wa -b -c "lambda: 'a'; lambda: b'a'" sys:1: BytesWarning: Comparison between bytes and string sys:1: BytesWarning: Comparison between bytes and string sys:1: BytesWarning: Comparison between bytes and string sys:1: BytesWarning: Comparison between bytes and string _PyCode_ConstantKey() should produce keys that don't allow comparing bytes with strings nested in tuples or frozensets. Currently it returns (tuple, ('a',), ((str, 'a'),)) for ('a',) and (tuple, (b'a',), ((bytes, b'a'),)) for (b'a',). Key tuples have the same size and the same first element. Comparing second elements emits a BytesWarning.