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Exceptions, Logging and Testing

Know exactly how try/except/else/finally behave, how exceptions chain and group, how logging decides what to emit, and how pytest and unittest.mock work so tests catch real bugs instead of hiding them.

Key points

1
except Exception misses BaseException subclasses (KeyboardInterrupt, SystemExit, GeneratorExit); a bare except: catches them all, which can swallow sys.exit() and Ctrl+C.
2
finally always runs. A return or exception inside it replaces the pending result or exception, so keep cleanup code simple and never return from it.
3
raise X from e sets __cause__ ("direct cause"), implicit raising sets __context__ ("During handling…"), and from None hides the context while still storing it.
4
except* (3.11+) can run several clauses for one ExceptionGroup, always binds a group, re-raises unmatched leaves as a new group, and forbids return/break/continue inside the block.
5
Logging: records pass the logger level, then each handler level, then propagate to ancestor handlers. basicConfig is ignored once handlers exist unless force=True. Use lazy %-style arguments.
6
pytest: fixtures with yield give setup/teardown, scopes control lifetime, parametrize multiplies, raises(match=) is a regex search, and approx compares floats.
7
unittest.mock: patch where the name is looked up, prefer spec/autospec, and remember side_effect beats return_value and raises StopIteration when an iterable runs out.

Common traps

The name bound by except ... as e is deleted when the block ends, even if e existed before.
assert statements disappear under python -O, so never use them for validation.
Patching module.attr does nothing for code that already did from module import attr; patch the importing module instead.

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