Exception groups in Python

Several failures at once, and the except* syntax that splits them.

def gather_failures():
    problems = [ValueError("bad number"), KeyError("missing"),
                ValueError("also bad")]
    raise ExceptionGroup("validation failed", problems)


try:
    gather_failures()
except* ValueError as group:
    print("values:", [str(e) for e in group.exceptions])
except* KeyError as group:
    print("keys:", [str(e) for e in group.exceptions])

try:
    gather_failures()
except ExceptionGroup as group:
    print(group.message, len(group.exceptions))

How it works

1. ExceptionGroup carries a list of exceptions.
2. except* handles the matching ones and re-raises the rest.
3. TaskGroup raises these, which is why the syntax exists.

Keywords and builtins used here

• as
• def
• except
• for
• gather_failures
• len
• print
• raise
• str
• try

The run, in numbers

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Step 4 of 4 in Async in depth, step 51 of 53 in Pythonic Python.

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