Python "StopIteration" inside a generator in CI

Calling next(it) on an exhausted iterator raises StopIteration. Inside a generator, PEP 479 converts an escaping StopIteration into a RuntimeError: generator raised StopIteration, which surfaces as a confusing failure.

What this error means

A test fails with "RuntimeError: generator raised StopIteration" or a bare "StopIteration" from a next() call inside generator or comprehension code.

RuntimeError: generator raised StopIteration

Common causes

next() called without a default on an empty iterator

When the underlying iterator is exhausted, next(it) raises StopIteration; inside a generator this becomes a RuntimeError.

Assuming an input always has at least one element

CI data happened to be empty where local data was not, so the first next() failed.

How to fix it

Pass a default to next() or catch the exhaustion

1. Use next(it, default) so exhaustion returns a sentinel instead of raising.
2. Where a missing element is a real error, catch StopIteration and raise a clear domain error.
3. Avoid raising StopIteration manually inside generators; use return instead.

first = next(iter(values), None)
if first is None:
    return  # nothing to do

How to prevent it

• Always supply a default to next() unless exhaustion is impossible.
• Use return (not raise StopIteration) to end a generator.
• Test generators against empty inputs.

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