PEP 479 (https://www.python.org/dev/peps/pep-0479/) changed the rules
around generators: if it would have leaked StopIteration, it instead
raises RuntimeError. This converts hard-to-debug premature termination
into easily-spotted exceptions, but it applies only to actual
generator functions.
Implementing a map-like function as a generator means you're safe:
def poof(x):
if x == 5: raise StopIteration # Leak!
return x * 3
def mymap(func, it):
for value in it:
yield func(value)
for n in mymap(poof, range(10)):
print(n)
Boom! RuntimeError that highlights the source of the StopIteration.
Similarly, using a list comprehension is safe:
[poof(n) for n in range(10)] # raises StopIteration
But using the built-in map() will silently terminate:
for n in map(poof, range(10)):
print(n)
I propose to grant PEP 479 semantics - namely, that a StopIteration
during the calling of the mapped function be translated into a
RuntimeError. Likewise for filter(), guarding the predicate function,
and all similar functions in itertools: accumulate, filterfalse,
takewhile/dropwhile, starmap, and any that I didn't notice.
ChrisA
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