This is my first post to this list, apologies in advance if this is the wrong
place or it's a known topic that my search failed to find.
As a Python user since 2.6, I make stupid mistakes with exhausted generators
more often than I'd like to admit:
numbers = (i for i in range(5))
assert 5 not in numbers
sorted(numbers)
# []
Then the empty list creates hard-to-track bugs. I'm familiar with the iterator
protocol and why the behavior above happens, but couldn't it be prevented? A
naive example of the behavior I wish it had:
class SafeGenerator:
def __init__(self, generator):
self.generator = generator
self.is_exhausted = False
def __iter__(self):
if self.is_exhausted:
raise ValueError("can't iterate over an already exhausted
generator")
return self
def __next__(self):
try:
return next(self.generator)
except StopIteration:
self.is_exhausted = True
raise
safe_generator = SafeGenerator(i for i in range(5))
assert 5 not in safe_generator
sorted(safe_generator)
# ValueError: can't iterate over an already exhausted generator
Note that the error is raised on `__iter__`, not `__next__`, so idioms like
`zip(generator, generator)` would still work. I can't imagine any sane code
that would break under this change, but even if there is, what's the downside
of at least emitting a warning when calling `__iter__` on an exhausted
generator?
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