Pablo Galindo Salgado <[email protected]> added the comment:
I don't think there is anything technically wrong here. The Seq class is
relying on the legacy version of the iterator protocol, which specifies that
you need to raise IndexError there to stop the iteration.
For instance, this version works:
import collections.abc
class Seq(collections.abc.Sequence):
def __getitem__(self, i):
if i >= len(self):
raise IndexError
return i
def __len__(self):
return 42
match Seq():
case [x, *w, y]:
z = 0
print(z)
Also, one could argue that Seq has exactly the same problem all over the
language. For instance
>> list(Seq())
will hang
>> [x,*y] = Seq()
will hang
and so on and so forth. So, if I am a user and I am trying to **predict** how
this would behave based on these two experiments my conclusion would be:
"Anything that tries to iterate on this thing will hang"
I think that whatever we do, we should make it *predictable* and consistency
across the language, and IMHO only fixing it in pattern matching doesn't make
it predictable. Being said that, I don't think these use cases justifies a
major overhaul of how this behaves everywhere.
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<https://bugs.python.org/issue44741>
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