David R. MacIverWhile we're talking about weird Python edge cases, what do you think this does?
```
class Foo:
def __getitem__(self, i):
return i
def __len__(self):
return 5
print(list(Foo()))
```
The answer is so counter-intuitive to me that I have independently discovered it twice, about a year apart, and posted outraged comments about it in discord both times, and the second time had to be reminded that this was not the first time I'd discovered the behaviour.
It's possible it's also not the first time I've posted it on mastodon.
CF Bolz-Tereick@DRMacIver hmmmmm, if it were the obvious [0, 1, 2, 3, 4] you wouldn't be outraged. so let me guess, it's an infinite loop? a MemoryError?
@cfbolz Correct, yes. It will hang until it eventually runs out of memory.
Now, do you know why?
Jonathan Hartley@DRMacIver @cfbolz Wild guess: the iteration doesn't consult `__len__`, but just keeps retrieving incremental indexes waiting for one to raise an exception.
UPDATE: my language is sloppy, but you get what I'm arm-waving about.
@tartley @DRMacIver yeah, it probably waits for an IndexError?
@cfbolz @tartley Yup, exactly that. Default iteration is like:
```
i = 0
while True:
try:
yield x[i]
except IndexError:
break
i += 1
```
Instead of:
```
for i in range(len(x)):
yield x[i]
```
In some sense you should never be able to tell the difference in correct code, so you'll rarely notice this, but I've at least twice managed to write incorrect code that doesn't raise for out of bounds access
@DRMacIver @tartley I think it's a bit of a general pattern in python, that the language mostly doesn't really use more than one special method in combination (with some exceptions of course)
@DRMacIver @tartley also, re the hang: is it actually not even ctrl-c-able? that's *really* annoying?!
@cfbolz @DRMacIver oh darn I hadn't expected that. I confess I don't know what determines that. Now you've nerd sniped me into figuring that out...
David Beazley@tartley @cfbolz @DRMacIver It's probably running a tight loop in some C code somewhere outside of the main eval loop where ctrl-c checking happens.
@dabeaz @tartley @DRMacIver nope, it was actually my own fault. ctrl-c works fine in CPython, but actually not in pypy3 😬 (it's my default python, so I tried there)
@tartley @dabeaz @DRMacIver I mean, you can definitely get CPython to enter an uninterrupted loop with the approach below. you get bonus points for telling me why operator.index works as `__getitem__` despite not taking a self?! (I don't know the answer to that). and why ctrl-c doesn't work here.
Jason Madden@cfbolz @tartley @dabeaz @DRMacIver `opeartor.index` is a builtin function, which means it isn't a descriptor like a regular function. In effect, it's the same as this: ``__getitem__ = staticmethod(lambda i: i)``
@ossmkitty @tartley @dabeaz @DRMacIver yeah, exactly, thank! I was just hunting that down too! basically it's because index.__get__ does not exist.
so this works for anything that's callable with one argument but without __get__. things I just tried:
danzin@cfbolz @ossmkitty @tartley @dabeaz @DRMacIver Ohoho, those look just like the kind of weirdness that can uncover interpreter and extension bugs!
Thank you for your contribution to my fuzzing habit :)