iterating or extending Function produces an infinite loop

[joshkaplan]

Problem

Both of these examples produce an infinite loop:

import peewee

pwf = peewee.fn.count()

# example 1:
for i in pwf:
    print(f'hello {str(i)}')

# example 2:
x = []
x.extend(pwf)

The offending code is ColumnBase.__getitem__: https://github.com/coleifer/peewee/blob/master/peewee.py#L1236-L1242

I'm not sure if this only applies to Function, or if it affects all non-slice ColumnBase classes.

For context, we did run into this in a real-world use-case.

Expected Behavior

I'm not 100% sure, but at the very least this probably should raise rather than infinitely loop, which is worse because it is silent and effectively a memory leak.

Proposed Solutions

(Disclaimer: I'm not deeply familiar with peewee's internals, so this is more directional than a fully thought-out solution.)

From the Python docs:

Note for loops expect that an IndexError will be raised for illegal indexes to allow proper detection of the end of the sequence.

So if raising in all non-slice cases doesn't make sense, perhaps just raising an IndexError at the appropriate time (e.g. after 1 loop)?

[coleifer]

We don't want to raise an IndexError since it is not inherently an iterable type, so I'm going with NotImplementedError for now.

[coleifer]

3.15.2 released now, which contains this fix.