Python: Modernize iter not returning self query

[joefarebrother]

Removes dependence on pointsto and adresses a couple common FP cases.

[github-actions]

QHelp previews:

python/ql/src/Functions/IterReturnsNonSelf.qhelp

Iterator does not return self from __iter__ method

Iterator classes (classes defining a __next__ method) should have an __iter__ method that returns the iterator itself. This ensures that the object is also an iterable; and behaves as expected when used anywhere an iterator or iterable is expected, such as in for loops.

Recommendation

Ensure that the __iter__ method returns self, or is otherwise equivalent as an iterator to self.

Example

In the following example, the MyRange class's __iter__ method does not return self. This would lead to unexpected results when used with a for loop or in statement.

class MyRange(object):
    def __init__(self, low, high):
        self.current = low
        self.high = high

    def __iter__(self):
        return (self.current, self.high) # BAD: does not return `self`.

    def __next__(self):
        if self.current > self.high:
            return None
        self.current += 1
        return self.current - 1

References

• Python Language Reference: object.iter.
• Python Standard Library: Iterators.

[tausbn]

A few minor comments, but otherwise this looks good to me. 👍

[tausbn]

This where clause requires that there be both an __iter__ and a __next__ method present. What if only the __next__ method is present? It might make sense to also flag this case.

[joefarebrother]

Potentially useful. The alert for that case would have to be different (doesn't link to the __iter__ method) which is a little trickier to handle.