Inspect stack frame by frame to improve performance

[rover-debug]

Addressing performance issue with the workaround used to address this issue: https://github.com/python/typing/issues/658.

Led to roughly 10x speedup in our project.

[Stewori]

If I understand it correctly, this change inverts the search order of the stack. Is that right? Probably this does not make a difference for the result. But programming is full of subtle problems. If it passes the tests I am fine with it.

[Stewori]

@bhavints Thank you for contributing this improvement!

[rover-debug]

If I understand it correctly, this change inverts the search order of the stack. Is that right? Probably this does not make a difference for the result. But programming is full of subtle problems. If it passes the tests I am fine with it.

Yes, the idea is that instead of collecting the entire stack via stack() and iterating through it, it is more performant to iterate backwards frame by frame until the class is found.

[mitar]

If I understand it correctly, this change inverts the search order of the stack.

No, the order is the same. stack() returns:

The first entry in the returned list represents the caller; the last entry represents the outermost call on the stack.

So calling currentframe() and then using f_back is walking in the same order. The difference is that it stops walking after you find the frame instead of first walking the whole stack.

[mitar]

@bhavints please also note of this. I suggest you make sure for frame to be deleted explicitly:

def handle_stackframe_without_leak():
    frame = inspect.currentframe()
    try:
        # do something with the frame
    finally:
        del frame

[Stewori]

If I understand it correctly while frame.f_back.f_back would skip the last two frames instead of the first two. Shouldn't rather the initialization change to frame = currentframe().f_back.f_back while the loop stays like before? As I understand @mitar currentframe() is stack[0].

[mitar]

Shouldn't rather the initialization change to frame = currentframe().f_back.f_back while the loop stays like before?

Yes, this is what I thought I am proposing. So the initialization should be done like you said. And then while frame.

[rover-debug]

OK thanks for the feedback, made some more edits.

[mitar]

Hm, CI tests have not run?

[Stewori]

Thank you for following all our comments patiently. And for moving the comment back to its proper location! Given also mitar approved I think this can be merged right away.

[Stewori]

Hm, CI tests have not run?

Yes, that puzzles me too. Is it not triggered by commits from new contributors?

[mitar]

I think you should first merge https://github.com/Stewori/pytypes/pull/108 and then ask @bhavints to rebase.