Backtrack if incompatibilities fail to apply

[uranusjr]

For pypa/pip#9180. The closure function looks so ugly but I can’t come up with something better :(

[pradyunsg]

I feel dumb -- I don't understand the need for the closure. :(

[pfmoore]

I assumed it was just to name the block of code, for readability. If it's more than that, I'm as dumb as you, and I clicked "Approve" as well 🙂

[uranusjr]

The closure is needed because we want to break a loop from a loop nested inside it. The high-level POV is

while len(self._states) >= 3:
    ...
    for k, incompatibilities in incompatibilities_from_broken:
        ...
        if criterion is None:
            # We want to break the "while" loop here.

And a closure with early return is the best way I can think of to do it.

[McSinyx]

I suppose we can use the following, but I'm not sure if it is any easier to read than a closure.

while ...:
    for ...:
        if ...:
            break
    else:
        continue
    break

[uranusjr]

@McSinyx I thought of using else: as well, but preferred the closure approach. Also we need to continue the outer while, and I couldn’t find a nice way to do it with else:.

[pfmoore]

I noticed that after I posted my comment. IMO using the closure for the early exit is fine, the code is perfectly readable as it stands.

[pradyunsg]

I'm not sure if it is any easier to read than a closure.

FWIW, I would find this form easier to read, but only marginally, so 🤷. This does look good to me now that I'm looking at it with that context! ^>^