Porting sort with lambda function to Python 3

Question

I'm porting code from Python 2.7 to 3. 2to3 does not convert the following lines and can't seem to figure it out. Any help is appreciated.

subpaths.sort(
    lambda x, y :
        int(pyx.unit.tocm(x.arclen() - y.arclen()) /
            math.fabs(pyx.unit.tocm(x.arclen() - y.arclen()))) )

Questions seeking debugging help ("why isn't this code working?") must include the desired behavior, a specific problem or error and the shortest code necessary to reproduce it in the question itself. Questions without a clear problem statement are not useful to other readers. See: How to create a Minimal, Complete, and Verifiable Example. — MattDMo
– MattDMo, Commented Jan 24, 2016 at 20:49
Without knowing what subpaths is and what x and y are, it's impossible to answer this question. — MattDMo
– MattDMo, Commented Jan 24, 2016 at 20:51

Régis B. · Accepted Answer · 2016-01-25 07:17:56Z

2

The sort method of a list requires a key, which is a function of just one argument. You need to convert your lambda function to a function of a single argument. There is a shortcut for that provided by functools.cmp_to_key. Thus, what you probably need is:

import functools
subpaths.sort(key=functools.cmp_to_key(lambda x, y: ...))

Note that if I understand your code correctly, you can simply sort the list using the following key:

subpaths.sort(key=lambda x: pyx.unit.tocm(x.arclen()))

edited Jan 25, 2016 at 7:17

answered Jan 24, 2016 at 20:57

Régis B.

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1 Comment

GingerPlusPlus Over a year ago

This solution is definetly the quickest to implement. But, if OP has time, IMO he should consider replacing cmp lambda with written from scratch key lambda, for improved efficency and readability.

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