[Solved] Python: Is there any reason a subset of a list would sort differently than the original list?

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I think I found why. That is because there are some nan in dpd.txt.

And nan is unable to compare:

float('nan') > 1 # False while float('nan') < 1 # False

So this totally breaks comparison.

If you change your key compare function to:

def _key(id_):
    import math
    result = -dpd[index_map[id_]], id_.lower()
    if math.isnan(result[0]):
        result = 0, id_.lower()
    return result

It will work.

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solved Python: Is there any reason a subset of a list would sort differently than the original list?