Python: sort function breaks in the presence of nan

Question

sorted([2, float(\'nan\'), 1]) returns [2, nan, 1]

(At least on Activestate Python 3.1 implementation.)

I understand nan

Answer 1

Regardless of standards, there are many cases where a user-defined ordering of float and NA values is useful. For instance, I was sorting stock returns and wanted highest to lowest with NA last (since those were irrelevant). There are 4 possible combinations

1. Ascending float values, NA values last
2. Ascending float values, NA values first
3. Descending float values, NA values last
4. Descending float values, NA values first

Here is a function that covers all scenarios by conditionally replacing NA values with +/- inf

import math 

def sort_with_na(x, reverse=False, na_last=True):
    """Intelligently sort iterable with NA values

    For reliable behavior with NA values, we should change the NAs to +/- inf
    to guarantee their order rather than relying on the built-in
    ``sorted(reverse=True)`` which will have no effect. To use the ``reverse``
    parameter or other kwargs, use functools.partial in your lambda i.e.

        sorted(iterable, key=partial(sort_with_na, reverse=True, na_last=False))

    :param x: Element to be sorted
    :param bool na_last: Whether NA values should come last or first
    :param bool reverse: Return ascending if ``False`` else descending
    :return bool:
    """
    if not math.isnan(x):
        return -x if reverse else x
    else:
        return float('inf') if na_last else float('-inf')

Testing out each of the 4 combinations

from functools import partial

a = [2, float('nan'), 1]
sorted(a, key=sort_with_na)                                         # Default
sorted(a, key=partial(sort_with_na, reverse=False, na_last=True))   # Ascend, NA last
sorted(a, key=partial(sort_with_na, reverse=False, na_last=False))  # Ascend, NA first
sorted(a, key=partial(sort_with_na, reverse=True, na_last=True))    # Descend, NA last
sorted(a, key=partial(sort_with_na, reverse=True, na_last=False))   # Descend, NA first