Max Length String in a List in Python

The max function in Python supports an optional “key” parameter which can be used to either access a property of the elements of a List or permute the elements in the List in a particular way.

The following example is a one-liner to get the maximum-length element from a Python List.

>>> x = ['1', '11', '111', '0000']
>>> max(x, key=len)
'0000'

The min function also supports a key parameter in the same way as max.

This also extends to any nested collection (since the string class supports __len__ just as a List or Dict does), so we can do other cool things like:

Return the shortest sublist in a nested list:

>>> x = [[0], [0,0], [0,0,0]]
>>> max(x, key=len)
[0]

Return an empty dictionary if it exists in a list:

>>> x = [{}, {1: 1}, {1: 1}]
>>> min(x, key=len)

Bonus: sets of sets (actually nested frozensets inside frozensets, since you can’t nest a set inside of a set since a set can’t be hashed).

>>> x = frozenset([frozenset([1]), frozenset([2, 3])])
>>> min(x, key=len)
frozenset([1])
>>> max(x, key=len)
frozenset([2, 3])