Python - return largest of N lists

Question

I would like have a function return the largest of N list. With two items in the list I can write:

l1 = [3, 4, 5]
l2 = [4, 5, 6, 7]

def f(L):
    if(len(L[0]) > len(L[1])):
        return L[0]
    else:
        return L[1]

which I run with f([l1, l2]) .

However, with more lists, it becomes a succession of if statements, and it is ugly.

How would you very efficiently return the largest of N lists ?

Answer 1

Use max with key=len .

In [3]: max([l1, l2], key=len)
Out[3]: [4, 5, 6, 7]

This will retrieve the (first) longest list, for a list of lists.

In fact, this will also work with strings (and other objects with a len attribute).

In [4]: max(['abcd', 'ab'], key=len)
Out[4]: 'abcd'

In [5]: max([(1, 2), (1, 2, 3), (1,)], key=len)
Out[5]: (1, 2, 3)

In [6]: max(['abc', [1, 2, 3]], key=len)
Out[6]: 'abc'

Note: we can also pass in the items as arguments:

In [7]: max(l1, l2, key=len)
Out[7]: [4, 5, 6, 7]

max reads: get me the largest item in the list when (if you pass key ) looking from the perspective of key .
It's roughly equivalent to the following code* (in python 3) but the actual source is written in C (so much more efficient, as well as actually tested, so please continue to use max and not this!):

def my_max(*L, key=None):  # in python 2 we'd need to grab from kwargs (and raise type error if rogue keywords are passed)
    L = list(L[0]) if len(L) == 1 else L  # max takes iterable as first argument, or two or more arguments...
    if not L:
        raise ValueError("my_max() arg is an empty sequence")
    if key is None:   # if you don't pass a key use the identity
        key = lambda x: x
    max_item, max_size = L[0], key(L[0])
    for item in L[1:]:
        if key(item) > max_size:
            max_item, max_size = item, key(item)
    return max_item

*I leave it as an exercise to write this using iterators rather than lists... and fix any other bugs!