How does iter and key in Python Max/Min Function work? [duplicate]

Question

I am relatively new to Python and i have been going over the documentation for various built-in functions.

When it comes to max/min functions:

 `max(arg1, arg2, *args[, key])`  or `max(iterable[, key])`

I know that arg1, arg2 etc or iterable could be a sequence of random values, however what is the role of the "key" element? Can someone give me an example of its application?

Normally when i see this built-in function, i automatically imagine a random list say x = [1,2,3] and going max(x) would yield the value of 3. However, what could the "key" feature offer me to manipulate this function some other way than just going through a simple straightforward list?

I am new to Python and not really up to speed with all the jargon in Python Docs.

Thanks, Ed

Answer 1

The key is used to pass a custom comparison function.

Example: output max by length of list, where arg1, arg2 are both lists.

>>> max([1,2,3,4], [3,4,5], key=len)
[1, 2, 3, 4]

Example: output max by sum of elements of args' lists

>>> max([1,2,3,4,5], [3,4,5], key=sum)
[1, 2, 3, 4, 5]

>>> max([1,2,3,4], [3,4,5], key=sum)
[3, 4, 5]

You can similarly use specific comparison functions for different arg objects.

Answer 2

You can use it when your you want to find the max of a sequence and you'd like a specific definition of max.

For example, say I have a list of tuple. If I just use max without the key argument, it will by default use the first item in each tuple

>>> l = [(1,3), (2,4), (1,9), (4,1)]

>>> max(l)
(4, 1)

But what if I want the max from the list, but by considering the second element of the tuple?

>>> max(l, key = lambda i : i[1])
(1, 9)

# Or

>>> import operator
>>> max(l, key = operator.itemgetter(1))
(1, 9)

Also what about a list of strings, and you want to find the max as if they were the numeric values?

>>> l = ['4', '11', '6', '31']

Just using max will sort them lexicographically

>>> max(l)
'6'

But again I can use key

>>> max(l, key = lambda i: int(i))
'31'