How to find the item in a list whose f(item) is the smallest?
Use key property of min

As @vaultah said, use min(l,key=f). min(l,key=f) takes the minimum of f(i) for i in l.

It is also possible to apply this to max, and sorted. For example, max(l,key=f) is the maximum of f(i) for i in l. For sorted, the usage would be: sorted(l,key=f).