Question

I am attempting to sort a Python list of ints and then use the .pop() function to return the highest one. I have tried a writing the method in different ways:

def LongestPath(T):    
    paths = [Ancestors(T,x) for x in OrdLeaves(T)]
    #^ Creating a lists of lists of ints, this part works
    result =[len(y) for y in paths ]
    #^ Creating a list of ints where each int is a length of the a list in paths
    result = result.sort()
    #^meant to sort the result
    return result.pop()
    #^meant to return the largest int in the list (the last one)

I have also tried

def LongestPath(T):
    return[len(y) for y in [Ancestors(T,x) for x in OrdLeaves(T)] ].sort().pop()

In both cases .sort() causes the list to be None (which has no .pop() function and returns an error). When I remove the .sort() it works fine but does not return the largest int since the list is not sorted.

Answer 1

Simply remove the assignment from

result = result.sort()

leaving just

result.sort()

The sort method works in-place (it modifies the existing list), so it returns None. When you assign its result to the name of the list, you’re assigning None. So no assignment is necessary.

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But in any case, what you’re trying to accomplish can easily (and more efficiently) be written as a one-liner:

max(len(Ancestors(T,x)) for x in OrdLeaves(T))

max operates in linear time, O(n), while sorting is O(nlogn). You also don’t need nested list comprehensions, a single generator expression will do.