Can you explain the following function?

Question

def a(n):
    return max([len(n)] + [a(i) for i in n]) if isinstance(n, list) else 0

this was in a recent test of mine and i just can't get list comprehension down. So basically this function was supposed to return the length of the largest list (which is what i'm assuming based on the correct answers) I would have understood that if it weren't for this part of the function:

+ [a(i) for i in n])

when i see that part, it looks like it adds something to the length of the list it was iterating over. Can someone shed some light on the purpose of that part? more specifically the reason for the addition.

edit: so after looking at it more carefully..it looks like the function puts the length of the first list in a list, then puts the length of the next list and returns the max?... is this how it works?

Answer 1

This function computes the length of the largest node in a tree (implemented as lists within lists). Perhaps with a bit of renaming and reording, it'll be clearer:

def longest_list_in_tree(tree):
    if not isinstance(tree, list):
        return 0 # This is a leaf-value of the tree, it has "length 0"

    own_length = len(tree)
    longest_descendant_of_each_subtree = [
        longest_list_in_tree(subtree) for subtree in tree
    ]

    return max([own_length] + longest_descendant_of_each_subtree)