return a list in python

Question

My question is : write a function that adds the length of the list to the list
My solution:
def add_list(lst):
return lst.append(len(lst))
print (add_list([2,1, 3]))

and it returns None but
def add_list(lst):
lst.append(len(lst))
return lst
print (add_list([2,1, 3]))
gives me the right answer : [3, 2, 1, 3]
It seems these two functions are identical but why they have different behavior?

Answer 1

lst.append will update lst directly... The return value of append is None.
that's why in your first example the returned value is just that - None...
in your second example you added another step, so the result is just as you expected it:)

Answer 2

With return lst.append(len(lst)) you return the return value of the method append() which is None.