len() to get the one-dimensional length of a list.
len(['one', 'two']) # returns 2 len(['one', [2, 3], 'four']) # returns 3, not 4
len() also works on strings, dictionaries, and other data structures similar to lists.
len() is a built-in function, not a method of a list object.
Also note that the cost of
O(1), meaning it will take the same amount of time to get the length of a list regardless of its length.