Cost of len() function Cost of len() function python python

Cost of len() function

It's O(1) (constant time, not depending of actual length of the element - very fast) on every type you've mentioned, plus set and others such as array.array.

Calling len() on those data types is O(1) in CPython, the most common implementation of the Python language. Here's a link to a table that provides the algorithmic complexity of many different functions in CPython:

TimeComplexity Python Wiki Page

All those objects keep track of their own length. The time to extract the length is small (O(1) in big-O notation) and mostly consists of [rough description, written in Python terms, not C terms]: look up "len" in a dictionary and dispatch it to the built_in len function which will look up the object's __len__ method and call that ... all it has to do is return self.length