How to Python count list length

Question

I want to know how len() works.

Does it count from beginning to end of a list every time I call len(), or, since list is also an class, does len() just return a variable in the list object which record the list's length?

Also, I hope someone can tell me where I can find the source code of those built-in functions like 'len()', 'map()', etc.

Answer 1

Well, you can find the documentation of the built-in functions here.

The list data-type keeps track of the number of elements it's holding, len(list) is an O(1) operation.

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For the source code, you can find the source code of Python at the download page.