Why does Python code use len() function instead of a length method?

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Strings do have a length method: __len__()

The protocol in Python is to implement this method on objects which have a length and use the built-in len() function, which calls it for you, similar to the way you would implement __iter__() and use the built-in iter() function (or have the method called behind the scenes for you) on objects which are iterable.

See Emulating container types for more information.

Here’s a good read on the subject of protocols in Python: Python and the Principle of Least Astonishment

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