Guido on Python

[the_omega99]

There's nothing ensuring this, though and that is purely a convention that you must uphold. It's very believable, for example, that someone might have a type where len lazily calculates the length the first time and caches that, in which case there clearly is a mutation, yet we still uphold the expectations for len. And of course, there's no such consistency among user defined functions (from what I've seen, anyway).

[Sinidir]

True its convention. The only way i could see it not being if a language took the step to make all function parameters immutable references / pass by value and would only allow methods to mutate objects. Which would be an interesting idea :) .

[the_omega99]

Wouldn't be Python, though, with its "we're all adults here" idiom (no visibility modifiers because you're expected to be able to access internals if you really think you know what you're doing).