What you mean to say is this - "The most common implementation of Python, CPython, happens to have this behavior, but it is not guaranteed."

There exist other implementations of Python, like PyPy and IronPython, and they don't necessarily do this. Or if you compile your Python using, say, Cython, it definitely won't work. And for all we know, future implementations of CPython won't do this, because there's no guarantee (though by now it's so ingrained that changing that would wreak havoc).

More information here: https://stackoverflow.com/a/15172182/43839