`<Javascript undefined>` when parameter name starts with double underscore in methods

[Matéo Duparc]

The following code produces a strange <Javascript undefined>:

class A:
    def f(self, __v):
        print(__v)

A().f(1)

I noticed it only happens when the parameter name starts with __. However functions defined outside classes work fine.

[Pierre Quentel]

Thanks again Matéo. It was a bug in the implementation of Python "name mangling".