> I also couldn't figure out yet why PyUnicode_Check() was necessary in the first place. Doesn't PyObject_Str() just increment the refcount if the argument is already a string?

`str()` returns its argument if it's exactly a `builtins.str` instance.  If it's a subclass of str, it will construct a `builtins.str` out of it.

>>> class mystr(str):
...     pass
>>> a = mystr('aaa')
>>> str(a) is a
False

So Eric is right, there's a small discrepancy between Python and C version.