Josh Rosenberg <shadowranger+pyt...@gmail.com> added the comment:

object.__ipow__ is documented to take an optional third argument (though there 
is no way to pass it aside from explicitly calling __ipow__ directly since 
there is no syntax support for three-arg pow, in place or otherwise), so it's 
not some incompatibility with object.__ipow__'s signature.

How are you seeing garbage passed? In the CPython C code base, I only see 
PyNumber_InPlacePower called in two places; ceval.c (to handle **=, which only 
handles two operands) and _operator.c (to implement operator.__ipow__, which 
unlike object.__ipow__, only takes two arguments, not three). In both cases, 
the third argument is explicitly passed in as Py_None.

PyNumber_InPlacePower itself then passes along that third argument to 
ternary_op as its third argument, and every code path that calls the retrieved 
slot consistently passes that argument along as the third argument to the 
slotted ternaryfunc.

I suppose an extension module might incorrectly call PyNumber_InPlacePower 
without passing the third argument, but that's a problem on their end (and 
should be caught by the compiler unless all diagnostics are suppressed).

But I'm not seeing the problem here. The code path is probably untested (given 
all numeric types in the CPython core are immutable, so none of them set 
nb_inplace_pow), but it looks correct at first glance. Do you have code that 
reproduces the error?

----------
nosy: +josh.r

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<https://bugs.python.org/issue36379>
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