x *= seq raises TypeError for int subclasses and IntEnum where x * seq works #161968

Bug report

When the left operand is an instance of a Python class that supports __index__ but has no __mul__/__imul__ that handles sequences (an int subclass, an IntEnum member, or any class with __index__), x *= [1] raises TypeError, while x * [1] and x = x * [1] work. With a plain int on the left it works.

import enum, operator

class MyInt(int):
    pass

class Number(enum.IntEnum):
    TWO = 2

x = MyInt(2)
print(x * [1])              # [1, 1]
x *= [1]                    # TypeError: unsupported operand type(s) for *=: 'MyInt' and 'list'

y = Number.TWO
y *= [1]                    # TypeError: unsupported operand type(s) for *=: 'Number' and 'list'

operator.imul(MyInt(2), [1])  # same TypeError

Expected: [1, 1] in each case, the same as x * [1].

The cause is in PyNumber_InPlaceMultiply (Objects/abstract.c): the right operand's sq_repeat is only tried in an else if (mw != NULL) branch, which is skipped whenever the left operand's type has a tp_as_sequence at all, even if that has no repeat slot. Every heap type has tp_as_sequence, so for these classes the list's repeat is never reached. PyNumber_Multiply does not have this problem because it checks the right operand independently.

CPython versions tested on:

3.15, CPython main branch

Operating systems tested on:

Linux

Found while building a Lean 4 model of CPython (https://broyojo.com/python-formalization/). AI tools (Claude) helped investigate the findings and prepare this report. The examples were rerun on CPython 3.15.2 and main, and the report received technical review. Record: corelibs-inplace-mul-heap-left

Linked PRs

• gh-161968: Fall back to the right operand's sq_repeat in PyNumber_InPlaceMultiply #161969
• [3.14] gh-161968: Fall back to the right operand's sq_repeat in PyNumber_InPlaceMultiply (GH-161969) #162008
• [3.15] gh-161968: Fall back to the right operand's sq_repeat in PyNumber_InPlaceMultiply (GH-161969) #162009