both of the following true division and multiplication operations crash the interpreter: import datetime class BadFloat(float): def as_integer_ratio(self): return (1 << 1000) - 1 datetime.timedelta() / BadFloat() datetime.timedelta() * BadFloat() this is because both multiply_float_timedelta() and truedivide_timedelta_float() (in Modules/_datetimemodule.c) assume as_integer_ratio() returns tuple.

i am working on a patch. BTW, is there anywhere a list of what counts as an extension module, and what counts as the interpreter core?

I don't know if there is a list, but definitely the parser, the compiler, the evaluation loop, builtin types and few builtin modules (like builtins and sys) are interpreter core. The _datetime module is an accelerator of the datetime module. It is optional and definitely is not in the core.

Please check if there is similar issue with Decimal.

I guess you meant for me to check whether the following has any problem: import decimal class BadFloat(float): def as_integer_ratio(self): return 1 << 1000 decimal.Decimal.from_float(BadFloat()) so it doesn't crash. if IIUC, this is because the C implementation of Decimal.from_float() is PyDecType_FromFloat(), while the relevant part of it is a call to PyDecType_FromFloatExact(). But PyDecType_FromFloatExact() uses float.as_integer_ratio(), and thus ISTM that the issue doesn't exist there. (also, the following doesn't raise an exception, as expected: import decimal class BadFloat(float): def as_integer_ratio(self): raise RuntimeError decimal.Decimal.from_float(BadFloat()) )

 New changeset 865e4b4f630e2ae91e61239258abb58b488f1d65 by Serhiy Storchaka (Oren Milman) in branch 'master': bpo-31293: Fix crashes in truediv and mul of a timedelta by a float with a bad as_integer_ratio() method. (#3227) https://github.com/python/cpython/commit/865e4b4f630e2ae91e61239258abb58b488f1d65 

 New changeset f37dd11f0d4832c15d49c2ddc83d533ddaa36e74 by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6': [3.6] bpo-31293: Fix crashes in truediv and mul of a timedelta by a float with a bad as_integer_ratio() method. (GH-3227) (#3654) https://github.com/python/cpython/commit/f37dd11f0d4832c15d49c2ddc83d533ddaa36e74