bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations by mjpieters · Pull Request #51 · python/cpython

When you use '%s' % SubClassOfStr(), where SubClassOfStr.__rmod__ exists, the reverse operation is ignored as normally such string formatting operations use the PyUnicode_Format() fast path. This patch tests for subclasses of str first and picks the slow path in that case.

Fixes bpo-28598.