Port, board and/or hardware

Unix port, Linux

MicroPython version

v1.27.0

Reproduction

class Foo:
    def __repr__(self):
        return True

print(repr(Foo()))

Expected behaviour

CPython v3.11.15:

TypeError: __repr__ returned non-string (type bool)

Observed behaviour

MicroPython v1.27.0:

True

Additional Information

repr() does not raise TypeError when a user-defined __repr__ method returns a non-string value. Instead, MicroPython silently converts and returns the value as a string. This is a CPython compatibility issue.

Root Cause

instance_print in objtype.c calls the user-defined __repr__ method via mp_call_function_1 and passes the result directly to mp_obj_print_helper without validating that the return value is a string.

CPython reference: Objects/object.c#L439-L445

// CPython validates that __repr__ returns a string, raises TypeError otherwise
if (!PyUnicode_Check(res)) {
    _PyErr_Format(tstate, PyExc_TypeError,
                  "__repr__ returned non-string (type %.200s)",
                  Py_TYPE(res)->tp_name);
    Py_DECREF(res);
    return NULL;
}

Fix

Add a mp_obj_is_str(r) check after calling the __repr__ method in instance_print, and raise TypeError if the return value is not a string.

Code of Conduct

Yes, I agree