[issue15801] Weird string interpolation behaviour

[Mike Graham]

Mike Graham added the comment:

Line 13464 of unicodeobject.c is

    if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
        !PyUnicode_Check(args))

Too lazy to check, but don't user-created types have a tp_as_mapping? If so, I think it mistakes Foo() for a dict.

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nosy: +Mike.Graham

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<http://bugs.python.org/issue15801>
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