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I have a Python extension module which creates a tuple as an attribute of another object, and sets items in the tuple. Whenever I execute this module in Python, I keep getting the error SystemError: bad argument to internal function After reading over the docs for PyTuple , and debugging my program for a few hours, I still couldn't figure out what the hell was going on. Running my program through a debugger indicated the problem was occurring within a library call inside the Python interpreter. So, finally, I looked at the Python source code, and at long last I realized the problem. The PyTuple_SetItem function has an interesting restriction which I didn't know about, and can't find explicitly documented. Here is the important function in the Python source (edited for clarity): int PyTuple_SetItem(register PyObject *op, register Py_ssize_t i, PyObject *newitem) { ..... if (!PyTuple_Check(op) || op->ob_refcnt != 1) { Py_XDECREF(newitem); PyErr_BadInternalCall(); return -1; } ..... } The important line here is the condition op->ob_refcnt != 1 . So here's the problem: you can't even call PyTuple_SetItem unless the Tuple has a ref-count of 1. It looks like the idea here is that you're never supposed to use PyTuple_SetItem except right after you create a tuple using PyTuple_New() . I guess this makes sense, since Tuples are supposed to be immutable, after all, so this restriction helps keep your C code more in line with the abstractions of the Python type system. However, I can't find this restriction documented anywhere. Relevant docs seem to be here and here

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