PyFile_WriteString() calls PyObject_Str() which calls PyErr_CheckSignals(). If a signal was catched, the signal handler is called. If the signal handler raises an error, PyObject_Str() and then PyFile_WriteString() return NULL.

mywrite() ignores all PyFile_WriteString() errors. It should maybe only ignores errors from the file (except IOError: ...) and not any error.

Another problem: mywrite() is called from PySys_WriteStdout() and PySys_WriteStderr() which are procedures. PySys_WriteStdout()/PySys_WriteStderr() caller cannot detect the error. There are 65 calls to PySys_WriteStd...