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I'm able to reproduce the RuntimeError on Windows 7, it comes from a pipe. The message is probably written by a child process, not by the main process. I suppose that Richard knows better than me how to fix this warning, so I don't want to investigate it :-)
I'm unable to reproduce the "Assertion failed: !PyErr_Occurred(), file ..\Python\ceval.c, line 4077" failure on Windows 7 on my AMD64 with Python compiled in debug mode in 32-bit mode (I only have Visual Studio Express, so no 64-bit binary). I'm interested by this one, but I need a traceback, the C traceback if possible.
An option would be to enable faulthandler by monkey-patching multiprocessing.spawn.get_command_line() (to add -X faulthandler). But in my exprerience, the Python traceback doesn't help to investigate such assertion error.
I added this assertion recently in Python 3.4 to detect bugs earlier. If PyEval_CallObjectWithKeywords() is called with an exception set, the exception may be cleared or replaced with a new exception, so the original exception can be lost, which is probably not expected. For example, PyDict_GetItem() raises a KeyError and then clears the current exception.