Maybe this is related to

    http://bugs.python.org/issue13673

which causes PyTraceback_Print() to fail if a signal is received but PyErr_CheckSignals() has not been called.

Note that wrapping in "try: ... except: raise" makes a traceback appear:

>>> try: input()
... except: raise
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyboardInterrupt

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyboardInterrupt