From the implementation:

     try:
         [...]
     except IOError:
         # Worst-case scenario we can only catch errors at a granularity
         # of the whole function.
         [...]
     else:
         # If we have access to the source, we can silence errors on a
         # per-expression basis, which is "better".
         [...]

and then:

    def __exit__(self, exc_type, exc_value, traceback):
        # Returning True prevents the error from propagating. Don't silence
        # KeyboardInterrupt or SystemExit. We aren't monsters.

Brilliant.