[Cython] doctest wraps stdout so file pointers can't convert

[Stefan Behnel]

Hi,

Jim Kleckner wrote:
> I ran across an annoyance and wondered if anyone else hit this.
>
> I have some compiled C code wrapped with Cython that writes to
> an ordinary file descriptor when the debug level is high enough.
> It directly references sys.stdout and sends output there.  When
> using a doctest, however, that object is replaced with _SpoofOut
> that subclasses StringIO.  Thus, when executing something like the
> following in a doctest:
>
>    ...
>    cdef file outFile = sys.stdout  # This fails for the spoofed output
>    callDebugCodeInC(PyFile_AsFile(outFile))
>    ...
>
> It (understandably) fails on the assignment with a message:
>    TypeError: Cannot convert instance to file

Note that PyFile_AsFile() was dropped in Py3 anyway, so your code will not
be future-proof if you use this.

Stefan