[pytest-dev] Preserving capture fd from fixture setup?

holger krekel holger at merlinux.eu
Wed Jun 1 01:17:16 EDT 2016

On Tue, May 31, 2016 at 10:17 -0700, Nikolaus Rath wrote:
> On May 28 2016, holger krekel <holger-TO2CgvPYN1Lsq35pWSNszA at public.gmane.org> wrote:
> > Hi Nikolaus,
> >
> > On Thu, May 19, 2016 at 12:34 -0700, Nikolaus Rath wrote:
> >> Hello,
> >> 
> >> I am testing a server. I am starting the server in a subprocess from a
> >> fixture, and then issue requests to it from various tests. Unfortunately
> >> this seems to have one big drawback: when a test fails, the capture
> >> plugin does not display any output from the server process.
> >>
> >> After looking at the code, I think this is because the capture plugin
> >> creates a new file descriptor for each test phase (setup, call,
> >> teardown).
> >
> > it reuses the same file descriptor for capturing IIRC.  After each
> > phase (setup, call, teardown) the FD is reset to position 0.
> Hmm. Yeah, on closer inspection my test case was flawed - adding a
> .flush() at the end of the setup code made the output appear in the
> right spot. But my original problem remains.
> > Is your problem that the server is started during fixture setup
> > and you'd like to see its standard out/err during the test call phase?
> Exactly.

I guess you could write some conftest.py hook which transfers the output
from the setup phase to the call phase so you can see it when a test call
phase fails.  Something like:


shows a bit of the mechanics for that although it is not
precisely yet what you are asking for.

It's probably a good idea to make it easier to show setup/fixture output
during a test call failure.  Showing all fixture output by default would 
be a bit much, though.  Also, if a fixture is setup with a non-function caching
scope one needs to memorize output differently because a test call phase then
relates to a setup phase of a different test ... this is probably all 
worth a discussion at the upcoming sprint ...


> Best,
> -Nikolaus
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