[Tutor] subprocess.Popen()

[Dave Hill]

The combination of installing Python 3.7 on RasPi and removing the 
StringIO() cures the error.

I now get the statistics and the 'have a nice day' output from omxplayer.

I just need to find out how to invoke the idle 3.7, rather than idle 
3.5, but I think that is a question for the RasPi forum.

Thank you.

Dave

On 03/12/2018 16:27, Mats Wichmann wrote:
> On 12/2/18 3:29 AM, Dave Hill wrote:
>
>> Having 'graduated' to Python 3.7, I thought I would explore
>> subprocess.Popen, and put the code in a Class, see code below. The video
>> runs, but an error occurs, which I do not understand, see further below
> the error happens in the except clause of your try block ...
>
>> Traceback (most recent call last):
> ...
>>    File "/home/pi/Code/VideoPlayer.py", line 51, in playVideo
>>      out = omxp.communicate()
> ...
>>    File "/usr/lib/python3.5/selectors.py", line 39, in _fileobj_to_fd
>>      "{!r}".format(fileobj)) from None
>> ValueError: Invalid file object: <_io.BufferedReader name=8>
> You can see that the call to communicate() is the one without arguments,
> so it's in your cleanup code.   You should catch the specific exception
> you expect could happen here, which is probably a timeout - but you
> can't tell because you don't specify it.
>
>>          try:
>>              out = omxp.communicate(timeout=self.timeout)
>>              print("Try outs =  ", StringIO(out))
>>          except:
>>              omxp.kill()
>>              out = omxp.communicate()
>>              print("Except outs =  ", StringIO(out))
> use
>
>        except TimeoutExpired:
>
> then it looks like the file descriptor for the second communicate is no
> longer valid... here's a larger chunk from subprocess that shows what it
> is raising and how it got there:
>
>      if isinstance(fileobj, int):
>          fd = fileobj
>      else:
>          try:
>              fd = int(fileobj.fileno())
>          except (AttributeError, TypeError, ValueError):
>              raise ValueError("Invalid file object: "
>                               "{!r}".format(fileobj)) from None
>
>
> In other words, it's raising the ValueError you see  because it's not a
> file object. The "from None" part suppresses the context from the
> original exception.
>
> Note that communicate returns a tuple so your 'out' is actually a tuple,
> and this is likely what is going wrong - by calling StringIO(out) in the
> try: you are causing an exception which isn't the exception one would
> expect; by not being explicit in your except: you catch that error and
> take cleanup steps that are not appropriate - your video probably
> finished normally.
>
> hopefully this will help debug things...
>
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