Running a GUI program haults the calling program (linux)

[Peter Otten]

Kryptxy via Python-list wrote:

> Sent with [ProtonMail](https://protonmail.com) Secure Email.
> 
>> -------- Original Message --------
>> Subject: Re: Running a GUI program haults the calling program (linux)
>> Local Time: 26 September 2017 12:09 PM
>> UTC Time: 26 September 2017 06:39
>> From: cs at cskk.id.au
>> To: Kryptxy <kryptxy at protonmail.com>
>> python-list <python-list at python.org>
>>
>> On 25Sep2017 20:38, Kryptxy <kryptxy at protonmail.com> wrote:
>>>I want to run a GUI program (transmission-gtk) from python. This is what
>>>I do:
>>>
>>>import subprocess
>>>subprocess.Popen(["transmission-gtk", link], stdout=subprocess.PIPE,
>>>stderr=subprocess.PIPE)
>>>
>>>Where `link` is some magnetic link.
>>>
>>>This command opens transmission-gtk, but it haults the calling program,
>>>and keeps the terminal busy till the time GUI is running. As soon as GUI
>>>is closed, the control goes back to the program.
>>
>> I do not believe this is all your code. Please post a _complete_ example.
>>
>> Popen dispatches a process. It does not wait for it to terminate. GUI
>> programs are not special. Therefore, if your calling python program is
>> blocking, it is blocking somewhere other that this line of code. Please
>> show us where.
>>
>> Consider putting print() calls through your code to locate where your
>> program stalls.
>>
>> Cheers,
>> Cameron Simpson <cs at cskk.id.au> (formerly cs at zip.com.au)
> 
> Here -
> 
> p = subprocess.Popen(['transmission-gtk', link], stdout=subprocess.PIPE,
> stderr=subprocess.PIPE)
> output, error = p.communicate()

That's the problem. communicate() *must* block to collect the program's 
entire output. You get what you ask for...

> error = error.decode('utf-8')
> output = output.decode('utf-8')
> if error != '':
>     print("\nUnable to load torrent.")
> else:
>     print("Torrent added successfully!")
> 
> Here, transmission-gtk is opened and the program haults (at
> subprocess.Popen() statement). When I close the GUI window, I get the
> message "Unable to load torrent".
> 
> Error message I get:
> `Unable to parse nameserver address 8.8.8.8,`
> `(transmission-gtk:3982): GLib-CRITICAL **: g_file_test: assertion
> 'filename != NULL' failed`