The systemd version in CentOS 7 (version 219) does not support the
StandardError=file:... syntax, which was only added in systemd version 236:
Controls where file descriptor 1 (STDOUT) of the executed processes is connected to. Takes one of inherit, null, tty, journal, syslog, kmsg, journal+console, syslog+console, kmsg+console or
(from the systemd.exec man page in CentOS 7).
I think since you've not specified a valid value, the default value of
journal is used and the journal forwards everything to syslog:
journal connects standard output with the journal which is accessible
via journalctl(1). Note that everything that is written to syslog or
kmsg (see below) is implicitly stored in the journal as well, the
specific two options listed below are hence supersets of this one.
syslog connects standard output to the syslog(3) system syslog
service, in addition to the journal. Note that the journal daemon is
usually configured to forward everything it receives to syslog anyway,
in which case this option is no different from journal.
If you are able to modify the python application, you can use pythons logging module to log to a file:
logging.basicConfig(filename='example.log', encoding='utf-8', level=logging.DEBUG)
logging.debug('This message should go to the log file')
logging.info('So should this')
logging.warning('And this, too')
logging.error('And non-ASCII stuff, too, like Øresund and Malmö')