How to test characters of a string

[2QdxY4RzWzUUiLuE at potatochowder.com]

On 2022-06-09 at 04:15:46 +1000,
Chris Angelico <rosuav at gmail.com> wrote:

> On Thu, 9 Jun 2022 at 04:14, <2QdxY4RzWzUUiLuE at potatochowder.com> wrote:
> >
> > On 2022-06-09 at 03:18:56 +1000,
> > Chris Angelico <rosuav at gmail.com> wrote:
> >
> > > On Thu, 9 Jun 2022 at 03:15, <2QdxY4RzWzUUiLuE at potatochowder.com> wrote:
> > > >
> > > > On 2022-06-08 at 08:07:40 -0000,
> > > > De ongekruisigde <ongekruisigde at news.eternal-september.org> wrote:
> > > >
> > > > > Depending on the problem a regular expression may be the much simpler
> > > > > solution. I love them for e.g. text parsing and use them all the time.
> > > > > Unrivaled when e.g. parts of text have to be extracted, e.g. from lines
> > > > > like these:
> > > > >
> > > > >   root:x:0:0:System administrator:/root:/run/current-system/sw/bin/bash
> > > > >   dhcpcd:x:995:991::/var/empty:/run/current-system/sw/bin/nologin
> > > > >   nm-iodine:x:996:57::/var/empty:/run/current-system/sw/bin/nologin
> > > > >   avahi:x:997:996:avahi-daemon privilege separation user:/var/empty:/run/current-system/sw/bin/nologin
> > > > >   sshd:x:998:993:SSH privilege separation user:/var/empty:/run/current-system/sw/bin/nologin
> > > > >   geoclue:x:999:998:Geoinformation service:/var/lib/geoclue:/run/current-system/sw/bin/nologin
> > > > >
> > > > > Compare a regexp solution like this:
> > > > >
> > > > >   >>> g = re.search(r'([^:]*):([^:]*):(\d+):(\d+):([^:]*):([^:]*):(.*)$' , s)
> > > > >   >>> print(g.groups())
> > > > >   ('geoclue', 'x', '999', '998', 'Geoinformation service', '/var/lib/geoclue', '/run/current-system/sw/bin/nologin')
> > > > >
> > > > > to the code one would require to process it manually, with all the edge
> > > > > cases. The regexp surely reads much simpler (?).
> > > >
> > > > Uh...
> > > >
> > > >     >>> import pwd # https://docs.python.org/3/library/pwd.html
> > > >     >>> [x for x in pwd.getpwall() if x[0] == 'geoclue']
> > > >     [pwd.struct_passwd(pw_name='geoclue', pw_passwd='x', pw_uid=992, pw_gid=992, pw_gecos='Geoinformation service', pw_dir='/var/lib/geoclue', pw_shell='/sbin/nologin')]
> > >
> > > That's great if the lines are specifically coming from your system's
> > > own /etc/passwd, but not so much if you're trying to compare passwd
> > > files from different systems, where you simply have the files
> > > themselves.
> >
> > In addition to pwent to get specific entries from the local password
> > database, POSIX has fpwent to get a specific entry from a stream that
> > looks like /etc/passwd.  So even POSIX agrees that if you think you have
> > to process this data manually, you're doing it wrong.  Python exposes
> > neither functon directly (at least not in the pwd module or the os
> > module; I didn't dig around or check PyPI).
> 
> So...... we can go find some other way of calling fpwent, or we can
> just parse the file ourselves. It's a very VERY simple format.

If you insist:

    >>> s = 'nm-iodine:x:996:57::/var/empty:/run/current-system/sw/bin/nologin'
    >>> print(s.split(':'))
    ['nm-iodine', 'x', '996', '57', '', '/var/empty', '/run/current-system/sw/bin/nologin']

Hesitantly, because this is the Python mailing list, I claim (a) ':' is
simpler than r'([^:]*):([^:]*):(\d+):(\d+):([^:]*):([^:]*):(.*)$', and
(b) string.split covers pretty much the same edge cases as re.search.