Writing your own apt-cdrom¶
| Author: | Julian Andres Klode <jak@debian.org> |
|---|---|
| Release: | 1.4.0~beta3 |
| Date: | January 15, 2019 |
This article explains how to utilise python-apt to build your own clone of the
apt-cdrom command. To do this, we will take a look at the
apt.cdrom and apt.progress.text modules, and we will learn how
to use apt_pkg.parse_commandline to parse commandline arguments. The code shown
here works on Python 2 and Python 3.
Basics¶
The first step in building your own apt-cdrom clone is to import the
apt package, which will import apt.cdrom and
apt.progress.text:
import apt
Now we have to create a new apt.cdrom.Cdrom object and pass to it an
apt.progress.text.CdromProgress object, which is responsible for
displaying the progress and asking questions:
cdrom = apt.Cdrom(apt.progress.text.CdromProgress())
Now we have to choose the action, depending on the given options on the
command line. For now, we simply use the value of sys.argv[1]:
import sys
if sys.argv[1] == 'add':
cdrom.add()
elif sys.argv[1] == 'ident':
cdrom.ident()
Now we have a basic apt-cdrom clone which can add and identify CD-ROMs:
import sys
import apt
cdrom = apt.Cdrom(apt.progress.text.CdromProgress())
if sys.argv[1] == 'add':
cdrom.add()
elif sys.argv[1] == 'ident':
cdrom.ident()
Advanced example with command-line parsing¶
Our example clearly misses a way to parse the commandline in a correct
manner. Luckily, apt_pkg provides us with a function to do this:
apt_pkg.parse_commandline(). To use it, we add import apt_pkg right
after import apt:
import sys
import apt_pkg
import apt
apt_pkg.parse_commandline() is similar to getopt functions, it
takes a list of recognized options and the arguments and returns all unknown
arguments. If it encounters an unknown argument which starts with a leading
‘-‘, the function raises an error indicating that the option is unknown. The
major difference is that this function manipulates the apt configuration space.
The function takes 3 arguments. The first argument is an
apt_pkg.Configuration object. The second argument is a list of tuples
of the form (shortopt, longopt, config, type), whereas shortopt is a
character indicating the short option name, longopt a string indicating the
corresponding long option (e.g. "--help"), config the name of the
configuration item which should be set and type the type of the argument.
For apt-cdrom, we can use the following statement:
arguments = apt_pkg.parse_commandline(apt_pkg.config,
[('h', "help", "help"),
('v', "version", "version"),
('d', "cdrom", "Acquire::cdrom::mount", "HasArg"),
('r', "rename", "APT::CDROM::Rename"),
('m', "no-mount", "APT::CDROM::NoMount"),
('f', "fast", "APT::CDROM::Fast"),
('n', "just-print", "APT::CDROM::NoAct"),
('n', "recon", "APT::CDROM::NoAct"),
('n', "no-act", "APT::CDROM::NoAct"),
('a', "thorough", "APT::CDROM::Thorough"),
('c', "config-file", "", "ConfigFile"),
('o', "option", "", "ArbItem")], args)
This allows us to support all options supported by apt-cdrom. The first option
is –help. As you can see, it omits the fourth field of the tuple; which means
it is a boolean argument. Afterwards you could use
apt_pkg.config.find_b("help") to see whether --help was specified. In
('d',"cdrom","Acquire::cdrom::mount","HasArg") the fourth field is
"HasArg". This means that the option has an argument, in this case the
location of the mount point. ('c',"config-file","","ConfigFile") shows how
to include configuration files. This option takes a parameter which points to
a configuration file which will be added to the configuration space.
('o',"option","","ArbItem") is yet another type of option, which allows users
to set configuration options on the commandline.
Now we have to check whether help or version is specified, and print a message
and exit afterwards. To do this, we use apt_pkg.Configuration.find_b()
which returns True if the configuration option exists and evaluates to
True:
if apt_pkg.config.find_b("help"):
print("This should be a help message")
sys.exit(0)
elif apt_pkg.config.find_b("version"):
print("Version blah.")
sys.exit(0)
Now we are ready to create our progress object and our cdrom object. Instead
of using apt.Cdrom like in the first example, we will use
apt_pkg.Cdrom which provides a very similar interface. We could also
use apt.Cdrom, but apt.Cdrom provides options like nomount which
conflict with our commandline parsing:
progress = apt.progress.text.CdromProgress()
cdrom = apt_pkg.Cdrom()
Now we have to do the action requested by the user on the commandline. To see
which option was requested, we check the list arguments which was returned
by apt_pkg.parse_commandline above, and afterwards call cdrom.add or
cdrom.ident:
if apt_pkg.config.find_b("help"):
print("This should be a help message")
sys.exit(0)
elif apt_pkg.config.find_b("version"):
print("Version blah.")
sys.exit(0)
if not arguments:
sys.stderr.write('E: No operation specified\n')
sys.exit(1)
elif arguments[0] == 'add':
cdrom.add(progress)
elif arguments[0] == 'ident':
cdrom.ident(progress)
else:
sys.stderr.write('E: Invalid operation %s\n' % arguments[0])
sys.exit(1)
After putting all our actions into a main() function, we get a completely
working apt-cdrom clone, which just misses useful --help and --version
options. If we add a function show_help(), we get an even more complete
apt-cdrom clone:
#!/usr/bin/python
import sys
import apt_pkg
import apt
def show_help():
print("apt %s compiled on %s %s" % (apt_pkg.VERSION,
apt_pkg.DATE, apt_pkg.TIME))
if apt_pkg.config.find_b("version"):
return 0
# Copied from apt-cdrom
print("Usage: apt-cdrom [options] command\n"
"\n"
"apt-cdrom is a tool to add CDROM's to APT's source list. The\n"
"CDROM mount point and device information is taken from apt.conf\n"
"and /etc/fstab.\n"
"\n"
"Commands:\n"
" add - Add a CDROM\n"
" ident - Report the identity of a CDROM\n"
"\n"
"Options:\n"
" -h This help text\n"
" -d CD-ROM mount point\n"
" -r Rename a recognized CD-ROM\n"
" -m No mounting\n"
" -f Fast mode, don't check package files\n"
" -a Thorough scan mode\n"
" -c=? Read this configuration file\n"
" -o=? Set an arbitrary configuration option, eg -o "
"dir::cache=/tmp\n"
"See fstab(5)")
return 0
def main(args):
arguments = apt_pkg.parse_commandline(apt_pkg.config,
[('h', "help", "help"),
('v', "version", "version"),
('d', "cdrom", "Acquire::cdrom::mount", "HasArg"),
('r', "rename", "APT::CDROM::Rename"),
('m', "no-mount", "APT::CDROM::NoMount"),
('f', "fast", "APT::CDROM::Fast"),
('n', "just-print", "APT::CDROM::NoAct"),
('n', "recon", "APT::CDROM::NoAct"),
('n', "no-act", "APT::CDROM::NoAct"),
('a', "thorough", "APT::CDROM::Thorough"),
('c', "config-file", "", "ConfigFile"),
('o', "option", "", "ArbItem")], args)
if apt_pkg.config.find_b("help") or apt_pkg.config.find_b("version"):
return show_help()
progress = apt.progress.text.CdromProgress()
cdrom = apt_pkg.Cdrom()
if not arguments:
return show_help()
elif arguments[0] == 'add':
cdrom.add(progress)
elif arguments[0] == 'ident':
cdrom.ident(progress)
else:
sys.stderr.write('E: Invalid operation %s\n' % arguments[0])
return 1
if __name__ == '__main__':
sys.exit(main(sys.argv))