Writing your own apt-cdrom

Author

Julian Andres Klode <jak@debian.org>

Release

2.3.0

Date

Oct 22, 2021

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/python3
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))