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Command-Line Interface (CLI)

The other side of executing programs with ease is writing CLI programs with ease. Python scripts normally use optparse or the more recent argparse, and their derivatives; but all of these are somewhat limited in their expressive power, and are quite unintuitive (and even unpythonic). Plumbum's CLI toolkit offers a programmatic approach to building command-line applications; instead of creating a parser object and populating it with a series of "options", the CLI toolkit translates these primitives into Pythonic constructs and relies on introspection.

From a bird's eye view, CLI applications are classes that extend :class:`plumbum.cli.Application`. They define a main() method and optionally expose methods and attributes as command-line :func:`switches <plumbum.cli.switch>`. Switches may take arguments, and any remaining positional arguments are given to the main method, according to its signature. A simple CLI application might look like this:

from plumbum import cli

class MyApp(cli.Application):
    verbose = cli.Flag(["v", "verbose"], help = "If given, I will be very talkative")

    def main(self, filename):
        print "I will now read", filename
        if self.verbose:
            print "Yadda " * 200

if __name__ == "__main__":

And you can run it:

$ python foo
I will now read foo

$ python --help v1.0

Usage: [SWITCHES] filename
    -h, --help                 Prints this help message and quits
    --version                  Prints the program's version and quits

    -v, --verbose              If given, I will be very talkative

So far you've only seen the very basic usage. We'll now start to explore the library.


The :class:`Application <plumbum.cli.Application>` class is the "container" of your application. It consists of the main() method, which you should implement, and any number of CLI-exposed switch functions or attributes. The entry-point for your application is the classmethod run, which instantiates your class, parses the arguments, invokes all switch functions, and then calls main() with the given positional arguments. In order to run your application from the command-line, all you have to do is

if __name__ == "__main__":

Aside from run() and main(), the Application class exposes two built-in switch functions: help() and version() which take care of displaying the help and program's version, respectively. By default, --help and -h invoke help(), and --version and -v invoke version(); if any of these functions is called, the application will display the message and quit (without processing any other switch).

You can customize the information displayed by help() and version by defining class-level attributes, such as PROGNAME, VERSION and DESCRIPTION. For instance,

class MyApp(cli.Application):
    PROGNAME = "Foobar"
    VERSION = "7.3"

Switch Functions

The decorator :func:`switch <plumbum.cli.switch>` can be seen as the "heart and soul" of the CLI toolkit; it exposes methods of your CLI application as CLI-switches, allowing them to be invoked from the command line. Let's examine the following toy application:

class MyApp(cli.Application):
    @cli.switch("--log-to-file", str)
    def log_to_file(self, filename):
        """Sets the file into which logs will be emitted"""

    @cli.switch(["-r", "--root"])
    def allow_as_root(self):
        """If given, allow running as root"""
        self._allow_root = True

    def main(self):
        if os.geteuid() == 0 and not self._allow_root:
            raise ValueError("cannot run as root")

When the program is run, the switch functions are invoked with their appropriate arguments; for instance, $ ./ --log-to-file=/tmp/log would translate to a call to app.log_to_file("/tmp/log"). After all switches were processed, control passes to main.


Methods' docstrings and argument names will be used to render the help message, keeping your code as DRY as possible.

There's also :func:`autoswitch <plumbum.cli.autoswitch>`, which infers the name of the switch from the function's name, e.g.

def log_to_file(self, filename):

Will bind the add the switch function to --log-to-file.


As demonstrated in the example above, switch functions may take no arguments (not counting self) or a single argument argument. If a switch function accepts an argument, it must specify the argument's type. If you require no special validation, simply pass str; otherwise, you may pass any type (or any callable, in fact) that will take a string and convert it to a meaningful object. If conversion is not possible, the type (or callable) is expected to raise either TypeError` or ``ValueError.

For instance

class MyApp(cli.Application):
    _port = 8080

    @cli.switch(["-p"], int)
    def server_port(self, port):
        self._port = port

    def main(self):
        print self._port
$ ./ -p 17
$ ./ -p foo
Argument of -p expected to be <type 'int'>, not 'foo':
    ValueError("invalid literal for int() with base 10: 'foo'",)

The toolkit includes two additional "types" (or rather, validators): Range and Set. Range takes a minimal value and a maximal value and expects an integer in that range (inclusive). Set takes a set of allowed values, and expects the argument to match one of these values. Here's an example

class MyApp(cli.Application):
    _port = 8080
    _mode = "TCP"

    @cli.switch("-p", cli.Range(1024,65535))
    def server_port(self, port):
        self._port = port

    @cli.switch("-m", cli.Set("TCP", "UDP", case_sensitive = False))
    def server_mode(self, mode):
        self._mode = mode

    def main(self):
        print self._port, self._mode
$ ./ -p 17
Argument of -p expected to be [1024..65535], not '17':
    ValueError('Not in range [1024..65535]',)
$ ./ -m foo
Argument of -m expected to be Set('udp', 'tcp'), not 'foo':
    ValueError("Expected one of ['UDP', 'TCP']",)


The toolkit also provides some other useful validators: ExistingFile (ensures the given argument is an existing file), ExistingDirectory (ensures the given argument is an existing directory), and NonexistentPath (ensures the given argument is not an existing path). All of these convert the argument to a :ref:`local path <guide-paths>`.

Repeatable Switches

Many times, you would like to allow a certain switch to be given multiple times. For instance, in gcc, you may give several include directories using -I. By default, switches may only be given once, unless you allow multiple occurrences by passing list = True to the switch decorator

class MyApp(cli.Application):
    _dirs = []

    @cli.switch("-I", str, list = True)
    def include_dirs(self, dirs):
        self._dirs = dirs

    def main(self):
        print self._dirs
$ ./ -I/foo/bar -I/usr/include
['/foo/bar', '/usr/include']


The switch function will be called only once, and its argument will be a list of items

Mandatory Switches

If a certain switch is required, you can specify this by passing mandatory = True to the switch decorator. The user will not be able to run the program without specifying a value for this switch.


Many time, the occurrence of a certain switch depends on the occurrence of another, e..g, it may not be possible to give -x without also giving -y. This constraint can be achieved by specifying the requires keyword argument to the switch decorator; it is a list of switch names that this switch depends on. If the required switches are missing, the user will not be able to run the program.

class MyApp(cli.Application):
    @cli.switch("--log-to-file", str)
    def log_to_file(self, filename):

    @cli.switch("--verbose", requires = ["--log-to-file"])
    def verbose(self):
$ ./example --verbose
Given --verbose, the following are missing ['log-to-file']


The toolkit invokes the switch functions in the same order in which the switches were given on the command line. It doesn't go as far as computing a topological order on the, but this will change in the future.

Mutual Exclusion

Just as some switches may depend on others, some switches mutually-exclude others. For instance, it does not make sense to allow --verbose and --terse. For this purpose, you can set the excludes list in the switch decorator.

class MyApp(cli.Application):
    @cli.switch("--log-to-file", str)
    def log_to_file(self, filename):

    @cli.switch("--verbose", requires = ["--log-to-file"], excludes = ["--terse"])
    def verbose(self):

    @cli.switch("--terse", requires = ["--log-to-file"], excludes = ["--verbose"])
    def terse(self):
$ ./example --log-to-file=log.txt --verbose --terse
Given --verbose, the following are invalid ['--terse']


If you wish to group certain switches together in the help message, you can specify group = "Group Name", where Group Name is any string. When the help message is rendered, all the switches that belong to the same group will be grouped together. Note that grouping has no other effects on the way switches are processed, but it can help improve the readability of the help message.

Switch Attributes

Many times it's desired to simply store a switch's argument in an attribute, or set a flag if a certain switch is given. For this purpose, the toolkit provides :class:`SwitchAttr <plumbum.cli.SwitchAttr>`, which is data descriptor that stores the argument in an instance attribute. There are two additional "flavors" of SwitchAttr: Flag (which toggles its default value if the switch is given) and CountingAttr (which counts the number of occurrences of the switch)

class MyApp(cli.Application):
    log_file = cli.SwitchAttr("--log-file", str, default = None)
    enable_logging = cli.Flag("--no-log", default = True)
    verbosity_level = cli.CountingAttr("-v")

    def main(self):
        print self.log_file, self.enable_logging, self.verbosity_level
$ ./ -v --log-file=log.txt -v --no-log -vvv
log.txt False 5


The main() method takes control once all the command-line switches have been processed. It may take any number of positional argument; for instance, in cp -r /foo /bar, /foo and /bar are the positional arguments. The number of positional arguments that the program would accept depends on the signature of the method: if the method takes 5 arguments, 2 of which have default values, then at least 3 positional arguments must be supplied by the user and at most 5. If the method also takes varargs (*args), the number of arguments that may be given is unbound

class MyApp(cli.Application):
    def main(self, src, dst, mode = "normal"):
        print src, dst, mode
$ ./ /foo /bar
/foo /bar normal
$ ./ /foo /bar spam
/foo /bar spam
$ ./ /foo
Expected at least 2 positional arguments, got ['/foo']
$ ./ /foo /bar spam bacon
Expected at most 3 positional arguments, got ['/foo', '/bar', 'spam', 'bacon']


The method's signature is also used to generate the help message, e.g.

Usage:  [SWITCHES] src dst [mode='normal']

With varargs:

class MyApp(cli.Application):
    def main(self, src, dst, *eggs):
        print src, dst, eggs
$ ./ a b c d
a b ('c', 'd')
$ ./ --help
Usage:  [SWITCHES] src dst eggs...
    -h, --help                 Prints this help message and quits
    -v, --version              Prints the program's version and quits
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