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Thin argparse wrapper for quick, clear and easy declaration of hierarchical console command interfaces
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argcmdr

The thin argparse wrapper for quick, clear and easy declaration of (hierarchical) console command interfaces via Python.

argcmdr:

  • handles the boilerplate of CLI
    • while maintaining the clarity and extensibility of your code
    • without requiring you to learn Yet Another argument-definition syntax
    • without reinventing the wheel or sacrificing the flexibility of argparse
  • enables invocation via
    • executable script (__name__ == '__main__')
    • setuptools entrypoint
    • command-defining module (like the Makefile of make)
  • determines command hierarchy flexibly and cleanly
    • command declarations are nested to indicate CLI hierarchy or
    • commands are decorated to indicate their hierarchy
  • includes support for elegant interaction with the operating system, via plumbum

Setup

argcmdr is developed for Python version 3.6.3 and above, and may be built via setuptools.

Python

If Python 3.6.3 or greater is not installed on your system, it is available from python.org.

However, depending on your system, you might prefer to install Python via a package manager, such as Homebrew on Mac OS X or APT on Debian-based Linux systems.

Alternatively, pyenv is highly recommended to manage arbitrary installations of Python, and may be most easily installed via the pyenv installer.

argcmdr

To install from PyPI:

pip install argcmdr

To install from Github:

pip install git+https://github.com/dssg/argcmdr.git

To install from source:

python setup.py install

Tutorial

The Command

argcmdr is built around the base class Command. Your console command extends Command, and optionally defines:

  • __init__(parser), which adds to the parser the arguments that your command requires, as supported by argparse (see argparse)
  • __call__([args, parser, ...]), which is invoked when your console command is invoked, and which is expected to implement your command's functionality

For example, let's define the executable file listdir, a trivial script which prints the current directory's contents:

#!/usr/bin/env python

import os

from argcmdr import Command, main

class Main(Command):
    """print the current directory's contents"""

    def __call__(self):
        print(*os.listdir())

if __name__ == '__main__':
    main(Main)

Should we execute this script, it will perform much the same as ls -A.

Let's say, however, that we would like to optionally print each item of the directory's contents on a separate line:

class Main(Command):
    """print the current directory's contents"""

    def __init__(self, parser):
        parser.add_argument(
            '-1',
            action='store_const',
            const='\n',
            default=' ',
            dest='sep',
            help='list one file per line',
        )

    def __call__(self, args):
        print(*os.listdir(), sep=args.sep)

We now optionally support execution similar to ls -A1, via listdir -1.

Fittingly, this is reflected in the script's autogenerated usage text – listdir -h prints:

usage: listdir [-h] [--tb] [-1]

print the current directory's contents

optional arguments:
  -h, --help         show this help message and exit
  --tb, --traceback  print error tracebacks
  -1                 list one file per line

The command decorator

For particularly trivial commands, the class declaration syntax may be considered verbose and unnecessary. The @cmd decorator manufactures the appropriate Command from a decorated function or method.

The first command may be rewritten to produce an identical result:

from argcmdr import cmd

@cmd
def main():
    """print the current directory's contents"""
    print(*os.listdir())

and, for the second, cmd optionally accepts an argparse argument definition:

@cmd('-1', action='store_const', const='\n', default=' ', dest='sep', help='list one file per line')
def main(args):
    """print the current directory's contents"""
    print(*os.listdir(), sep=args.sep)

Further arguments may be added via additional decoration:

@cmd('-a', ...)
@cmd('-1', ...)
def main(args):
    ...

Local execution

As much as we gain from Python and its standard library, it's quite typical to need to spawn non-Python subprocesses, and for that matter for your script's purpose to be entirely to orchestrate workflows built from operating system commands. Python's – and argcmdr's – benefit is to make this work easier, debuggable, testable and scalable.

In fact, our above, trivial example could be accomplished easily with direct execution of ls:

import argparse

from argcmdr import Local, main

class Main(Local):
    """list directory contents"""

    def __init__(self, parser):
        parser.add_argument(
            'remainder',
            metavar='arguments for ls',
            nargs=argparse.REMAINDER,
        )

    def __call__(self, args):
        print(self.local['ls'](args.remainder))

local, bound to the Local base class, is a dictionary which caches path look-ups for system executables.

This could, however, still be cleaner. For this reason, the Local command features a parallel invocation interface, prepare([args, parser, ...]):

class Main(Local):
    """list directory contents"""

    def __init__(self, parser):
        parser.add_argument(
            'remainder',
            metavar='arguments for ls',
            nargs=argparse.REMAINDER,
        )

    def prepare(self, args):
        return self.local['ls'][args.remainder]

Via the prepare interface, standard output is printed by default, and your command logic may be tested in a "dry run," as reflected in the usage output of the above:

usage: listdir [-h] [--tb] [-q] [-d] [-s] [--no-show] ...

list directory contents

positional arguments:
  arguments for ls

optional arguments:
  -h, --help         show this help message and exit
  --tb, --traceback  print error tracebacks
  -q, --quiet        do not print command output
  -d, --dry-run      do not execute commands, but print what they are (unless
                     --no-show is provided)
  -s, --show         print command expressions (by default not printed unless
                     dry-run)
  --no-show          do not print command expressions (by default not printed
                     unless dry-run)

To execute multiple local subprocesses, prepare may either return an iterable (e.g. list) of the above plumbum bound commands, or prepare may be defined as a generator function, (i.e. make repeated use of yield – see below).

Managing execution

Handling exit codes

Subprocess commands emitted by Local.prepare are executed in order and, by default, failed execution is interrupted by a raised exception:

class Release(Local):
    """release the package to pypi"""

    def __init__(self, parser):
        parser.add_argument(
            'part',
            choices=('major', 'minor', 'patch'),
            help="part of the version to be bumped",
        )

    def prepare(self, args):
        yield self.local['bumpversion'][args.part]
        yield self.local['python']['setup.py', 'sdist', 'bdist_wheel']
        yield self.local['twine']['upload', 'dist/*']

Should the bumpversion command fail, the deploy command will not proceed.

In some cases, however, we might like to disable this functionality, and proceed regardless of a subprocess's exit code. We may pass arguments such as retcode to plumbum by setting this attribute on the prepare method:

def prepare(self, args):
    yield self.local['bumpversion'][args.part]
    yield self.local['python']['setup.py', 'sdist', 'bdist_wheel']
    yield self.local['twine']['upload', 'dist/*']

prepare.retcode = None

Subprocess commands emitted by the above method will not raise execution exceptions, regardless of their exit code. (To allow only certain exit code(s), set retcode as appropriate – see plumbum.)

Inspecting results

Having disabled execution exceptions – and regardless – we might need to inspect a subprocess command's exit code, standard output or standard error. As such, (whether we manipulate retcode or not), argcmdr communicates these command results with prepare generator methods:

def prepare(self, args):
    (code, out, err) = yield self.local['bumpversion']['--list', args.part]

    yield self.local['python']['setup.py', 'sdist', 'bdist_wheel']

    if out is None:
        version = 'DRY-RUN'
    else:
        (version_match,) = re.finditer(
            r'^new_version=([\d.]+)$',
            out,
            re.M,
        )
        version = version_match.group(1)

    yield self.local['twine']['upload', f'dist/*{version}*']

In the above, prepare stores the results of bumpversion execution, in order to determine from its standard output the version to be released.

Handling exceptions

Moreover, we might like to define special handling for execution errors; and, perhaps rather than manipulate retcode for all commands emitted by our method, we might like to handle them separately. As such, execution exceptions are also communicated back to prepare generators:

def prepare(self, args):
    try:
        (_code, out, _err) = yield self.local['bumpversion']['--list', args.part]
    except self.local.ProcessExecutionError:
        print("execution failed but here's a joke ...")
        ...
Modifying execution

Commands are run in the foreground by default, their outputs printed, as well as recorded for inspection, via the plumbum modifier, TEE.

To execute a command in the background (and continue), we may specify the BG modifier:

def prepare(self, args):
    future = yield (self.local.BG, self.local['bumpversion']['--list', args.part])

Alternatively, we may wish to execute a command in the foreground only, (and not record its output) – e.g. to best support processes which require TTY:

def prepare(self):
    return (self.local.FG, self.local['ipython']['-i', 'startup.py'])

The local decorator

Local is an alternate command base class, and a subclass of Command. Any base class may be substituted for Command when using the command decorator:

@cmd(base=CustomCommand)
def main():
    ...

Moreover, Local functionality may be requested via keyword flag local:

@cmd(local=True)
def main(self):
    ...

And in support of the above, common case, the @local decorator is provided:

from argcmdr import local

@local
def main(self):
    ...

Note that in the last two examples, our command function's call signature included self.

Decorated command functions are in fact replaced with manufactured subclasses of Command, and the function is invoked as this command's functionality – either __call__ or prepare. It is assumed that, by default, this function should be treated as a staticmethod, and given no reference to the manufactured Command instance. However, in the case of local decoration, this is not the case; the binding is left up to the decorated object, which, according to Python descriptor rules, means that a decorated function is treated as a "method" and receives the instance. This way, local command functions may access the instance's local dictionary of operating system executables.

Binding may be explicitly controlled via the decorator keyword binding, e.g.:

@cmd(binding=True, base=CustomCommand)
def main(self):
    ...

See Method commands for further examples of decorator-defined commands and alternative bindings.

Command invocation signature

Note that in our last trivial examples of listing directory contents, we made our script dependent upon the ls command in the operating environment. argcmdr will not, by default, print tracebacks, and it will colorize unhandled exceptions; however, we might prefer to print a far friendlier error message.

One easy way of printing friendly error messages is to make use of argparse.ArgumentParser.error(). As we've seen, Command invocation, via either __call__ or prepare, may accept zero arguments, or it may require the parsed arguments argparse.Namespace. Moreover, it may require a second argument, and receive the argument parser:

class Main(Local):
    """list directory contents"""

    def __init__(self, parser):
        parser.add_argument(
            'remainder',
            metavar='arguments for ls',
            nargs=argparse.REMAINDER,
        )

    def prepare(self, args, parser):
        try:
            local_exec = self.local['ls']
        except self.local.CommandNotFound:
            parser.error('command not available')

        yield local_exec[args.remainder]

If ls is not available, the user is presented the following message upon executing the above:

usage: listdir [-h] [--tb] [-q] [-d] [-s] [--no-show] ...
listdir: error: command not available

Access to the parsed argument namespace

The command invocation's parsed arguments are most straight-forwardly accessible as the first argument of the Command invocation signature, either __call__ or prepare. However, in less-than-trivial implementations, wherein command methods are factored for reusability, passing the argument namespace from method to method may become tedious. To support such scenarios, this object is made additionally available via the Command property, args.

Consider a class of commands which require a database password. We don't want to store this password anywhere in plain text; rather, we expect it to be input, either via (piped) standard input or the TTY:

class DbSync(Command):
    """sync databases"""

    def __init__(self, parser):
        parser.add_argument(
            '-p', '--password',
            action='store_true',
            dest='stdin_password',
            default=False,
            help="read database password from standard input",
        )

    def __call__(self, args):
        engine = self.dbengine(args)
        ...

    def dbcreds(self, args):
        dbcreds = {
            'username': os.getenv('PGUSER'),
            'host': os.getenv('PGHOST'),
            'port': os.getenv('PGPORT'),
            'database': os.getenv('PGDATABASE'),
        }

        missing = [key for (key, value) in dbcreds.items() if not value]
        if missing:
            raise RuntimeError(
                "database connection information missing from "
                "environmental configuration: " + ', '.join(missing)
            )

        if args.stdin_password:
            dbcreds['password'] = sys.stdin.read().rstrip('\n\r')

            # we're done with the (pipe) stdin, so force it back to TTY for
            # any subsequent input()
            sys.stdin = open('/dev/tty')
        else:
            dbcreds['password'] = os.getenv('PGPASSWORD')
            if not dbcreds['password']:
                dbcreds['password'] = getpass.getpass(
                    'enter password for '
                    + ('{username}@{host}:{port}'.format_map(dbcreds) | colors.bold)
                    + ': '
                    | colors.yellow
                )

        return dbcreds

    def dburi(self, args):
        return sqlalchemy.engine.url.URL('postgres', **self.dbcreds(args))

    def dbengine(self, args):
        return sqlalchemy.create_engine(self.dburi(args))

Not only were we forced to verbosely daisy-chain the arguments namespace, args, from method to method; moreover, we were prevented from (trivially) caching the result of dbcreds, to ensure that the password isn't ever requested more than once.

Now, let's reimplement the above, making use of the property args:

class DbSync(Command):
    """sync databases"""

    def __init__(self, parser):
        parser.add_argument(
            '-p', '--password',
            action='store_true',
            dest='stdin_password',
            default=False,
            help="read database password from standard input",
        )

    def __call__(self):
        engine = self.dbengine
        ...

    @cachedproperty
    def dbcreds(self):
        dbcreds = {
            'username': os.getenv('PGUSER'),
            'host': os.getenv('PGHOST'),
            'port': os.getenv('PGPORT'),
            'database': os.getenv('PGDATABASE'),
        }

        missing = [key for (key, value) in dbcreds.items() if not value]
        if missing:
            raise RuntimeError(
                "database connection information missing from "
                "environmental configuration: " + ', '.join(missing)
            )

        if self.args.stdin_password:
            dbcreds['password'] = sys.stdin.read().rstrip('\n\r')

            # we're done with the (pipe) stdin, so force it back to TTY for
            # any subsequent input()
            sys.stdin = open('/dev/tty')
        else:
            dbcreds['password'] = os.getenv('PGPASSWORD')
            if not dbcreds['password']:
                dbcreds['password'] = getpass.getpass(
                    'enter password for '
                    + ('{username}@{host}:{port}'.format_map(dbcreds) | colors.bold)
                    + ': '
                    | colors.yellow
                )

        return dbcreds

    @property
    def dburi(self):
        return sqlalchemy.engine.url.URL('postgres', **self.dbcreds)

    @property
    def dbengine(self):
        return sqlalchemy.create_engine(self.dburi)

In this form, args needn't be passed from method to method; in fact, methods of the DbSync command needn't worry about arguments which don't directly interest them at all. And, using cachedproperty from Dickens, the database credentials are trivially cached, ensuring they aren't needlessly re-requested.

Note that attempting to access the args property before invocation arguments have been parsed – e.g. within __init__ – is not allowed, and will raise RuntimeError.

Command hierarchy

Our tools should be modular and composable, favoring atomicity over monolithism. Nevertheless, well-designed, -structured and -annotated code and application interfaces pay their users and developers tremendous dividends over time – no less in the case of more extensive interfaces, and particularly so for project management libraries (consider the Makefile).

argcmdr intends to facilitate the definition of argparse-based interfaces regardless of their structure. But it's in multi-level, or hierarchical, command argumentation that argcmdr shines.

Nested commands

Rather than procedurally defining subparsers, Command class declarations may simply be nested.

Let's define an executable file manage for managing a codebase:

#!/usr/bin/env python

import os

from argcmdr import Local, main

class Management(Local):
    """manage deployment"""

    def __init__(self, parser):
        parser.add_argument(
            '-e', '--env',
            choices=('development', 'production'),
            default='development',
            help="target environment",
        )

    class Build(Local):
        """build app"""

        def prepare(self, args):
            req_path = os.path.join('requirements', f'{args.env}.txt')
            yield self.local['pip']['-r', req_path]

    class Deploy(Local):
        """deploy app"""

        def prepare(self, args):
            yield self.local['eb']['deploy', args.env]

if __name__ == '__main__':
    main(Management)

Local command Management, above, defines no functionality of its own. As such, executing manage without arguments prints its autogenerated usage:

usage: manage [-h] [--tb] [-q] [-d] [-s] [--no-show]
              [-e {development,production}]
              {build,deploy} ...

Because Management extends Local, it inherits argumentation controlling whether standard output is printed and offering to run commands in "dry" mode. (Note, however, that it could have omitted these options by extending Command. Moreover, it may override class method base_parser().)

Management adds to the basic interface the optional argument --env. Most important, however, are the related, nested commands Build and Deploy, which define functionality via prepare. Neither nested command extends its subparser – though they could; but rather, they depend upon the common argumentation defined by Management.

Exploring the interface via --help tells us a great deal, for example manage -h:

usage: manage [-h] [--tb] [-q] [-d] [-s] [--no-show]
              [-e {development,production}]
              {build,deploy} ...

manage deployment

optional arguments:
  -h, --help            show this help message and exit
  --tb, --traceback     print error tracebacks
  -q, --quiet           do not print command output
  -d, --dry-run         do not execute commands, but print what they are
                        (unless --no-show is provided)
  -s, --show            print command expressions (by default not printed
                        unless dry-run)
  --no-show             do not print command expressions (by default not
                        printed unless dry-run)
  -e {development,production}, --env {development,production}
                        target environment

management commands:
  {build,deploy}        available commands
    build               build app
    deploy              deploy app

And manage deploy -h:

usage: manage deploy [-h]

deploy app

optional arguments:
  -h, --help  show this help message and exit

As such, a "dry run":

manage -de production deploy

prints the following:

> /home/user/.local/bin/eb deploy production

and without the dry-run flag the above operating system command is executed.

Root commands

There is no artificial limit to the number of levels you may add to your command hierarchy. However, application interfaces are commonly "wider" than they are "deep". For this reason, as an alternative to class-nesting, the hierarchical relationship may be defined by a class decorator provided by the RootCommand.

Let's define the executable file git with no particular purpose whatsoever:

#!/usr/bin/env python

from argcmdr import Command, RootCommand, main

class Git(RootCommand):
    """another stupid content tracker"""

    def __init__(self, parser):
        parser.add_argument(
            '-C',
            default='.',
            dest='path',
            help="run as if git was started in <path> instead of the current "
                 "working directory.",
        )

@Git.register
class Stash(Command):
    """stash the changes in a dirty working directory away"""

    def __call__(self, args):
        self['save'](args)

    class Save(Command):
        """save your local modifications to a new stash"""

        def __init__(self, parser):
            parser.add_argument(
                '-p', '--patch',
                dest='interactive',
                action='store_true',
                default=False,
                help="interactively select hunks from the diff between HEAD "
                     "and the working tree to be stashed",
            )

        def __call__(self, args):
            interactive = getattr(args, 'interactive', False)
            print("stash save", f"(interactive: {interactive})")

    class List(Command):
        """list the stashes that you currently have"""

        def __call__(self):
            print("stash list")

if __name__ == '__main__':
    main(Git)

We anticipate adding many subcommands to git beyond stash; and so, rather than nest all of these command classes under Git:

  • we've defined Git as a RootCommand
  • we've declared Stash at the module root
  • we've decorated Stash with Git.register

The RootCommand functions identically to the Command; it only adds this ability to extend the listing of its subcommands by those registered via its decorator. (Notably, LocalRoot composes the functionaliy of Local and RootCommand via multiple inheritance.)

The stash command, on the other hand, has opted to contain the entirety of its hierarchical functionality, nesting its own subcommands list and save.

Nevertheless, you are not limited to a single RootCommand. Any command whose hierarchy you would like to extend via the register decorator may inherit it. Moreover, the @cmd decorator accepts the keyword flag root.

Method commands

Decorator-manufactured commands are no less capable than those derived from class declaration syntax, except in that other commands cannot, syntactically, be nested beneath them. (For that reason the @cmd decorator's root flag is of note.) Decorator-manufactured commands can nonetheless themselves extend hierarchies, either by being further decorated by register or nested under command class declarations:

@Git.register
class Stash(Command):
    """stash the changes in a dirty working directory away"""

    def __call__(self, args):
        self['save'](args)

    @cmd('-p', '--patch', dest='interactive', action='store_true', default=False,
         help="interactively select hunks from the diff between HEAD "
              "and the working tree to be stashed")
    def save(args):
        """save your local modifications to a new stash"""
        interactive = getattr(args, 'interactive', False)
        print("stash save", f"(interactive: {interactive})")

    @cmd
    def list():
        """list the stashes that you currently have"""
        print("stash list")

Above we've rewritten the trivial stash commands save and list as @cmd-decorated functions.

Say, however, that we needed to invert the factoring of save logic between that command and its parent:

@Git.register
class Stash(Command):
    """stash the changes in a dirty working directory away"""

    def perform_save(self, args):
        interactive = getattr(args, 'interactive', False)
        print("stash save", f"(interactive: {interactive})")

    def __call__(self, args):
        self.perform_save(args)

    @cmd('-p', '--patch', dest='interactive', action='store_true', default=False,
         help="interactively select hunks from the diff between HEAD "
              "and the working tree to be stashed")
    @cmd(binding=True)
    def save(self, args):
        """save your local modifications to a new stash"""
        self[-1].perform_save(args)

    @cmd
    def list():
        """list the stashes that you currently have"""
        print("stash list")

(Note that cmd can accept both an argparse argument specification and command feature-defining arguments at once; however, this is of use mainly to the definition of helpers such as the local decorator, as this style is difficult to read and otherwise discouraged. Moreover, only the firsti.e. inner-most – cmd decorator's command features are respected.)

In this version, save functionality is shared as a method of Stash. save is able to access this method only by ascending the command hierarchy. This might make particular sense when multiple nested commands must share functionality, which is defined on the command class under which they are nested. (Note, however, that in such a case as this one, where the shared method could be defined as a staticmethod, it is no less advisable to do so, and for nested commands to access it directly as, e.g. Stash.perform_save.)

Our above reference to self in save, however, is at first glance misleading. This command looks like an instance method of Stash; yet, it's its own Command, and the save function receives as its first invocation argument an instance of the Command class save. Moreover, in this case, save gains nothing from this self-reference; its class defines no special attributes or functionality of its own beyond argument-parsing.

To improve on the above, we may instead decorate our command function with cmdmethod:

@Git.register
class Stash(Command):
    """stash the changes in a dirty working directory away"""

    def perform_save(self, args):
        interactive = getattr(args, 'interactive', False)
        print("stash save", f"(interactive: {interactive})")

    def __call__(self, args):
        self.perform_save(args)

    @cmdmethod('-p', '--patch', dest='interactive', action='store_true', default=False,
               help="interactively select hunks from the diff between HEAD "
                    "and the working tree to be stashed")
    def save(self, args):
        """save your local modifications to a new stash"""
        self.perform_save(args)

The cmdmethod decorator – as well as the complementary localmethod decorator – alter the binding of the decorated function such that it receives the instance of its parent command – not itself – upon invocation. Much cleaner.

As with the local decorator, cmdmethod is merely a wrapper of cmd. Identical functionality can be achieved via the binding keyword, though far more verbosely:

from argcmdr import CommandDecorator

@cmd(binding=CommandDecorator.Binding.parent)
def save(self, args):
    ...

Walking the hierachy

Unlike the base command git in the example above, the command git stash – despite defining its own subcommands – also defines its own functionality, via __call__. This functionality, however, is merely a shortcut to the stash command save. Rather than repeat the definition of this functionality, Stash "walks" its hierarchy to access the instantiation of Save, and invokes this command by reference.

Much of argcmdr is defined at the class level, and as such many Command methods are classmethod. In the static or class context, we might walk the command hierarchy by reference, e.g. to Stash.Save; or, from a class method of Stash, as cls.Save. Moreover, Command defines the class-level "property" subcommands, which returns a list of Command classes immediately "under" it in the hierarchy.

The hierarchy of executable command objects, however, is instantiated at runtime and cached within the Command instance. To facilitate navigation of this hierarchy, the Command object is itself subscriptable. Look-up keys may be:

  • strings – descend the hierarchy to the named command
  • negative integers – ascend the hierarchy this many levels
  • a sequence combining the above – to combine "steps" into a single action

In the above example, Stash may have (redundantly) accessed Save with the look-up key:

(-1, 'stash', 'save')

that is with the full expression:

self[-1, 'stash', 'save']

(The single key 'save', however, was far more to the point.)

Because command look-ups are relative to the current command, Command also offers the property root, which returns the base command. As such, our redundant expression could be rewritten:

self.root['stash', 'save']

The management file

In addition to the interface of custom executables, argcmdr endeavors to improve the generation and maintainability of non-executable but standardized files, intended for management of code development projects and operations.

Similar to a project's Makefile, we might define our previous codebase-management file as the following Python module, manage.py:

import os

from argcmdr import Local, main

class Management(Local):
    """manage deployment"""

    def __init__(self, parser):
        parser.add_argument(
            '-e', '--env',
            choices=('development', 'production'),
            default='development',
            help="target environment",
        )

    class Build(Local):
        """build app"""

        def prepare(self, args):
            req_path = os.path.join('requirements', f'{args.env}.txt')
            yield self.local['pip']['-r', req_path]

    class Deploy(Local):
        """deploy app"""

        def prepare(self, args):
            yield self.local['eb']['deploy', args.env]

Unlike our original script, manage, manage.py is not executable, and need define neither an initial shebang line nor a final __name__ == '__main__' block.

Rather, argcmdr supplies its own, general-purpose manage executable command, which loads Commands from any manage.py in the current directory, or as specified by option --manage-file PATH. As such, the usage and functionality of our manage.py, as invoked via argcmdr's installed manage command, is identical to our original manage. We need only ensure that argcmdr is installed, in order to make use of it to manage any or all project tasks, in a standard way, with even less boilerplate.

Entrypoint definition

In lieu of an explicitly defined execution path, manage infers the base command – and hence the entrypoint – of the manage.py management file module.

The entrypoint of a management file defining – at the module level – only one Command, or multiple commands but only one RootCommand, is assumed to be this one command. Otherwise, the intended entrypoint must be decorated with @entrypoint:

from argcmdr import entrypoint, RootCommand

class GoodCommand(RootCommand):

    def good_function(self):
        ...

@entrypoint
class CommandEhh(GoodCommand):

    def __call__(self):
        self.good_function()
        ...

@CommandEhh.register
class CommandBeh(GoodCommand):

    def __call__(self):
        self.good_function()
        ...

We may infer from the above that GoodCommand is merely a base class extension, and that the module's CLI begins with the most "root" command, CommandEhh, which is extended by CommandBeh. However, rather than go out on a limb, when presented with these three subclasses of Command and RootCommand, argcmdr requires that the intended entrypoint is explicitly marked.

Note, however, that only commands declared at the module, or "top" level, are considered potential entrypoints:

class CommandEhh(Command):

    class CommandBeh(Command):

        ...

Presented with a module containing only the above commands, argcmdr would identify CommandEhh as the entrypoint; CommandBeh would never be considered, even if decorated @entrypoint.

The management package

Python packages, no less than stand-alone modules, may also be defined for use with the manage command, to aid in maintenance and development.

Consider the following example directory layout:

manage/
├── __init__.py
├── cloud.py
├── db.py
├── main.py
├── morale.py
├── server.py
└── util.py

argcmdr will load the above top-level Python module, manage, just as well as it would the manage module defined by a manage.py file, (whether these are available on the PYTHONPATH or not).

Furthermore, detecting that this manage is in fact a package, argcmdr will automatically and recursively load all of the modules this package contains.

This allows the developer to provide argcmdr the minimum that it requires at manage/__init__.py – access to an interface entrypoint, i.e. the base Command – and to organize the development of that interface in whatever maintainable way suits them.

To wit, the developer simply might write, in manage/__init__.py:

from .main import Main  # noqa

(…And they will have no need of the @entrypoint decorator, as argcmdr will only see the one top-level command.)

Of course, that top-level command might have been defined in __init__.py, or as you might prefer, in manage/main.py:

from argcmdr import RootCommand

class Main(RootCommand):
    """your one-stop shop for devops"""

    ...

And, each subcommand may be defined in a submodule, such as manage/cloud.py:

from argcmdr import Command

from .main import Main

@Main.register
class Cloud(Command):
    """manage cloud computing resources"""

    ...

Thanks to automatic loading, the Cloud subcommand, (which will resolve to manage cloud), will be picked up, without additional boilerplate and without needing to consider circular imports.

To disable automatic submodule loading, set the following in manage/__init__.py:

__auto_init_package__ = False

And to make (custom) use of this feature, see: argcmdr.init_package().

Bootstrapping

To ensure that such a friendly – and relatively high-level – project requirement as argcmdr is satisfied, consider the expressly low-level utility install-cli, with which to guide contributors through the process of provisioning your project's most basic requirements.

Shell completion

argcmdr supports shell command argument completion via argcomplete (see argcomplete).

As explained by its documentation, your user (perhaps in executing the installation of your command), may enable argument completion, either:

  • specifically for your shell command
  • or generally for any script containing the string PYTHON_ARGCOMPLETE_OK in its first 1024 bytes

For flexibility, (and, e.g., in support of installation into virtual environments, or otherwise where system- or user-global installation is undesirable or impossible), argcmdr does not currently insist on a particular scheme to enable argument completion.

Rather, for example, to enable argument completion system-wide, specifically for the manage command (provisioned by argcmdr), you might execute the following from a Bash shell (as the root user):

register-python-argcomplete --shell bash manage > /etc/bash_completion.d/python-argcomplete-manage.sh

Alternatively, the same argument completion may be enabled, but only for the current user:

mkdir -p ~/.bash_completion.d
register-python-argcomplete --shell bash manage > ~/.bash_completion.d/python-argcomplete-manage.sh

Only in the latter case, the user must have the file ~/.bash_completion, including contents of the following form:

if [ -d ~/.bash_completion.d/ ] && [ ! -z "$(ls ~/.bash_completion.d/)" ]; then
  for bcfile in ~/.bash_completion.d/*; do
    . "$bcfile"
  done
fi

(Bash will load this file automatically.)

In the case that neither system-wide nor user-only installation is appropriate, the same argument completion may be enabled, but only for the current shell:

eval "$(register-python-argcomplete --shell bash manage)"

Regardless of the method, having so enabled argument completion (for your command), in your shell, argcmdr will handle the rest, generating completion suggestions based on your command definition.

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