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An interactive console application library for Python

Project Description


PyMander (short for Python Commander) is a library for writing interactive command-line interface (CLI) applications in Python.

Quick Start

Let’s say, we need a CLI app that has two commands: date and time that print the current date and time respectively. Then you would do something like this:

import time
from pymander.handlers import LineHandler
from pymander.exceptions import CantParseLine
from pymander.shortcuts import run_with_handler

class DatetimeLineHandler(LineHandler):
    def try_execute(self, line):
        if line.strip() == 'time':
        elif line.strip() == 'date':
            raise CantParseLine(line)


And you’ll get… (just type exit to exit the loop)

>>> date
>>> time
>>> exit

Let’s spice things up and add some time travel functionality to your app. Adding a lot of commands to the same function as if-blocks is not a very good idea, besides you might want to keep warping of the Universe separate from the code that just shows the date and time, so go ahead and create a new handler:

import re

class TimeTravelLineHandler(LineHandler):
    def try_execute(self, line):
        cmd_match = re.match('go to date (?P<new_date>.*?)\s*$', line)
        if cmd_match:
            new_date = line.split(' ', 2)[-1]
            self.context.write('Traveling to date: {0}\n'.format('new_date')))
            raise CantParseLine(line)

At this point we have a problem: how do we use the two handlers in our app simultaneously?

Command contexts are a way of combining several handlers in a single scope so that they can work together. Having said that, let’s run it using a StandardPrompt command context:

from pymander.contexts import StandardPrompt
from pymander.shortcuts import run_with_context


And back to the future we go!

>>> date
>>> go to date October 10 2058
Traveling to date: October 10 2058

It’s worth mentioning that run_with_handler(handler) is basically a shortcut for run_with_context(StandardPrompt([handler])).

StandardPrompt is a simple command context that includes the following features:

  • prints the ">>> " when prompting for a new command
  • writes “Invalid command: …” when it cannot recognize a command
  • adds the EchoLineHandler and ExitLineHandler handlers, which implement the echo and exit commands, which do pretty much what you expect them to do

More Examples

Moving on to more complicated examples…

Using regular expresssions (RegexLineHandler)


from pymander.decorators import bind_command

class BerryLineHandler(RegexLineHandler):
    @bind_command(r'pick a (?P<berry_kind>\w+)')
    def pick_berry(self, berry_kind):
        self.context.write('Picked a {0}\n'.format(berry_kind))

    @bind_command(r'make (?P<berry_kind>\w+) jam')
    def make_jam(self, berry_kind):
        self.context.write('Made some {0} jam\n'.format(berry_kind))


>>> pick a strawberry
Picked a strawberry
>>> make blueberry jam
Made some blueberry jam

Using argparse (ArgparseLineHandler)


from pymander.decorators import bind_command

class GameLineHandler(ArgparseLineHandler):
    @bind_command('play', [
        ['game', {'type': str, 'default': 'nothing'}],
        ['--well', {'action': 'store_true'}],
    def play(self, game, well):
        self.context.write('I play {0}{1}\n'.format(game, ' very well' if well else ''))

    def win(self):
        self.context.write('I just won!\n')


>>> play chess --well
I play chess very well
>>> play monopoly
I play monopoly
>>> win
I just won!

Combining argparse and regexes using PrebuiltCommandContext

Sometimes you might find it useful to be able to use both approaches together or be able to switch from one to another without making a mess of a whole bunch of handlers.

PrebuiltCommandContext allows you to use decorators to assign its own methods as either argparse or regex commands in a single (command context) class without having to define the handlers yourself:

from pymander.contexts import PrebuiltCommandContext, StandardPrompt
from pymander.shortcuts import run_with_context
from pymander.decorators import bind_argparse, bind_regex

class SaladContext(PrebuiltCommandContext, StandardPrompt):
    @bind_regex(r'(?P<do_what>eat|cook) caesar')
    def caesar_salad(self, do_what):
        self.write('{0}ing caesar salad...\n'.format(do_what.capitalize()))

    @bind_argparse('buy', [
        ['--price', '-p', {'default': None}]
    def buy_salad(self, kind_of_salad, price):
        self.write('Buying {0} salad{1}...\n'.format(
            kind_of_salad, ' for {0}'.format(price) if price else '')



>>> cook caesar
Cooking caesar salad...
>>> buy greek
Buying greek salad...
>>> buy russian --price $5
Buying russian salad for $5...

The PrebuiltCommandContext class can be used with three decorators for assigning methods to specific handlers:

  • bind_exact(command) binds to ExactLineHandler (matches the line exactly to the specified string, e.g. the exit command)
  • bind_argparse(command, options) binds to ArgparseLineHandler (uses argparse to evaluate the line)
  • bind_regex(regex) binds to RegexLineHandler (matches the line to regular expressions)

and one generic decorator:

  • bind_to_handler(handler_class, *bind_args, **bind_kwargs)

binds to any given LineHandler subclass. The handler class can then access its autogenerated methods via the self.command_methods attribute:

class MyLineHandler(LineHandler):
    def try_execute(self, line):
        for command_info in self.command_methods:
            # where: command_info = {"method": <callable>, "args": <bind_args>, "kwargs": <bind_kwargs>}
            # your logic goes here:
            #     determine whether <line> matches the <args> and <kwargs> options)
            #     and call the callable if it does

        # if no suitable match was found:
        raise CantParseLine

And then use it like this:

class MyPrebuiltContext(PrebuiltCommandContext, StandardPrompt):
    @bind_to_handler(MyLineHandler, 'some', 'arguments')
    def do_whatever(self, *your_method_args):
        self.write('Whatever, bro\n')

At this point you might be wondering, why we always also use StandardPrompt when inheriting from PrebuiltCommandContext. That’s because PrebuiltCommandContext is an abstract class and does not implement some of the required CommandContext methods. So this is where I’d normally send you to the full documentation of the project, but it’s not finished yet, so, for now, you can just browse the source code of the examples and the pymander package itself :)

Using Nested Contexts

An obvious extension would be the ability to enter a new context on some commands and then exit them (multi-step commands, entering and exiting a file editor, etc.). All you have to do to use this is return an instance of a new CommandContext from your command, and you’re in! Just don’t forget to supply this context with an exit, or you’ll be stuck in there forever.

See DeeperLineHandler in the simple example.

Using Multiline Commands (text input)

Check out the multi and fswalk examples.

Major TODOs

Here I’ll be listing some of the major fetures that are not yet implemented, but are crucial to the library’s usability.

  1. an easy to use help mechanism. It should be able to list possible commands and how they should be used (like in argparse)
  2. read input by character instead of by line to handle special characters (Esc, Ctrl, arrows keys, etc.). This might also mean using OS-specific adapters for the console
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