sh2py

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‌ !!! DEPRECATION NOTICE: this project is discontinued. Use Python Fire instead !!!

‌

‌ sh2py is a shell to Python command line mapper. In other words, it is the easiest way to expose Python functions to the shell. Main features:

• Very small code size. A single Python module. About a hundred lines of code.
• The public API consists of a Python class, CommandLineMapper; plus two methods: add and run.
• No external dependencies.
• Support for Python 2 and 3.

Installation

pip install sh2py

Basic Use

Expose a single Python function. Consider the snippet below.

from sh2py import CommandLineMapper

def myfunc(arg1, arg2, arg3='val'):
    print(arg1)
    print(arg2)
    print(arg3)

if __name__ == '__main__':
    cli = CommandLineMapper()

    cli.add(myfunc)

    cli.run()

The last three instructions demonstrate the basic operation of the API.

1. Instantiate the CommandLineMapper.

2. Register the function you want to expose, using the CommandLineMapper.add method.

3. Call CommandLineMapper.run. It will process the command line, sys.argv, and invoke the matching function.

Suppose the snippet above is saved as myscript.py.

$ python myscript.py val1 val2 arg3=val3
val1
val2
val3

The output reveals that the shell command above actually called myfunc('val1', 'val2', arg3='val3'). A couple of notes.

1. Only one function was exposed via CommandLineMapper.add, so its name could be omitted from the command line. If exposing more than one function, it is recommended to pass its name as the first program argument, otherwise the first exposed function will be called. See next section.

2. There is no type validation/coercion. The run method will only pass strings on actual function calls.

Multiple Commands

If your script is large, you can organize it as a suite of sub-commands. Create one function for each command you expect to handle and register all of them with CommandLineMapper.add. Consider the code below.

from sh2py import CommandLineMapper

def subcommand1(*args1, **kwargs1):
    pass

def subcommand2(*args2, **kwargs2):
    pass

if __name__ == '__main__':
    cli = CommandLineMapper()

    cli.add(subcommand1)
    cli.add(subcommand2)

    cli.run()

You can then specify which function to run via command line. For example, to invoke subcommand2 with arguments val1 and val2, type the following.

python mysuite.py subcommand2 val1 val2

Notice that the subcommand2 entry itself is extracted from the argument line. The remainder of the line is processed as usual. In this case, args2 is ['val1', 'val2'].

Complete example

Here is a complete example.

from sys import exit, stderr
from time import sleep
from random import sample

from sh2py import HALT, CommandLineMapper

def hello(case='', shuffle='no', pace='0'):
    '''
    Emits a "Hello, world!" greeting message on standard error.

    Usage:

        python hello.py help
        python hello.py [case=upper/lower] [shuffle=yes/no] [pace=0]

    Arguments:

        help,              print this usage help and exit;
        case=upper/lower,  convert the message to uppercase, or lowercase;
        shuffle=yes/no,    shuffle the letters of the message;
        pace=0,            pace at which the letters are shown, in miliseconds.

    By default, there is no case conversion and no shuffling. Also, the pace is
    zero, so the entire message is shown immediately on standard error.
    '''

    if case in ['', 'upper', 'lower'] and shuffle in ['yes', 'no']:
        if pace.isdigit():
            greet = ['Hello', 'world']
            delay = int(pace)

            if shuffle == 'yes':
                greet[0] = ''.join(sample(greet[0], len(greet[0])))
                greet[1] = ''.join(sample(greet[1], len(greet[1])))

            greet = '{}, {}!'.format(*greet)

            if case == 'upper':
                greet = greet.upper()

            elif case == 'lower':
                greet = greet.lower()

            if delay == 0:
                print(greet, file=stderr)

            else:
                for x in greet:
                    stderr.write(x)

                    stderr.flush()

                    sleep(delay / 1000.0)

                print(file=stderr)

        else:
            return HALT

    else:
        return HALT

if __name__ == '__main__':
    cli = CommandLineMapper()

    cli.add(hello)

    if cli.run() is HALT:
        print(hello.__doc__, file=stderr)

        exit(1)

Observe that the hello function does not have a help argument, but the docstring says different. The help argument is automatically recognized and, when used, will print the docstring of the default, hello, function. A default help text will be printed if a function does not supply a docstring.

The call to CommandLineMapper.run will return HALT if a command is invoked with an invalid syntax. Your functions can also use it to signal error conditions. This allows the program to exit with a non-zero status, or trigger some error handling function.

Q&A

Q. I want POSIX arguments.

A. Wrong tool. There are plenty of Python command line parsing utilities supporting POSIX. Try one of them.

• Argparse/Optparse/Getopt. Built into Python. Complex.
• Compago. Very nice, but unmaintained. Also, does not run on Python 3.
• Docopt.
• Clint.
• Click.

Q. I need type coercion.

A. Implement it yourself, or use some third party library.

Q. I can use Python itself as a command line interpreter. Why all of this?

A. Yes you can. In this case all of this becomes irrelevant. I prefer not to, that is one of the reasons I wrote this tool. Another is that everything else is too complex. YMMV.

Q. Why not use sys.argv directly.

A. Good question. I do this a lot of times. Actually, if your script is small enough and requires zero, one, or maybe two positional parameters, there is no need for command line processing tools. Write a couple of conditionals and print a simple help message in case of errors.