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generate command-line interfaces from function signatures

Project description

lazycli is a module which provides a decorator which will generate cli scripts from function signatures. The intention is to allow the creation of cli-scripts with as little extra work as possible. It was originally going to be called sig2cli, but someone else already had the same idea and got the name on PyPI in ten months before I did.

The one and only goal of lazycli is to facilitate the creation of CLI interfaces with minimum effort.

lazycli wraps argparse from the Python standard library and exposes some parts of the argparse api. The abstraction it provides is a little leaky, but it’s not too bad, because it’s relatively simple and is not intended to provide the full range functionality. If you need flexibility, use argparse directly or something more powerful like click.

Basics

Consider this simple clone of the cp command, cp.py:

#!/usr/bin/env python3
import lazycli
import shutil
import sys


@lazycli.script
def cp(*src, dst, recursive=False):
    """copy around files"""
    for path in src:
        try:
            shutil.copy2(path, dst)
        except IsADirectoryError as err:
            if recursive:
                shutil.copytree(path, dst)
            else:
                print(err, file=sys.stderr)


if __name__ == '__main__':
    cp.run()
$ ./cp.py -h
usage: cp.py [-h] [-r] [src [src ...]] dst

copy around files

positional arguments:
  src
  dst

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

It works like you’d expect. I chose cp because shutil can do all the heavy lifting, and the body of the function isn’t important. The important thing in this script are these three lines:

@lazycli.script
def cp(*src, dst, recursive=False):

# ... and ...

cp.run()
  • All parameters without defaults become positional arguments.
  • All parameters with defaults become optional arguments.
  • *args arguments will translate into variadic arguments at the command line as well. There can always be zero of them.
  • Parameters with boolean default values are treated as boolean flags and don’t accept arguments.
  • Short versions of flags are generated automatically from the first letter of the parameter.
  • A .run function is tacked on to the cp function which triggers argument parsing and applies the results to cp. The cp function itself is unaltered and can be called elsewhere if desired.

I’m not entirely sure how useful this last point is, since script entry-point functions tend not to be very general-purpose, but, eh, who knows.

Be aware that, presently, **kwargs-style parameters are ignored by lazycli.

Note on short flags:
Short flags are generated for optional arguments based on the first letter of parameter names. If that flag has been used by a previous parameter, the flag will be uppercased. If that has already been used, no short flag is generated. Because of this, changing the order of arguments can potentially break the backward compatibility of your CLI.
Note on boolean defaults:

A boolean default set to False produces the output seen above. If we change the parameter default to recursive=True, the name of the flag is inverted:

optional arguments:
  -h, --help          show this help message and exit
  -r, --no-recursive

Types

lazycli attempts to determine argument types based first on type annotations in the function signature and then based on the type of the default argument.

  • If the type of parameter is an iterable (besides mappings, strings and files), it will become a variadic when interpreted. If it’s a subscripted type from the typing module, like typing.Iterable[int], the subscript will be used as the type.
  • If the type is determined to be a mapping or is annotated as object, the argument should be a json literal (though it could theoretically be a string, number, array or object).

The inferred type is then used as a constructor to parse the argument string. This means only constructors that can take strings as input may be used.

#!/usr/bin/env python3
import typing as t
import lazycli

@lazycli.script
def mysum(numbers: t.Iterable[float]):
    return sum(numbers)

if __name__ == '__main__':
    mysum.run()
$ ./sum.py -h
usage: sum.py [-h] [numbers [numbers ...]]

positional arguments:
  numbers     type: float

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

$ ./sum.py 5 8
13.0

Though the style is questionable, this means you can use arbitrary callables as type annotations:

#!/usr/bin/env python3
import sys
import lazycli


@lazycli.script
def upcat(
        infile: open = sys.stdin,
        outfile: lambda f: open(f, 'w') = sys.stdout
):
    """cat, but upper-cases everything."""
    for line in infile:
        outfile.write(line.upper())


if __name__ == '__main__':
    upcat.run()
usage: upcat.py [-h] [-i INFILE] [-o OUTFILE]

cat, but upper-cases everything.

optional arguments:
  -h, --help            show this help message and exit
  -i INFILE, --infile INFILE
                        type: open; default: <stdin>
  -o OUTFILE, --outfile OUTFILE
                        type: <lambda>; default: <stdout>

This looks pretty bad, and mypy is going to hate it. A better way to do this is probably just parsing the string inside the script.

However, because the pattern of having an optional file argument and falling back to standard streams is so common, lazycli provides special classes for making this less ugly:

#!/usr/bin/env python3
import sys
from lazycli import script, ReadFile, WriteFile


@script
def upcat2(infile:ReadFile=sys.stdin, outfile:WriteFile=sys.stdout):
    """cat, but upper-cases everything."""
    for line in infile:
        outfile.write(line.upper())


if __name__ == '__main__':
    upcat2.run()

These classes will provide users more helpful type information and will return true if used in instance checks of text file types (including sys.{stdin,stdout,stderr} and non-bytes output of the open builtin function). These classes don’t create instances of themselves, but rather instances of io.TextIOWrapper. However, they still break mypy. Funny how metaclasses will do that.

In addition to ReadFile and WriteFile, there is also an AppendFile constructor.

Output

So far, output has simply been printed. However, If the function has a return value, that will also be printed. If it is an iterable (besides a string or mapping), each item will be printed on a new line.

Subcommands

I’ll expand this section of the documentation later, but here’s a sample script, modeled on info in this blog post

#!/usr/bin/env python3
import lazycli


@lazycli.script
def main(version=False):
  if version:
    return 1.0


@main.subcommand
def hello(name, greeting="Hello", caps=False):
    return greet(name, greeting, caps)


@main.subcommand
def goodbye(name, greeting="Goodbye", caps=False):
    return greet(name, greeting, caps)


def greet(name, greeting, caps):
    if caps:
        return f'{greeting}, {name}!'.upper()
    return f'{greeting}, {name}!'


if __name__ == '__main__':
    script.run()

Any parameters definied on the script function (main, in this case) will be passed to the script function, and parameters definied on the subcommands are parsed to them. If one wishes to pass values from the top-level function to subcommands, and external mechanism must be used. (e.g. a global variable)

$ ./test_sub.py -h
usage: test_sub.py [-h] [-v] {hello,goodbye} ...

positional arguments:
  {hello,goodbye}

optional arguments:
  -h, --help       show this help message and exit
  -v, --version
$
$
$ ./test_sub.py hello -h
usage: test_sub.py hello [-h] [-c] [-g GREETING] name

positional arguments:
  name

optional arguments:
  -h, --help            show this help message and exit
  -c, --caps
  -g GREETING, --greeting GREETING
                        default: Hello

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