calf: Command Argument Loading Function for Python

Calf lets you remove all your command argument parsing code, at least for simple cases. Only the implementation function is left, with initialization code that uses calf to call this function. The command argument parser is configured with a proper docstring, and perhaps some annotations (argument type) and default values for the parameters. In other words, stuffs that you would write anyway.

The docstring can be written in Google, Sphinx, epydoc or Numpy style, and the design is that it is easy to swap the parsing function with yours. In fact, you can customize such a wide range of characteristics of calf, that you can treat it as a slightly restricted frontend to the ArgumentParser under the hood. Used in this way, you can treat calf as a cute way to configure argparse.

This package shamelessly stole a lot of ideas from plac, but hopes to be more focused on creating comfortable command line interfaces rather than becoming a Swiss knife for programs with text-only user interface.

Basic example

Hello-world looks like this:

def hello(name) -> None:
    """Say hello

    Args:

        name: name of to say hello to

    """
    print('Hello,', name)

if __name__ == '__main__':
    import calf
    calf.call(hello)

The first thing to notice is that the program uses Google docstring style. If you want to use another style, just add doc_parser=<parser> to calf.call. Here <parser> may be calf.google_doc_parser, calf.sphinx_doc_parser (for Sphinx or Epydoc) or calf.numpy_doc_parser. You can run this program with:

hello.py Isaac

Here name is a positional command line argument: a normal function argument always maps to a positional command line argument. If you want an option instead, you can replace the function argument like this:

def hello(*, name: str = 'Isaac') -> None:
    """Say hello

    Args:

        name: (-n) name of to say hello to

    """
    print('Hello,', name)

Then the program is run like one of the following:

hello.py
hello.py --name Cathy
hello.py -n Cathy

Now name is an option: a keyword-only function argument always maps to a function. In this version we are explicit about the type of the argument. Note also that the leading -n in the docstring describing the argument, enclosed in parentheses, becomes the short option name.

It is usually a good idea to allow options not to be specified, by providing a default value. Positional arguments can also be provided a default value, but it doesn't mix well with variable arguments described below.

It is also possible to specify a default which provides no value (so the program knows that no value is provide). This is done by either using a default value of None, or setting in parameter a type of a parameterized Typing.Optional (without setting a default). In this case the normal construction of the target type will not happen.

There is a special case: any boolean function argument becomes a default-off flag. I cannot find a natural way to have a default-on flag, so it is not provided. (Let me know if you think otherwise!)

Variable arguments and keyword arguments can also be used. Variable arguments will become a list of the specified type:

def do_sum(*arg: int) -> None:
    """Sum numbers"""
    print('Sum =', sum(arg, 0))

Here the argument type is "int". The string passed in the command line argument will be converted to this type, and in the help message there will be a little hint (looking like "[int]") indicating the needed type. Also note that in this example I don't add documentation for the arguments: the docstring information is optional, without them there is no help string but everything else still works.

Keyword arguments cause command line arguments like "=" to be stolen from the var-arg and form a map. A type can still be provided. For example, if you have:

import urllib.parse
def get_query_str(url, **item) -> None:
    "Create URL with parameters"
    qstr = urllib.parse.urlencode(item)
    if qstr:
        url += '?' + qstr
    print(url)

Then you can run something like

get_query_str.py http://a/b x=a=c y=/

to get http://a/b?y=%2F&x=a%3Dc.

Finally, if you're tired of writing initialization code, you have an additional option to directly place your module under your PYTHONPATH. Then you can run your program simply like

calf hello.hello -n Isaac

Advanced capability

You can have your function to accept other types. Calf normally uses one positional argument or option for each function argument, and whatever string you specified in the argument will be passed to the type you specified (via default argument or annotation) as constructor. In cases that passing the string to the type constructor doesn't do the right thing (e.g., datetime), you can create your own conversion function and add it to calf.CONVERTERS. This has been done for datetime.date, datetime.time and datetime.datetime, and you can change how they behave by modifying calf.CONVERTERS (see the nextday.py example in the docs directory).

But you can also extend calf by creating a subclass of "selector" which selects function arguments based on name and type. It then specifies how to create a "loader" to handle the function argument, which may use multiple command line arguments (or do any other interaction with the ArgumentParser). See composite.py in the docs directory to see how this is done, for the common case.

Other parts of the module can also be overridden. For example, you can change the docstring parser and parameter doc parser. See the design document in the docs directory to understand the design and do all sorts of things with calf.