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A fast way to turn your python function into a script

Project description


A fast way to turn your python function into a script.

Part of's toolkit for delightful developer experiences. Written by Jeremy Howard.


pip install fastscript


Sometimes, you want to create a quick script, either for yourself, or for others. But in Python, that involves a whole lot of boilerplate and ceremony, especially if you want to support command line arguments, provide help, and other niceties. You can use argparse for this purpose, which comes with Python, but it's complex and verbose.

fastscript makes life easier. There are much fancier modules to help you write scripts (we recommend Python Fire, and Click is also popular), but fastscript is very fast and very simple. In fact, it's <50 lines of code! Basically, it's just a little wrapper around argparse that uses modern Python features and some thoughtful defaults to get rid of the boilerplate.


Here's a complete example - it's provided in the fastscript repo as examples/

from fastscript import *
def main(msg:Param("The message", str),
         upper:Param("Convert to uppercase?", bool_arg)=False):
    print(msg.upper() if upper else msg)

When you run this script, you'll see:

$ python examples/
usage: [-h] [--upper UPPER] msg error: the following arguments are required: msg

As you see, we didn't need any if __name__ == "__main__", we didn't have to parse arguments, we just wrote a function, added a decorator to it, and added some annotations to our function's parameters. As a bonus, we can also use this function directly from a REPL such as Jupyter Notebook - it's not just for command line scripts!


Each parameter in your function should have an annotation Param(...) (as in the example above). You can pass the following when calling Param: help,type,opt,action,nargs,const,choices,required . Except for opt, all of these are just passed directly to argparse, so you have all the power of that module at your disposal. Generally you'll want to pass at least help (since this is provided as the help string for that parameter) and type (to ensure that you get the type of data you expect). opt is a bool that defines whether a param is optional or required (positional) - but you'll generally not need to set this manually, because fastscript will set it for you automatically based on default values.

You should provide a default (after the =) for any optional parameters. If you don't provide a default for a parameter, then it will be a positional parameter.

setuptools scripts

There's a really nice feature of pip/setuptools that lets you create commandline scripts directly from functions, makes them available in the PATH, and even makes your scripts cross-platform (e.g. in Windows it creates an exe). fastscript supports this feature too. To use it, follow this example from fastscript/ in the repo. As you see, it's basically identical to the script example above, except that we can treat it as a module. The trick to making this available as a script is to add a console_scripts section to your setup file, of the form: script_name=module:function_name. E.g. in this case we use: test_fastscript=fastscript.test_cli:main. With this, you can then just type test_fastscript at any time, from any directory, and your script will be called (once it's installed using one of the methods below).

You don't actually have to write a yourself. Instead, just copy the we have in the fastscript repo, and copy settings.ini as well. Then modify settings.ini as appropriate for your module/script. Then, to install your script directly, you can type pip install -e .. Your script, when installed this way (it's called an editable install, will automatically be up to date even if you edit it - there's no need to reinstall it after editing.

You can even make your module and script available for installation directly from pip by running make. There shouldn't be anything else to edit - you just need to make sure you have an account on pypi and have set up a .pypirc file.

Importing Command Line Functions

Sometimes it might be useful to also be able to import command-line functions and use them as regular functions.

from fastscript.test_cli import main
main("This can also be used as a regular imported function.", upper=True)

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