Should argparse allow abbreviations?

In Python, argparse is the default module for parsing arguments from command line. The optional arguments in argparse can either be short options (single letter) such as -s or long options such as --output_folder which are usually defined with words.

For long options, there is an interesting property of argparse which might be seen as a bug at first sight. Let’s assume, we have a Python script that accepts two optional command line arguments -s as bool and output_path as str. Normally, these arguments are provided to the script as follows:

    python3 main.py -s True --output_folder <folder_name>

What happens if you try to run the same code like this:

    python3 main.py -s True --out /home/user/Documents

Surprisingly, the script does not complain about unrecognized argument name --out and runs normally. This is because argparse.ArgumentParser method has a parameter called allow_abbrev which is by default True. Thus, even if we don’t type the whole long option name, it accepts the parameter unless it is unambiguous which means the first letter of the input argument out matches with the first letter of the optional argument output_folder . You can test this behavior with abbreviated inputs such as --o --outp etc.

If you want to allow only exact typing of your long options, you should initialize the argument parser as follows:

    parser = argparse.ArgumentParser(allow_abbrev=False)

This will disable the usage of abbreviated arguments and raise an unrecognized argument error when there is a mismatch between the input and given argument names. Here is an example Colab notebook that shows the behavior of allow_abbrev parameter.