Reducing code smelling with a help from first class citizens

Photo by Mae Mu on Unsplash

Recently I've encountered a situation where elif's stacked on top of each other like pancakes. For example, we are parsing different file formats. Our parsers are ready and we just need a higher level interface.

if file_format == 'xml':
    parse_xml(file_name)
elif file_format == 'csv':
    parse_csv(file_name)
elif file_format == 'json':
    parse_json(file_name)
else:
    parse_txt(file_name)

At first there were just if and else. Then things started to add up and I realized that multiple elif's was just a smelling piece of code.

To deal with that we can leverage a python concept that functions are first class citizens. Meaning we can return them or use them as a dictionary values. So we need to map our file formats with corresponding parsing functions:

parsers = {
 'xml': parse_xml,
 'csv': parse_csv,
 'json': parse_json,
}

Notice that I did not include our default txt parser. It's like we rewrote
if and elif's of our previous code. Now we need a function to return a parser for a given file format:

def get_parser(file_format):
    return parsers.get(file_format, parse_txt)

This will return a parser function from our parsers dictionary for a given file format. Or, if file_format is not a key of that dictionary, default parse_txt. As an analogy before, it's like we rewrote the last else statement.

To use that we need to call get_parser function and we can also immediately execute it:

get_parser('csv')(file_name)

But if you want to be more transparent, you can expand this to multiple lines:

parser = get_parser('csv')
parser(file_name)

I think this code looks better than at the beginning of the article, although might not be obvious for everyone, so don't forget to place some comments :)

And I want to believe that it works a little faster because we're just getting a value from a dictionary instead of checking every single if case.