DEV Community

Cover image for Avoid multiple ElIf situation
Constantine
Constantine

Posted on • Updated on

Avoid multiple ElIf situation

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)
Enter fullscreen mode Exit fullscreen mode

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,
}
Enter fullscreen mode Exit fullscreen mode

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)
Enter fullscreen mode Exit fullscreen mode

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)
Enter fullscreen mode Exit fullscreen mode

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

parser = get_parser('csv')
parser(file_name)
Enter fullscreen mode Exit fullscreen mode

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.

Top comments (5)

Collapse
 
patarapolw profile image
Pacharapol Withayasakpunt • Edited

I still don't really understand why elif or else if smells, when dictionary with functions just make the code more complicated.

Danger 1 - Dictionary with non-function gets compiled
Danger 2 - not in Python. Fall through in switch case statement

Collapse
 
c_v_ya profile image
Constantine • Edited

I have nothing against elifs, it's a powerful and useful tool. And I use it a lot in my projects. But when I see 4-5 or more of them - Idk, it makes me cringe. Dictionary can help with that giving you a neat structure of what to do when you have this condition.

Collapse
 
jhermann profile image
Jürgen Hermann • Edited

Because the number of code paths is reduced, and the intent of the code is way more clearer (get me a parser for this format). The code is actually simpler, because less chance for the unexpected.

Complexity is not just what is, but what could be.

Collapse
 
sobolevn profile image
Nikita Sobolev

I absolutely agree. In wemake-python-styleguide we have a special violation called TooManyElifViolations. We raise a violation when developers use more then 3 elifs.

More docs: wemake-python-stylegui.de/en/lates...

Check it out:

GitHub logo wemake-services / wemake-python-styleguide

The strictest and most opinionated python linter ever!

wemake-python-styleguide

wemake.services Supporters Build Status Coverage Status Python Version wemake-python-styleguide


Welcome to the strictest and most opinionated python linter ever.

wemake-python-styleguide logo

wemake-python-styleguide is actually a flake8 plugin with some other plugins as dependencies.

Quickstart

pip install wemake-python-styleguide

You will also need to create a setup.cfg file with the configuration.

We highly recommend to also use:

  • flakehell for easy integration into a legacy codebase
  • nitpick for sharing and validating configuration across multiple projects

Running

flake8 your_module.py

This app is still just good old flake8 And it won't change your existing workflow.

invocation resuts

See "Usage" section in the docs for examples and integrations.

We also support Github Actions as first class-citizens Try it out!

What we are about

The ultimate goal of this project is to make all people write exactly the same python code.

flake8 pylint black mypy wemake-python-styleguide
Formats code?
Finds style issues? 🤔 🤔
Finds bugs? 🤔
Collapse
 
c_v_ya profile image
Constantine • Edited

I agree with you. But this code was just an example. I could not come up with something more useful but had an idea to write about.