How to avoid very long if-elif-elif-else statements in Python function

Question

Is there a smart way to shorten very long if-elif-elif-elif... statements?

Let's say I have a function like this:

def very_long_func():
  something = 'Audi'
  
  car = ['VW', 'Audi', 'BMW']
  drinks = ['Cola', 'Fanta', 'Pepsi']
  countries = ['France', 'Germany', 'Italy']
  
  if something in car:
    return {'type':'car brand'}
  elif something in drinks:
    return  {'type':'lemonade brand'}
  elif something in countries:
    return {'type':'country'}
  else:
    return {'type':'nothing found'}
  

very_long_func()
>>>> {'type': 'car brand'}

The actual function is much longer than the example. What would be the best way to write this function (not in terms of speed but in readability)

I was reading this, but I have trouble to apply it to my problem.

"I was reading this, but I have trouble to apply it to my problem." What happened when you tried? What code did you write, and where did you get stuck? — Karl Knechtel, Feb 05 '21 at 15:15
in this case you could have a dictionary where "VW", "Audi", and "BMW" were keyed to "car brand", and "Cola", "Fanta", and "Pepsi" were keyed to "lemonade brand" and so forth. — Andrew Allaire, Feb 05 '21 at 15:17
I was trying to apply option 4 from the master solution. One problem is, that it doens't work to put ``return ....`` into a python dictionary — PParker, Feb 05 '21 at 15:18

score 3 · Accepted Answer · answered Feb 05 '21 at 15:28

You can't hash lists as dictionary values. So go other way round. Create a mapping of type -> list. And initialize your output with the default type. This allows you to keep on adding new types to your mapping without changing any code.

def very_long_func():
  something = 'Audi'
  
  car = ['VW', 'Audi', 'BMW']
  drinks = ['Cola', 'Fanta', 'Pepsi']
  countries = ['France', 'Germany', 'Italy']
  
  out = {'type': 'nothing found'}  # If nothing matches
  mapping = {
      'car brand': car,
      'lemonade brand': drinks,
      'country': countries
    }
  for k,v in mapping.items() :
    if something in v:
      out['type'] = k    # update if match found
      break
  return out             # returns matched or default value

This answer solves the OP's question as asked and has the cleanest coding style out of the numerous similar answers so this one should be the accepted answer IMO. — Biggsy, Feb 05 '21 at 15:35
Thank you very much to ALL OF YOU. I think this is my prefered answer, because it is the most readable (at least for a Python beginner like me) — PParker, Feb 05 '21 at 15:40

score 2 · Answer 2 · answered Feb 05 '21 at 15:30

you can create dictionary like this and then use map_dict.

from functools import reduce
car = ['VW', 'Audi', 'BMW']
drinks = ['Cola', 'Fanta', 'Pepsi']
countries = ['France', 'Germany', 'Italy']

li = [car, drinks, countries]
types = ['car brand', 'lemonade brand', 'country', 'nothing found']
dl = [dict(zip(l, [types[idx]]*len(l))) for idx, l in enumerate(li)]

map_dict = reduce(lambda a, b: dict(a, **b), dl)

score 1 · Answer 3 · answered Feb 05 '21 at 15:29

Try this:

def create_dct(lst, flag):
    return {k:flag for k in lst}

car = ['VW', 'Audi', 'BMW']
drinks = ['Cola', 'Fanta', 'Pepsi']
countries = ['France', 'Germany', 'Italy']

merge_dcts = {}
merge_dcts.update(create_dct(car, 'car brand'))
merge_dcts.update(create_dct(drinks, 'lemonade brand'))
merge_dcts.update(create_dct(countries, 'country'))

something = 'Audi'
try:
    print("type: ", merge_dcts[something])
except:
    print("type: nothing found")

Alain T. · Answer 4 · 2021-02-05T18:03:38.903

You can simulate a switch statement with a helper function like this:

def switch(v): yield lambda *c: v in c

The your code could be written like this:

something = 'Audi'

for case in switch(something):
    if case('VW', 'Audi', 'BMW'):          name = 'car brand'      ; break
    if case('Cola', 'Fanta', 'Pepsi'):     name = 'lemonade brand' ; break
    if case('France', 'Germany', 'Italy'): name = 'country'        ; break
else:                                      name = 'nothing found'
return {'type':name}

If you don't have specific code to do for each value, then a simple mapping dictionary would probably suffice. For ease of maintenance, you can start with a category-list:type-name mapping and expand it before use:

mapping = { ('VW', 'Audi', 'BMW'):'car brand',
            ('Cola', 'Fanta', 'Pepsi'):'lemonade brand',
            ('France', 'Germany', 'Italy'):'country' }
mapping = { categ:name for categs,name in mapping.items() for categ in categs }

Then your code will look like this:

something = 'Audi'
return {'type':mapping.get(something,'nothing found')}

using a defaultdict would make this even simpler to use by providing the 'nothing found' value automatically so you could write: return {'type':mapping[something]}

How to avoid very long if-elif-elif-else statements in Python function

4 Answers4