Python default (mutable) parameter initialization style

Question

Let's have a function make_sandwich that takes a list of ingredients which has a default value of ['ham', 'ham', 'bacon', 'ham']

def make_sandwich(ingredients=['ham', 'ham', 'bacon', 'ham']):
    print("Making a sandwich with ", ingredients)

However, since this default value is susceptible to this python "mutable default argument" ~~bug~~ feature, we should use an immutable instead like this:

def make_sandwich(ingredients=None):
    # initialized ingredients here
    print("Making a sandwich with ", ingredients)

So here's the question. There're two ways that I am aware of to do this, but I am not sure which one is considered a better practice.

The first one:

if not ingredients:
    ingredients = ['ham', 'ham', 'bacon', 'ham']

The second one:

ingredients = ingredients or ['ham', 'ham', 'bacon', 'ham']

Personally I use the second one more often. Sometimes, I even inline that if the argument is used only once. e.g.

print("Making a sandwich with ", ingredients or ['ham', 'ham', 'bacon', 'ham'])

Is there any solid reason to prefer one over the others?

Olivier Melançon · Accepted Answer · 2023-12-03 19:42:36Z

4

None of them is actually the right way to do. What if you want to pass an empty list of ingredients?

A better solution would be

ingredients = ['ham', 'bacon', 'ham'] if ingredients is None else ingredients

edited Dec 3, 2023 at 19:42

answered Dec 8, 2017 at 6:41

Olivier Melançon

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2 Comments

Uduse Over a year ago

good point! I guess the other solution of mine suffers from the same reason.

Olivier Melançon Over a year ago

Indeed, they have the exact same behavior and are just a matter of preference. Don't forget to select the answer if you consider it answers your question the best.

ShpielMeister · Accepted Answer · 2017-12-08 06:44:43Z

1

a matter of personal style. you could also do

ingredients = ingredients if ingredients else ['ham', 'ham', 'bacon', 'ham']

just depends on who will be reading your code. personally i'm fine with your second

ingredients = ingredients or ['ham', 'ham', 'bacon', 'ham']

answered Dec 8, 2017 at 6:44

ShpielMeister

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