Null coalescing on a dict entry in python

Question

So, I'm aware that, in Python I can do this:

variable_name = other_variable or 'something else'

...and that that will assign 'something else' to variable_name if other_variable is falsy, and otherwise assign other_variable value to variable.

Can I do a nice succinct similar thing with a dict:

variable_name = my_dict['keyname'] or 'something else'

...or will a non-existent keyname always raise an error, cause it to fail?

use the get method my_dict.get('keyname', 'something else') — user8060120
– user8060120, Commented Nov 12, 2018 at 13:18

jpp · Accepted Answer · 2018-11-12 13:20:27Z

9

You will see KeyError if 'keyname' does not exist in your dictionary. Instead, you can use:

variable_name = my_dict.get('keyname', False) or 'something else'

With this logic, 'something else' is assigned when either 'keyname' does not exist or my_dict['keyname'] is Falsy.

answered Nov 12, 2018 at 13:20

jpp

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6 Comments

Gabriel Over a year ago

You answer is correct, but I'd like to point out that what we usually need in this scenario is my_dict.get('keyname', 'something else').

jpp Over a year ago

@Gabriel, No, my_dict.get('keyname', 'something else') will be incorrect for my_dict = {'key_name':0}. In this scenario, OP will want 'something else'.

Gabriel Over a year ago

I know and that's why I said your answer is correct. I just noted that when dealing with dicts we USUALLY (not always) want a default for missing keys, not falsey values.

Gabriel Over a year ago

OP was 100% clear and that's why I can say you're correct (otherwise I couldn't say that for sure). My previous advise is for anyone who can read your answer, not only OP. For example, if someone had optional arguments in a function (as in f(a=None)) and converted to kwargs, one would want to test for missing keys, not falsey values. It may not be obvious by reading your answer, so I left an advice. I wasn't implying your answer is wrong.

Owen C. Jones Over a year ago

Thanks! To be honest, I didn't think I meant this, but it's made me realise that this scenario is exactly what I need. :)

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