initialize and add value to a list in dictionary at one step in Python 3.7

Question

I have a dictionary where key is string and value is list.

Now while adding a value associated with given key, I always have to check if there is any list yet, otherwise

Answer 1

You can use dict.setdefault:

>>> d = {}
>>> d.setdefault('k', []).append(1)
>>> d
{'k': [1]}
>>> d.setdefault('k', []).append(2)
>>> d
{'k': [1, 2]}

Help on method_descriptor in dict:

dict.setdefault = setdefault(...) D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D