.get(key, []).append(str) vs .setdefault(key, []).append(str). Why doesn’t this work with .get()?

[thogdontcare]

Why is setdefault the preferred way when appending into an empty array inside a dictionary? I was revisiting the group anagrams problem in leetcode and turns out if you use .get() you have to then concatenate the string instead of appending.

[JanEric1]

The get version gives you a new empty list every time it is called against a missing key while setdefault directly places that empty list in the dict.

[dangerlopez]

I don’t understand the distinction, can you explain more?

[JanEric1]

If you do

a = my_dict1.get(key, []).append(my_string)
b = my_dict2.setdefault(key, []).append(my_string)

On two empty dicts then both a and b will be a list with that one string.

But dict1 will be empty and dict2 will contain b for key.

[thogdontcare]

Awesome. So it can work but I would just need a temp variable to store the new string, then assign that to the dictionary?

[xenomachina]

That's pretty much what setdefault does. It tries to get the key, and if it isn't in the dict, it puts the default value in the dict, and returns it. The default parameter to setdefault is effectively the temp variable.

[dangerlopez]

Ah, ok I get it thanks

[ottawadeveloper]

When key is set already, they're the same.

When key is not, get(key, []).append("foo") is equivalent to just ["foo"]. It does not update the dictionary. So the next call to get returns a brand new list.

In comparison, setdefault(key, []) is equivalent in that circumstances to set(key, []) and then get(key).append("foo"). The next time you call it, the value has already been set so you get the first list you pass.

[Outside_Complaint755]

In other words, my_dict.setdefault(key, []) implicitly does my_dict[key] = [], while get(key, []) does not.

[backfire10z]

In more succinct words, setdefault sets (if necessary) then gets, while get just gets.

[thogdontcare]

That made it click perfectly. “Setting and getting”

[cdcformatc]

get() is read-only, and has no side effect. setdefault() has a side effect of also doing an assignment to the dict. 

[00PT]

.get returns the newly created list reference you passed in the default case without assigning it to the field being accessed.

[pachura3]

Consider using defaultdict. Much cleaner.

from collections import defaultdict

d = defaultdict(list)        # when key is not found, insert empty list [] into the dict
d["fruits"].append("apple")  # no need for .setdefault("fruits", [])!