Hacker News Reader

Comment by luizfzs | Hacker News Reader

This post shows up in the same day I was looking exactly at it. Quite interesting.

Another way of avoiding KeyError is using

  dict.get(val, default_val)

I find it a bit cleaner, since you don't have to create a function or import collection.

[1] https://docs.python.org/3/library/stdtypes.html?highlight=di...

View on HN

> Another way of avoiding KeyError is using dict.get(val, default_val)

Except that

   my_dict = {}
   my_dict[val] = my_dict.get(val, []).append('foo')

doesn't work because list.append() doesn't return anything, while

   my_dict = defaultdict(list)
   my_dict[val].append('foo')

does the right thing.

Defaultdict is sugar for dict.setdefault, not dict.get.

Shouldn't the walrus operator solve this, or did they limit that to conditionals only for some reason?

    (my_dict[val] := my_dict.get(val, [])).append('foo')

Haha, probably. But counterpoints:

1) the walrus operator is stupid

2) the walrus operator is dangerous

3) `(my_dict[val] := my_dict.get(val, [])).append` is the opposite of simpler compared to `my_dict.setdefault(val, []).append` or (cleanestly) just `my_default_dict.append`

That is a very good point! Thank you for raising that.

You may also be interested in `dict.setdefault`. It has the same behavior as above but also sets the value for the key if it was previously missing.

    dict.setdefault(val, default_val)

[1] https://docs.python.org/3/library/stdtypes.html#dict.setdefa...

But of course doing it the way you described is subtly unintuitive because it leads to either (1) duplication of the default value (or the need to pass it around as a constant to every access point) and (2) does not work anywhere the dict is accessed directly like:

    d['foo']

whereas defaultdict handles the default value for you correctly independently of the access method.

IMO defaultdict is ideal for this use case.

Reply on news.ycombinator.com