I think the first line means that when I call
x[k]
for a nonexistent keyk
(such as a statement likev=x[k]
), the key-value pair(k,0)
will be automatically added to the dictionary, as if the statementx[k]=0
is first executed.
That’s right. This is more idiomatically written
x = defaultdict(int)
In the case of y
, when you do y["ham"]["spam"]
, the key "ham"
is inserted in y
if it does not exist. The value associated with it becomes a defaultdict
in which "spam"
is automatically inserted with a value of 0
.
I.e., y
is a kind of “two-tiered” defaultdict
. If "ham" not in y
, then evaluating y["ham"]["spam"]
is like doing
y["ham"] = {}
y["ham"]["spam"] = 0
in terms of ordinary dict
.