How does .setdefault() work?

Question

I am using \'setdefault()\' in my program but I\'m not sure what it actually does... so I\'m asking on here if anyone one knows?

set default

th

Answer 1

The docs are here.

dict.setdefault() is something like the "lvalue" analog of dict.get(), which is typically an "rvalue" (occurs on the righthand side of assignments — it retrieves a value, as opposed to mutating the state of something).

Informally, d.setvalue(key, value) means "Give me d[key]. What, key isn't in d? OK, then assign d[key] = value, just in time; now give me d[key]."

Suppose d = {'a': 17}. Then

>>> d.setdefault('a', 0)
17

Here, 'a' is in d, so d.setdefault('a', 0) leaves d unchanged and returns d['a']. However,

>>> d.setdefault('b', 100)
100

Because b is not in d, d.setdefault('b', 100) returns 100 and sets d[b] = 100. d now has two items, and both a and b are keys:

>>> len(d), d['a'] == 17, d['b'] == 100
(2, True, True)