python setdefault in setdefault behavior

Question

I tries to understand how nested setdefault works in code. I know how the basic setdefault works but I have a problem with figuring out such an code:

there are no keys in d, why both keys and values are added? "333" not in d so we're going to defult value d.setdefault("444",[]) -> we put a "444" key with a value [], setdefault return [] which we insert to "333" key which is returned by first setdefault and append is made(why for both new keys, I thought only for "333")

d = {'111': ['aaa', 'bbb'], '222': ['ccc']}
d.setdefault("333", d.setdefault("444",[])).append("ddd") 
{'111': ['aaa', 'bbb'], '222': ['ccc'], '444': ['ddd'], '333': ['ddd']}

first is already in d, "ddd" is appended to the existing "111" key but why is there a new key "444"?

d = {'111': ['aaa', 'bbb'], '222': ['ccc']}
d.setdefault("111", d.setdefault("444",[])).append("ddd") 
{'111': ['aaa', 'bbb', 'ddd'], '222': ['ccc'], '444': []}

first and the second key is already in d, only first updated?

d = {'111': ['aaa', 'bbb'], '222': ['ccc']}
d.setdefault("111", d.setdefault("222",[])).append("ddd") 
{'111': ['aaa', 'bbb', 'ddd'], '222': ['ccc']}

second is in d,..?

d = {'111': ['aaa', 'bbb'], '222': ['ccc']}
d.setdefault("333", d.setdefault("222",[])).append("ddd") 
{'111': ['aaa', 'bbb'], '222': ['ccc', 'ddd'], '333': ['ccc', 'ddd']} 

thanks for the explanation

Answer 1

It works like this:

>>> d = {'111': ['aaa', 'bbb'], '222': ['ccc']}
>>> first = d.setdefault("444",[])
>>> first
[]
>>> d
{'111': ['aaa', 'bbb'], '222': ['ccc'], '444': []}
>>> second = d.setdefault("333", first)
>>> second
[]
>>> d
{'111': ['aaa', 'bbb'], '222': ['ccc'], '444': [], '333': []}
>>> second.append("ddd")
>>> second
['ddd']
>>> d
{'111': ['aaa', 'bbb'], '222': ['ccc'], '444': ['ddd'], '333': ['ddd']}

Does that make it clear?

Note that first and second refer to the same list.

>>> first is second
True
>>> first.append('test')
>>> first
['ddd', 'test']
>>> second
['ddd', 'test']
>>> d
{'111': ['aaa', 'bbb'], '222': ['ccc'], '444': ['ddd', 'test'], '333': ['ddd', 'test']}