Issue 25016: defaultdict's pop gives a KeyError (original) (raw)

Issue25016

Created on 2015-09-07 06:27 by rob.anyone, last changed 2022-04-11 14:58 by admin. This issue is now closed.

Messages (3)
msg250080 - (view)	Author: Chase Albert (rob.anyone)	Date: 2015-09-07 06:27
`defaultdict(list).pop(1)` raises a KeyError, this is not what I expected (I expected an empty list).
msg250081 - (view)	Author: Martin Panter (martin.panter) *	Date: 2015-09-07 06:37
I think this is by design. Under <https://docs.python.org/3/library/collections.html#collections.defaultdict.__missing__> it says “__missing__() is _not_ called for any operations besides __getitem__().” What gave you the impression that pop() should return a default value?
msg250140 - (view)	Author: R. David Murray (r.david.murray) *	Date: 2015-09-07 23:59
It would not be sensible for pop to return the default value, since the point of pop is to *remove* a key from the dictionary (while returning the existing value), whereas the point defaultdict is to *add* key to the dictionary with the default value if you retrieve a non-existent key. As Martin said, an important aspect of defaultdict is that *only* getattr has this default behavior; things like 'in' still work like a regular dictionary with a defaultdict.