I just tried playing with this idea:
from collections import UserDict
class InsertOrIgnoreDict(UserDict):
__setitem__ = UserDict.setdefault
print(InsertOrIgnoreDict([(1, 2), (3, 4)]))
It caused an infinite chain of exceptions:
Traceback (most recent call last):
File
"/home/alex/.pyenv/versions/3.8.0/lib/python3.8/_collections_abc.py", line
845, in setdefault
return self[key]
File
"/home/alex/.pyenv/versions/3.8.0/lib/python3.8/collections/__init__.py",
line 1003, in __getitem__
raise KeyError(key)
KeyError: 1
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File
"/home/alex/.pyenv/versions/3.8.0/lib/python3.8/_collections_abc.py", line
845, in setdefault
return self[key]
File
"/home/alex/.pyenv/versions/3.8.0/lib/python3.8/collections/__init__.py",
line 1003, in __getitem__
raise KeyError(key)
KeyError: 1
During handling of the above exception, another exception occurred:
...
I thought that was amusing.
Anyway, one could come up with infinitely many variations of potentially
useful semantics. Why include any of them in the language instead of
letting users implement what they need?
On Thu, Apr 16, 2020 at 2:37 PM Soni L. <fakedme...@gmail.com> wrote:
> currently dicts have insert-or-update semantics, e.g.:
>
> >>> dict([((), 1), ((), 2)])
> {(): 2}
>
> this is fine. however, in many cases insert or insert-or-ignore are more
> useful: (hypothetical examples)
>
> >>> InsertOrIgnoreDict([((), 1), ((), 2)])
> {(): 1}
>
> >>> InsertDict([((), 1), ((), 2)])
> Traceback: [...]
> ValueError: key exists
>
> these are useful when dealing with custom config file formats, config
> overrides, among other things.
>
> additionally we could also get an UpdateDict, a *view* into a dict that
> only allows updating existing entries:
>
> >>> a = {'a': 1}
> >>> b = {'a': 2, 'b': 3}
> >>> UpdateDict(a).update(b)
> >>> a
> {'a': 2}
>
> but I don't see an obvious usefulness to this one. it'd just be for
> consistency with other SQL-esque dict wrappers (like the above
> InsertOrIgnoreDict and InsertDict).
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