what about a dict.setdefaults (note the s) that takes in an iterable or a dict, and uses insert-or-ignore behaviour? (unfortunately a lot of iterables and all generators are hashable, and we can't break that.)

I could really use something like that tbh.

On 2020-04-16 9:47 a.m., Alex Hall wrote:
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+py@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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