dict: on retrieval, key allowed to be null

[refack]

> uv run --with pyright pyright  .\dict-none-key.py 
e:\1work\CFF\dict-none-key.py
  e:\1work\CFF\dict-none-key.py:4:12 - error: Argument of type "int | None" cannot be assigned to parameter "key" of type "int" in function "get"
    Type "int | None" is not assignable to type "int"
      "None" is not assignable to "int" (reportArgumentType)

But

> python -c "a = {'A': 1};b = {1: 'א'};k1 = a.get('A');k2 = b.get(k1);print('no runtime error')"                                                                                                                                   
no runtime error      

[refack]

It just came to me that this might have been a design decision as:

a = {'A': 1}
b: dict[int | None, str] = {1: 'א'}
k1 = a.get('A')
k2 = b.get(k1)

gives:

> uv run --with pyright pyright  .\dict-none-key.py                                             
0 errors, 0 warnings, 0 informations

I searched manually, and with copilot and found no direct mention...

Is this PR acceptable in principal, which in that case I'll cross all the I's and dot all the T's

[JelleZijlstra]

It's sort of questionable to me to treat None specially here. I think we've had someone before trying this (specifically for dict.get) and we rejected it, but can't immediately find it back. dict.get has seen a lot of efforts to change things: https://github.com/python/typeshed/pulls?q=is%3Apr+is%3Aclosed+dict.get+

[refack]

dict.get has seen a lot of efforts to change things:

I assumed so, but still somewhere there is an incongruency since:

a = {'A': 1}
b = {1: 'א'}

k1 = 'B'
kv1 = a.get(k1)
v1 = b.get(kv1)
print(f"'{k1}' -> '{kv1}' -> '{v1}'")

k2 = 'A'
kv2 = a.get(k2)
v2 = b.get(kv2)
print(f"'{k2}' -> '{kv2}' -> '{v2}'")

gets:

> python .\dict-none-key.py
'B' -> 'None' -> 'None'
'A' -> '1' -> 'א'

and IMHO is reasonable use case, where .get is used specifically for fall trough cascade

yet pyright and mypy report "error"

> uv run --with pyright pyright  .\dict-none-key.py 
dict-none-key.py
  dict-none-key.py:6:12 - error: Argument of type "int | None" cannot be assigned to parameter "key" of type "int" in function "get"
    Type "int | None" is not assignable to type "int"
      "None" is not assignable to "int" (reportArgumentType)
  dict-none-key.py:11:12 - error: Argument of type "int | None" cannot be assigned to parameter "key" of type "int" in function "get"
    Type "int | None" is not assignable to type "int"
      "None" is not assignable to "int" (reportArgumentType)
2 errors, 0 warnings, 0 informations

> uv run mypy  .\dict-none-key.py                   
dict-none-key.py:6: error: Argument 1 to "get" of "dict" has incompatible type "int | None"; expected "int"  [arg-type]
dict-none-key.py:11: error: Argument 1 to "get" of "dict" has incompatible type "int | None"; expected "int"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

You could argue it's a mypy/pyright/jetbrains bug where they are being presumptuous in inferring the type of b as dict[int,str]

[refack]

It's sort of questionable to me to treat None specially here. I think we've had someone before trying this (specifically for dict.get)

IMHO only .get and .pop should be treated this way.