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csv.DictReader and DictWriter not subscriptable at runtime #92391
Description
Feature or enhancement
Add PEP 585 style __class_getitem__ to csv.DictReader and csv.DictWriter.
Pitch
With PEP 585 most generic stdlib types are now subscriptable at runtime. Both DictReader and DictWriter are generic in typeshed but don't yet implement __class_getitem__.
from csv import DictWriter
from typing import IO
def f(fp: IO[str]) -> DictWriter[str]:
return DictWriter(fp, ["id", "name"])Previous discussion
Activity
serhiy-storchaka commentedon May 7, 2022
Why are they generic in typeshed?
AlexWaygood commentedon May 7, 2022
DictReaderhas been generic for a while.DictWriterwas made generic in python/typeshed#5366.The classes are slightly complicated to type, so I am not sure the stubs are 100% correct at the moment (I have a PR open at the moment to improve them). But I think it is correct that they are both generic. Copying what I wrote in python/typeshed#7787 (comment):
serhiy-storchaka commentedon May 7, 2022
I am not sure that they were designed this way. Why not return just
dict[Any, Any]inDictReader.__next__? The type of the result depends not only on the type of field names (which can have different types), but on types and values of other parameters and on the content of the file.AlexWaygood commentedon May 7, 2022
Can you give an example? If the stubs are incorrect, I would be interested in improving them.
serhiy-storchaka commentedon May 7, 2022
The type of keys: the type of any item in
fieldnames(str | floatiffieldnamesis None), or the type ofrestkey.The type of values:
str | float,list[str|float], or the type ofrestval.