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wrong types for csv.DictWriter #4800
Description
Hi, it seems that csv.DictWriter is assuming dictionary keys will always be strings
I've found it useful to use IntEnum keys with DictWriter to avoid having magic strings and write metadata rows positionally
I think it would make sense to change csv.pyi to change fieldnames and the key of _DictRow to be parameterize around a TypeVar('T')
I can add a pull request if you all think this is the right way to go
import csv
import enum
import io
from typing import TextIO
class MyEnum(enum.IntEnum):
foo = 0
bar = 1
baz = 2
def write_data(sink: TextIO) -> None:
writer = csv.DictWriter(sink, fieldnames=list(MyEnum))
writer.writerow({0: "<timestamp>", 1: "metadata"})
writer.writerow({MyEnum.foo: 1000, MyEnum.bar: 200, MyEnum.baz: 300})
def test_write_data() -> None:
sio = io.StringIO()
write_data(sio)
sio.seek(0)
assert sio.readlines() == [
"<timestamp>,metadata,\r\n",
"1000,200,300\r\n",
]mypy ouput:
$ mypy --strict csv_writer.py
csv_writer.py:16: error: Dict entry 0 has incompatible type "int": "str"; expected "str": "Any"
csv_writer.py:16: error: Dict entry 1 has incompatible type "int": "str"; expected "str": "Any"
csv_writer.py:17: error: Dict entry 0 has incompatible type "MyEnum": "int"; expected "str": "Any"
csv_writer.py:17: error: Dict entry 1 has incompatible type "MyEnum": "int"; expected "str": "Any"
csv_writer.py:17: error: Dict entry 2 has incompatible type "MyEnum": "int"; expected "str": "Any"
Found 5 errors in 1 file (checked 1 source file)
This is on Python 3.8.0 with mypy 0.790
Activity
Akuli commentedon Jan 11, 2021
The dict keys can be any type compatible with
_csv._writer.writerow, which is currentlyAny-typed in the stubs. But at runtime, it doesn't work with e.g.enum.Enum, even though it works with at leastint,floatandstr(and also withIntEnumbecause it inherits fromint). I guess you could use a typevar like this:This typevar can be for example
str,float,int(because mypy thinks thatintinherits fromfloateven though it actually doesn't) andIntEnum(because it inherits fromint). But it can't beenum.Enum.I guess
DictWritershould be made generic. Then your code would type-check as is becauseDictWriter(...)defaults toDictWriter[Any], and if you want better type checking, you could doDictWriter[MyEnum].kanetkarster commentedon Jan 20, 2021
Awesome! I will take a stab at implementing this
Akuli commentedon May 7, 2021
Actually, it accepts any type, because the underlying
_csv.writer.writerowconverts everything to string:In your case, you needed
IntEnumto ensure thatMyEnum.foo == 0. It needs that for finding the key0from thefieldnameslist.