How to type-annotate object returned by csv.writer?

Question

I want to apply type annotation to the returned object of csv.writer in order to comply with a larger codebase. Unfortunately I can not figure out the fitting return type.

>>> import csv
>>> writer = csv.writer(open('outfile.csv', 'w'))
>>> type(writer)
<class '_csv.writer'>

If I try to use this classname:

>>> import _csv
>>> writer: _csv.writer = csv.writer(open('outfile.csv', 'w'))

I get the following mypy error:

Invalid type "_csv.writer"

Does someone know which type to use in this case. Of course I could use typing.Any but this nullify the sense of a type annotation.

Answer 1

Often when things are acting weird it's a sign typeshed doesn't exactly map to runtime. If you look at _csv in typeshed you will see the type is named _writer. So you should can the annotation to _csv._writer.

Answer 2

Short answer is there is no way to directly access the type. Reading the C source of the _csv module will show that the types of reader and writer are not exposed. Even in Pypy where the _csv module is implemented in Python, doesn't expose the types.

Therefore, if you need to use it, you will need to use a work-around, by instantiating a temporary instance of the writer and getting it's type.

import csv
# We'll need a temporary file-like object, so use a tempfile
from tempfile import TemporaryFile

with TemporaryFile() as t:
    CSVReader = type(csv.reader(t))
    CSVWriter = type(csv.writer(t))

w: CSVWriter = csv.writer('path/to/data.csv')

If you want keep this logic separate, I would suggest creating the types in a separate module

from csv_types import CSVReader, CSVWriter

The other solution (which also involves writing your own module of types), is to follow the example of the typing module in it's definition of types for io and re.