On , I learnt ...
How to use type annotations with Python’s
Adding a type annotation for a “writer” object created by
fiddlier than you might think.
The type of the writer object is
_csv.writer which you verify with:
>>> import csv, sys >>> type(csv.writer(sys.stdout)) <class '_csv.writer'>
but if you try this:
import _csv def generate_report(writer: _csv.writer): pass
mypy complains that:
Function "_csv.writer" is not valid as a type [valid-type].
The correct approach is to use
_csv._writer as the type annotation. A string
literal is required otherwise you’ll get a
AttributeError: module '_csv' has no attribute '_writer' exception.
import _csv def generate_report(writer: '_csv._writer'): pass