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How to use type annotations with Python’s csv module

Adding a type annotation for a “writer” object created by csv.writer(...) is 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):

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'):