How to mock flask.request object in Python

Here is an example on how to mock flask.request module and test headers value of it in Python using pytest

# test_module.py

import pytest
import flask

def test_user_name(mocker):
    # GIVEN: user is provided in the request.headers
    given_user_name = "Some_User"
    request_mock = mocker.patch.object(flask, "request")
    request_mock.headers.get.return_value = given_user_name

    # WHEN: request.header.get method is called
    result = common.user_name()

    # THEN: user name should be returned
    request_mock.headers.get.assert_called_once_with("USERNAME", "Invalid User")
    assert result == given_user_name
# common.py module

import flask

def user_name():
    return flask.request.headers.get("USERNAME", "Invalid User")