Using unittest.mock to patch input() in Python 3

Question

How do you use the @patch decorator to patch the built-in input() function?

For example, here's a function in question.py that I'd like to test, which contains a call to input():

def query_yes_no(question, default="yes"):
""" Adapted from http://stackoverflow.com/questions/3041986/python-command-line-yes-no-input """

    valid = {"yes": True, "y": True, "ye": True, "no": False, "n": False}
    if default is None:
        prompt = " [y/n] "
    elif default == "yes":
        prompt = " [Y/n] "
    elif default == "no":
        prompt = " [y/N] "
    else:
        raise ValueError("invalid default answer: '%s'" % default)

    while True:
        sys.stdout.write(question + prompt)
        choice = input().lower()

        if default is not None and choice == '':
            return valid[default]
        elif choice in valid:
            return valid[choice]
        else:
            sys.stdout.write("Please respond with 'yes' or 'no' "
                             "(or 'y' or 'n').\n")

Here's my test, which gives me the error "ImportError: No module named 'builtins'":

import unittest
from unittest.mock import patch

import question

class TestQueryYesNo(unittest.TestCase):

    @patch('__builtins__.input.return_value', 'y')
    def test_query_y(self):
        answer = question.query_yes_no("Blah?")
        self.assertTrue(answer)

Answer 1

__builtin__ module is renamed to builtins in Python 3. Replace as follow:

@patch('builtins.input', lambda *args: 'y')

UPDATE

input has an optional parameter. updated the code to accept the optional parameter.

Answer 2

Or use Mock's return_value attribute. I couldn't get it to work as a decorator, but here's how to do it with a context manager:

>>> import unittest.mock
>>> def test_input_mocking():
...     with unittest.mock.patch('builtins.input', return_value='y'):
...         assert input() == 'y'
...
>>> def test_input_mocking():
...     with unittest.mock.patch('builtins.input', return_value='y'):
...         assert input() == 'y'
...         print('we got here, so the ad hoc test succeeded')
...
>>> test_input_mocking()
we got here, so the ad hoc test succeeded
>>>