How to test a function with input call?

Question

I have a console program written in Python. It asks the user questions using the command:

some_input = input('Answer the question:', ...) 

How would I test a function containing a call to input using pytest? I wouldn't want to force a tester to input text many many times only to finish one test run.

Answer 1

As The Compiler suggested, pytest has a new monkeypatch fixture for this. A monkeypatch object can alter an attribute in a class or a value in a dictionary, and then restore its original value at the end of the test.

In this case, the built-in input function is a value of python's __builtins__ dictionary, so we can alter it like so:

def test_something_that_involves_user_input(monkeypatch):      # monkeypatch the "input" function, so that it returns "Mark".     # This simulates the user entering "Mark" in the terminal:     monkeypatch.setattr('builtins.input', lambda _: "Mark")      # go about using input() like you normally would:     i = input("What is your name?")     assert i == "Mark" 

Answer 2

You should probably mock the built-in input function, you can use the teardown functionality provided by pytest to revert back to the original input function after each test.

import module  # The module which contains the call to input  class TestClass:      def test_function_1(self):         # Override the Python built-in input method          module.input = lambda: 'some_input'         # Call the function you would like to test (which uses input)         output = module.function()           assert output == 'expected_output'      def test_function_2(self):         module.input = lambda: 'some_other_input'         output = module.function()           assert output == 'another_expected_output'              def teardown_method(self, method):         # This method is being called after each test case, and it will revert input back to original function         module.input = input   

A more elegant solution would be to use the mock module together with a with statement. This way you don't need to use teardown and the patched method will only live within the with scope.

import mock import module  def test_function():     with mock.patch.object(__builtins__, 'input', lambda: 'some_input'):         assert module.function() == 'expected_output'