How to test a function with input call?

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Question :

How to test a function with input call?

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:

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'
Answered By: Forge

Answer #2:

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"
Answered By: mareoraft

Answer #3:

You can replace sys.stdin with some custom Text IO, like input from a file or an in-memory StringIO buffer:

import sys

class Test:
    def test_function(self):
        sys.stdin = open("preprogrammed_inputs.txt")

    def setup_method(self):
        self.orig_stdin = sys.stdin

    def teardown_method(self):
        sys.stdin = self.orig_stdin

this is more robust than only patching input(), as that won’t be sufficient if the module uses any other methods of consuming text from stdin.

This can also be done quite elegantly with a custom context manager

import sys
from contextlib import contextmanager

def replace_stdin(target):
    orig = sys.stdin
    sys.stdin = target
    sys.stdin = orig

And then just use it like this for example:

with replace_stdin(StringIO("some preprogrammed input")):
Answered By: Felk

Answer #4:

This can be done with mock.patch and with blocks in python3.

import pytest
import mock
import builtins

The function to test (would usually be loaded
from a module outside this file).
def user_prompt():
    ans = input('Enter a number: ')
        import sys
    return 'Your number is {}'.format(ans)

This test will mock input of '19'
def test_user_prompt_ok():
    with mock.patch.object(builtins, 'input', lambda _: '19'):
        assert user_prompt() == 'Your number is 19'

The line to note is mock.patch.object(builtins, 'input', lambda _: '19'):, which overrides the input with the lambda function. Our lambda function takes in a throw-away variable _ because input takes in an argument.

Here’s how you could test the fail case, where user_input calls sys.exit. The trick here is to get pytest to look for that exception with pytest.raises(SystemExit).

This test will mock input of 'nineteen'
def test_user_prompt_exit():
    with mock.patch.object(builtins, 'input', lambda _: 'nineteen'):
        with pytest.raises(SystemExit):

You should be able to get this test running by copy and pasting the above code into a file tests/ and running pytest from the parent dir.

Answered By: AlexG

Answer #5:

You can do it with mock.patch as follows.

First, in your code, create a dummy function for the calls to input:

def __get_input(text):
    return input(text)

In your test functions:

import my_module
from mock import patch

@patch('my_module.__get_input', return_value='y')
def test_what_happens_when_answering_yes(self, mock):
    Test what happens when user input is 'y'
    # whatever your test function does

For example if you have a loop checking that the only valid answers are in [‘y’, ‘Y’, ‘n’, ‘N’] you can test that nothing happens when entering a different value instead.

In this case we assume a SystemExit is raised when answering ‘N’:

def test_invalid_answer_remains_in_loop(self, mock):
    Test nothing's broken when answer is not ['Y', 'y', 'N', 'n']
    with self.assertRaises(SystemExit):
        mock.side_effect = ['k', 'l', 'yeah', 'N']
        # call to our function asking for input
Answered By: fernandezcuesta

Answer #6:

Since I need the input() call to pause and check my hardware status LEDs, I had to deal with the situation without mocking. I used the -s flag.

python -m pytest -s

The -s flag essentially means: shortcut for –capture=no.

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