Infinite loop testing

Question

How could I test an infinite loop? For example:

func.py

def func():
    while True:
        char = input().lower()
        if char == 'q':
            break 
        elif char.isalpha():
            print("It's not a number")
        if int(char) == #some_number
            break
        else:
            print('Try again')
        

test.py

def test_func():
    ???

Is there any solution for testing this?

Answer 1

You can use unittest to mock the user's input:

import builtins
from unittest.mock import patch

def func():
    while True:
        char = input().lower()
        if char == 'q':
            break 
        elif char.isalpha():
            print("It's not a number")
        if int(char) == 3:
            break
        else:
            print('Try again')

def test_func(inputs):
    with patch("builtins.input") as input_mock:
        input_mock.side_effect = inputs
        func()

test_func(["q"])
test_func(["3"])  
test_func(["4", "5", "6"])  # StopIteration error indicates this input is not sufficient for the function to return
test_func(["a", "b", "c", "q"])  # ValueError indicates a bug in the function
test_func(["4", "5", "6", "q"])  # Try again 3 times as expected

EDIT: You could also use unittest to capture the printed output and return it, so that you can check this output systematically against the expected output.

import builtins
from unittest.mock import patch
import io

def test_func(inputs):
    with patch("builtins.input") as input_mock, \
         patch("sys.stdout", new_callable=io.StringIO) as output_mock:
        input_mock.side_effect = inputs
        try:
            func()
        except StopIteration:
            print("FUNCTION DID NOT RETURN")
        return output_mock.getvalue().strip().split("\n")

print(test_func(["q"]) == [""]) # True
print(test_func(["4", "5", "6", "q"]) == ['Try again', 'Try again', 'Try again']) # True