How do I test a get_date function using pytest?

I got this function called get_date in a project and I'm requested to test it via pytest.

def get_date(string):
    try:
        date_str = input(string)
        date = datetime.datetime.strptime(date_str, "%Y-%m-%d").date()
        return date
    except ValueError:
        print("Please enter a date in the YYYY-MM-DD format")
        return get_date(string)

I previously had:

def get_date(string):
    while True:
        try:
            date_str = input(string)
            date = datetime.datetime.strptime(date_str, "%Y-%m-%d").date()
            return date
        except ValueError:
            print("Please enter a date in the YYYY-MM-DD format")
            raise

def test_get_date(monkeypatch):
    inputs = [
        "12/06/2023",
        "12-06-2023",
        "2023/06/12",
        "2023/17/87",
        "hello",
        "2023-06-12",
    ]
    input_values = iter(inputs)
    monkeypatch.setattr("builtins.input", lambda _: next(input_values))

    # Test first input ("12/06/2023")
    with pytest.raises(ValueError):
        get_date("Date (YYYY-MM-DD): ")

    # Test second input ("12-06-2023")
    with pytest.raises(ValueError):
        get_date("Date (YYYY-MM-DD): ")

    # Test third input ("2023/06/12")
    with pytest.raises(ValueError):
        get_date("Date (YYYY-MM-DD): ")

    # Test fourth input ("hello")
    with pytest.raises(ValueError):
        get_date("Date (YYYY-MM-DD): ")

    # Test fifth output ("2023-06-12")
    assert get_date("Date (YYYY-MM-DD): ") == datetime.date(2023, 6, 12)

The issue with this is that when the ValueError is raised the program crashes instead of re-prompting the user for new input, which is something that really can't happen.

I thought of changing the function to a is_date function that would return a bool, but that would involve changing the whole program, which is something I'm not looking for since everything works just fine.

Also, this is a void function and because of it I can't use the usual assert method. I did some research and found this iter method which I'm not sure is the optimal way to do so.

I really hope you can help me solve this problem

By adding raise, you re-raise the ValueError after printing the error msg and thus it crashes.

so if you want the program to repromt for the user to re-enter data remove the 'raise' statement in the get-date function.

Hope This Helps

Here is the whole code:

import datetime
import pytest

def get_date(string):
    while True:
        try:
            date_str = input(string)
            date = datetime.datetime.strptime(date_str, "%Y-%m-%d").date()
            return date
        except ValueError:
            print("Please enter a date in the YYYY-MM-DD format")

def test_get_date(monkeypatch, capfd):
    inputs = [
        "12/06/2023",
        "12-06-2023",
        "2023/06/12",
        "2023/17/87",
        "hello",
        "2023-06-12",
    ]
    input_values = iter(inputs)
    monkeypatch.setattr("builtins.input", lambda _: next(input_values))

    # invalid inputs test
    with pytest.raises(ValueError):
        get_date("Date (YYYY-MM-DD): ")
        captured = capfd.readouterr()
        assert "Please enter a date in the YYYY-MM-DD format" in captured.out

    # valid input test
    assert get_date("Date (YYYY-MM-DD): ") == date(2023, 6, 12)

capfd.readouterr() allows you to capture the output that would have been printed to the console.
if you wanna more about it doc

Hope This Is Helpful

Here's an approach that encapsulates the re-prompting the user in case of an input error inside the get_input function.

def get_input(string):
    date_str = input(string)
    while True:
        try:
            date = datetime.datetime.strptime(date_str, "%Y-%m-%d").date()
        except ValueError:
            print("Please enter a date in the YYYY-MM-DD format")
            date_str = input(string)
    return date

You don't need to change the function to a boolean. Instead just take the date string as an argument and move the user input outside to a separate function.

get_date function without user input:

def get_date(date_str):
    try:
        date = datetime.datetime.strptime(date_str, "%Y-%m-%d").date()
        return date
    except ValueError:
        raise

Testing:

def test_get_date():
    assert get_date("2023-06-12") == datetime.date(2023, 6, 12)

    with pytest.raises(ValueError):
        get_date("stuff")

    with pytest.raises(ValueError):
        get_date("2023/06/12")

    with pytest.raises(ValueError):
        get_date("2023/69/420")

Add the user input back into your code before you call get_date and hand it off as as the data_str argument.

You have the correct idea in your original code - you just need to skip calling get_date for all the intermediate cases, because the execution of get_date() will handle those bad inputs, and then prompt for the next input.

Then at the end we can assert that the list of inputs was exhausted.

#script.py
import datetime

def get_date(string):
    # Use a loop, not recursion
    while True:
        try:
            date_str = input(string)
            date = datetime.datetime.strptime(date_str, "%Y-%m-%d").date()
        except ValueError:
            print("Please enter a date in the YYYY-MM-DD format")
    
    return date

#test_script.py
import pytest
import datetime
from script import get_date

def test_get_date(monkeypatch):
    inputs = [
        "12/06/2023",
        "12-06-2023",
        "2023/06/12",
        "2023/17/87",
        "hello",
        "2023-06-12",
    ]
    input_values = iter(inputs)
    monkeypatch.setattr("builtins.input", lambda _: next(input_values))

    # Test that all other inputs were handled and we accept the last one
    assert get_date("Date (YYYY-MM-DD): ") == datetime.date(2023, 6, 12)

    with pytest.raises(StopIteration):
        _ = input("Verify inputs list is exhausted")

If the inputs list is not exhausted (there are values after an "accepted" input), then Pytest will fail with

FAILED test_script.py::test_get_date - Failed: DID NOT RAISE <class 'StopIteration'>