Pytest: mocking datetime date

import datetime
import math

def get_years_passed_category(date_string):
    date = datetime.date.fromisoformat(date_string)
    time_passed = datetime.date.today() - date
    years_passed = time_passed.days // 365
    years_passed_start_and_end_range_tuples_to_category = {
        (0, 1):    "Less than 1 year",
        (1, 5):    "1 - 5 years",
        (5, 10):   "5 - 10 years",
        (10, 20):  "10 - 20 years",
        (20, 30):  "20 - 30 years",
        (30, math.inf): "More than 30 years"
    }
    for (start, end), category in years_passed_start_and_end_range_tuples_to_category.items():
        if start <= years_passed < end:
            return category

    raise ValueError(f"Could not find a years_passed_category for '{date_string}'")

import app

for date in ['2000-01-01', '1990-06-02', '2020-01-01']:
    cat = app.get_years_passed_category(date)
    print(f"{date} : {cat}")

import app
import datetime
import pytest

def test_app(monkeypatch):
    mydt = datetime.date
    class MyDate():
        def today():
            return mydt(2021, 2, 15)
        def fromisoformat(date_str):
            return mydt.fromisoformat(date_str)

    monkeypatch.setattr(app.datetime, 'date', MyDate)

    assert app.get_years_passed_category('1990-06-02') == 'More than 30 years'
    assert app.get_years_passed_category('2000-01-01') == '20 - 30 years'
    assert app.get_years_passed_category('2011-01-01') == '10 - 20 years'
    assert app.get_years_passed_category('2016-01-01') == '5 - 10 years'
    assert app.get_years_passed_category('2020-01-01') == '1 - 5 years'
    assert app.get_years_passed_category('2021-02-14') == 'Less than 1 year'
    assert app.get_years_passed_category('2021-02-15') == 'Less than 1 year'


    with pytest.raises(Exception) as err:
        app.get_years_passed_category('2021-02-16')
    assert err.type == ValueError
    assert str(err.value) == "Could not find a years_passed_category for '2021-02-16'"