Mocking Python datetime In Tests With FreezeGun

This post will use the FreezeGun library to demonstrate how to mock the datetime.datetime.now return value to set up consistent test environments.

The repo code can be found on GitHub at okeeffed/hello-python-datetime

Prerequisites

Getting started

The previous code will built on top of the code from Datetime In Python. The final repo is at okeeffed/hello-python-datetime.

For the sake of simplicity, we will operate as if we are building a brand new repo:

Let's create the hello-python-datetime directory and install the required dependencies.

# Make the `hello-python-datetimes` directory
$ mkdir hello-python-datetimes
$ cd hello-python-datetimes

# Init the virtual environment
$ pipenv --three
$ pipenv install --dev ipython freezegun types-freezegun

# Create a folder to place files
$ mkdir src tests
# Create the required files
$ touch src/datetimes.py src/__init__.py tests/datetimes_test.py tests/__init__.py main.py
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At this stage, we are now ready to update our main.py file and src/datetimes.py to be up to par with what we need for testing.

Add the following to src/datetimes.py:

from datetime import date


def is_date_before_today(date_str: str):
    """Check if date is before today

    Args:
        date_str (str): String of a date to pass

    Returns:
        bool: Value of if date is before today
    """
    try:
        date_obj = date.fromisoformat(date_str)
        return date_obj < date.today()
    except Exception:
        return False
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Add the following to main.py:

from src.datetimes import is_date_before_today
from datetime import datetime, timedelta

print(is_date_before_today("2019-01-01"))
print(is_date_before_today("2022-01-01"))
print(is_date_before_today("2021-08-03"))
print(is_date_before_today("2021-08-04"))

now = datetime.now()
now_str = now.strftime('%Y-%m-%d')
print(now_str)
print(is_date_before_today(now_str))

now_subtract_one_day = now - timedelta(days=2)

now_subtract_one_day_str = now_subtract_one_day.strftime('%Y-%m-%d')
print(now_subtract_one_day_str)
print(is_date_before_today(now_subtract_one_day_str))

now_add_one_day = now + timedelta(days=1)

now_add_one_day_str = now_add_one_day.strftime('%Y-%m-%d')
print(now_add_one_day_str)
print(is_date_before_today(now_add_one_day_str))
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Running python main.py should bring us up to par with the following:

$ python main.py
True
False
True
True
2021-08-05
False
2021-08-03
True
2021-08-06
False
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The output matches up to us printing values out from main.py. We are at a stage now where we are up to par and able to start writing tests.

Exploring FreezeGun with PyTest

FreezeGun is a library that helps with mocking out the datetime.datetime.now function. It is a very useful tool for testing code that uses the datetime library.

We can use the library with a decorator for the test or creating a with block.

To demonstrate, add the following code to tests/datetimes_test.py:

import datetime
from freezegun import freeze_time
from src.datetimes import is_date_before_today


def test_freeze_time():
    assert datetime.datetime.now() != datetime.datetime(2012, 1, 14)
    # Mocking the time to be 2012-01-14
    with freeze_time("2012-01-14"):
        assert datetime.datetime.now() == datetime.datetime(2012, 1, 14)
    # Without the mock, the time should be back to normal
    assert datetime.datetime.now() != datetime.datetime(2012, 1, 14)


@freeze_time("2012-01-14")
def test_freeze_time_with_decorator():
    # Testing with a decorator that mocks throughout the test
    assert datetime.datetime.now() == datetime.datetime(2012, 1, 14)
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The first test demonstrates the with block will the second test demonstrates usage with a decorator.

Running pipenv run pytest will now run the tests and display the results.

$ pipenv run pytest
pipenv run pytest
================================== test session starts ===================================
platform darwin -- Python 3.9.6, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /path/to/hello-python-datetimes
collected 2 items

tests/test_datetimes.py ..                                                          [100%]

=================================== 2 passed in 0.21s ====================================
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Now we are ready to test our is_date_before_today function in a manner similar to how our main.py invokes the functions.

Testing the is_date_before_today function

Let's update our tests/datetimes_test.py file to test the is_date_before_today function.

import datetime
from freezegun import freeze_time
from src.datetimes import is_date_before_today


def test_freeze_time():
    assert datetime.datetime.now() != datetime.datetime(2012, 1, 14)
    # Mocking the time to be 2012-01-14
    with freeze_time("2012-01-14"):
        assert datetime.datetime.now() == datetime.datetime(2012, 1, 14)
    # Without the mock, the time should be back to normal
    assert datetime.datetime.now() != datetime.datetime(2012, 1, 14)


@freeze_time("2012-01-14")
def test_freeze_time_with_decorator():
    # Testing with a decorator that mocks throughout the test
    assert datetime.datetime.now() == datetime.datetime(2012, 1, 14)

# Converting the output we expected from main.py into a set of tests.
# Mocking time unnecessary, but done for the sake of completion.


@freeze_time("2021-08-05")
def test_is_date_before_today():
    """Should return False"""
    now = datetime.datetime.now()
    now_str = now.strftime('%Y-%m-%d')
    assert is_date_before_today(now_str) is False


@freeze_time("2021-08-05")
def test_is_one_day_ago_before_today():
    """Should return True"""
    now_subtract_one_day = datetime.datetime.now() - datetime.timedelta(days=1)
    now_subtract_one_day_str = now_subtract_one_day.strftime('%Y-%m-%d')
    assert is_date_before_today(now_subtract_one_day_str) is True


@freeze_time("2021-08-05")
def test_is_one_day_ahead_before_today():
    """Should return False"""
    now_add_one_day = datetime.datetime.now() + datetime.timedelta(days=1)
    now_add_one_day_str = now_add_one_day.strftime('%Y-%m-%d')
    assert is_date_before_today(now_add_one_day_str) is False
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In our test, we are freezing time (using the decorator) to the date of this blog post 2021-08-05 checking the following scenarios:

We can confirm this to be true by once again running pipenv run pytest:

$ pipenv run pytest
================================== test session starts ===================================
platform darwin -- Python 3.9.6, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /Users/dennisokeeffe/code/blog-projects/hello-python-datetimes
collected 5 items

tests/test_datetimes.py .....                                                      [100%]

=================================== 5 passed in 0.21s ====================================
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Summary

Today's post demonstrated how to use the FreezeGun package to mock the date for testing with PyTest.

Resources and further reading

Photo credit: pawel_czerwinski