Dennis O'Keeffe
3 min readSep 7, 2021


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This is Day 17 of the #100DaysOfPython challenge.

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

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


  1. Familiarity with Pipenv. See here for my post on Pipenv.
  2. Familiarity with JupyterLab. See here for my post on JupyterLab.
  3. My previous blog post Datetime In Python
  4. Familiarity with PyTest. See my blog post Python Unit Testing With PyTest for a start post.

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.

At this stage, we are now ready to update our file and src/ to be up to par with what we need for testing.

Add the following to src/

Add the following to

Running python should bring us up to par with the following:

The output matches up to us printing values out from We are at a stage now where we are up to par and able to start writing tests.

Note: If you are using a virtual environment, you will need to run pipenv shell to enter the virtual environment.

Exploring FreezeGun with PyTest

FreezeGun is a library that helps with mocking out the 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/

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.

Now we are ready to test our is_date_before_today function in a manner similar to how our invokes the functions.

Testing the is_date_before_today function

Let’s update our tests/ file to test the is_date_before_today function.

In our test, we are freezing time (using the decorator) to the date of this blog post 2021-08-05 checking the following scenarios:

  1. is_date_before_today when compared to today should be False.
  2. is_date_before_today when compared to one day ago should be True.
  3. is_date_before_today when compared to one day ahead should be False.

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


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

Resources and further reading

  1. The ABCs of Pipenv
  2. Hello, JupyterLab
  3. FreezeGun
  4. Pipenv
  5. Python Unit Testing With PyTest

Photo credit: pawel_czerwinski

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