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Learn how to retain default values in Pydantic models even when `None` is assigned to a field. Understand the role of `validate_assignment` and other tips to improve your model handling in FastAPI and Pydantic.
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How to Pass a Default Value to a Variable in Pydantic When None is Passed

When working with Pydantic models, particularly in FastAPI applications, a common problem developers face is handling default values for fields effectively. Specifically, what if you want your model to assign a default value when None is passed during initialization or assignment? Let’s delve into the problem and its solution.

The Problem

Imagine you have a Pydantic model where you want to set a default value for a field, say name, to 'foo' whenever None is provided. While you can achieve this during initialization using a validator, what happens when the field is reassigned later in your code?

Example Code

Here's an example of how one might typically set up a Pydantic model:

[[See Video to Reveal this Text or Code Snippet]]

What You Encounter

You might test this model like so:

[[See Video to Reveal this Text or Code Snippet]]

As you can see, after the model's initialization, setting name to None does not trigger the validator, thus letting it become None rather than reverting back to 'foo'.

The Solution

Enabling validate_assignment

To ensure that the validators are applied when attributes are reassigned, you need to enable the validate_assignment option in your Pydantic model's configuration. This feature ensures that any time you assign a new value to a field, Pydantic runs the validators again.

Updated Example Code

Here’s how you can modify your model to utilize this feature:

[[See Video to Reveal this Text or Code Snippet]]

Testing the Updated Model

Now, let’s test this new setup:

[[See Video to Reveal this Text or Code Snippet]]

This change allows the name property to maintain the default value of 'foo' even when None is assigned after the initial creation of the User instance.

Conclusion

When working with Pydantic, managing default values effectively can save you from unexpected bugs down the line. By enabling validate_assignment, you can ensure your fields remain robust, defaulting back to their intended values even after being explicitly set to None. This minor adjustment leads to better data integrity within your FastAPI applications.

With these guidelines and code examples, you'll be well on your way to effectively managing your Pydantic models!