Understanding Deep Copy Behavior with Default Parameters in Pydantic Models

What will you learn?

In this tutorial, we will delve into the behavior of custom objects used as default parameters in Pydantic models. We’ll explore why deep copying may not occur as expected and how to resolve this issue effectively.

Introduction to Problem and Solution

When working with Pydantic, a powerful data validation library in Python, handling default parameter values, especially when they involve custom objects, can lead to unexpected behavior. The root of the problem lies in how Python functions and methods evaluate their default arguments – only once at function definition time. If these defaults are mutable objects like lists or instances of custom classes, shared state issues can arise among instances.

The solution lies in ensuring that each model instance receives its own unique copy of the default parameter. By utilizing factory functions as default values instead of directly assigning mutable objects, we gain control over the instantiation process. This approach guarantees that instances have independent copies of default parameters, avoiding inadvertent shared-state problems.


from pydantic import BaseModel
from typing import List
from copy import deepcopy

def generate_default_object():
    # Replace `YourCustomObject` with your actual object class.
    return deepcopy(YourCustomObject())

class YourModel(BaseModel):
    your_field: List[YourCustomObject] = []

# Usage example:
model_instance = YourModel()

# Copyright PHD


In this solution:

  • generate_default_object: A factory function that generates a new instance of our custom object with proper deep copying using deepcopy.

  • Assignment in YourModel: Instead of setting mutable objects directly as default values (e.g., your_field: List[YourCustomObject] = []), we opt for the factory pattern to ensure unique copies per instance.

This strategy guarantees that each YourModel instance has its distinct copy of default parameters without sharing state unless intentionally designed otherwise.

    1. What is a mutable object? Mutable objects can change their state or contents after creation; examples include lists, dictionaries, sets, and most user-defined classes.

    2. Why isn’t my custom object deep-copied by default? Python evaluates function/method default arguments only once at definition time, leading to shared instances if they are mutable types.

    3. What is Pydantic? Pydantic is a data validation and settings management library using Python type annotations for runtime data parsing and validation.

    4. How does deepcopy() work? The deepcopy() function creates independent copies of the original object and all nested objects it contains to prevent unintended shared-state issues.

    5. Can I always use factory functions for defaults? While using factory functions helps avoid issues with mutable defaults, consider overhead from additional function calls; use judiciously where needed.

    6. Does this approach affect performance? There may be minimal performance implications due to creating new instances through factories but benefits often outweigh concerns where strict separation between instances is required.


Understanding the nuances of managing mutable vs immutable types within Pydantic models is essential for maintaining clean and predictable codebases. By implementing the strategies discussed here and employing thorough testing practices, navigating challenges related to shared-state issues becomes a more straightforward task.

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