Understanding Iterators in Pydantic Models

Making a Pydantic Field an Iterator

We frequently encounter scenarios where working with iterators within our data models becomes essential. Let’s delve into how we can make a field in a Pydantic model act as an iterator.

What You Will Learn

In this comprehensive tutorial, you will grasp the art of seamlessly integrating iterators into your Pydantic models for enhanced efficiency and flexibility.

Introduction to the Problem and Solution

At times, there arises a necessity to have one of the fields in a Pydantic model behave like an iterator. This requirement might stem from reasons such as lazily loading items on demand or efficiently processing streams of data. However, by default, Pydantic fields are not inherently designed to function as iterators.

To address this challenge effectively, we will explore customizing our Pydantic model by leveraging root validators or custom types. By adopting this approach, we can guide our model on intelligently handling iterable fields. Additionally, we will emphasize the significance of type annotations in ensuring that our solution is not only robust but also scalable.


from pydantic import BaseModel, validator
from typing import Iterator

class IterableFieldModel(BaseModel):
    numbers: list[int]  # This will hold the initial data

    @validator('numbers', pre=True)
    def convert_to_iterator(cls, v):
        if isinstance(v, list):
            return iter(v)
        raise ValueError("numbers must be a list")

# Usage example
model = IterableFieldModel(numbers=[1, 2, 3])
for num in model.numbers:

# Copyright PHD


In this implementation:

  • IterableFieldModel defines a numbers field that expects input as a list.
  • A validator is linked to the numbers field triggering before other validation processes (pre=True).
  • Within the convert_to_iterator validator method, it verifies if the provided value is of type list. If true, it transforms this list into an iterator using Python’s built-in iter() function.
  • Any attempt to assign a non-list value raises a ValueError, ensuring only lists can be converted into iterators for this field.

This strategy enables dynamic transformation of incoming data into an iterable format while capitalizing on Pydantic’s robust validation capabilities.

  1. Can I use generators instead of lists?

  2. Yes! Generators are inherently iterable objects; however, once consumed they cannot regenerate unless explicitly redefined.

  3. How do I handle iteration over dictionaries?

  4. You can customize the validator method based on your needs � converting values or items into iterables rather than just lists.

  5. What happens if my iterator runs out of values?

  6. Iterators do not raise explicit errors when depleted but cease yielding new values; gracefully managing empty iterations is recommended.

  7. Can I revert my iterator back into a list?

  8. Certainly! Simply utilize Python’s built-in list() function around your iterator object whenever required.

  9. Are there performance benefits using iterators over lists?

  10. Iterators offer memory efficiencies since they do not store all elements simultaneously during iteration unlike lists which occupy more space depending on their size immediately upon creation.


Incorporating iterable functionality within Pydantic models paves the way for versatile possibilities especially when handling large datasets or streaming inputs. By judiciously applying validators and grasping Python�s iterative protocols � efficient data processing patterns seamlessly align with contemporary software practices.

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