How to get the max_length value of a field in a Pydantic model through code?

This article discusses how to extract max_length value of a field from a Pydantic model. We will demonstrate how to avoid common errors and provide solutions for different versions of Pydantic.

Suppose we have a Pydantic model called MetaCollection where description field sets max_length to 1024:

 from pydantic import BaseModel, Field

class MetaCollection(BaseModel):
    id: int | None = Field(default=None)
    collection_name: str = Field(max_length=64)
    description: str | None = Field(None, max_length=1024)
    # ...Other fields

It is wrong to try directly to access the extra dictionary to get max_length value, which will result in KeyError . The correct way is to directly access max_length property of field_info object:

 # Get the max_length of the description field
description_max_length = MetaCollection.model_fields["description"].field_info.max_length
print(description_max_length) # Output 1024

Note that we used MetaCollection.model_fields instead of __fields__ . model_fields is a recommended way in Pydantic to access model field information.

Pydantic version differences:

The above method is suitable for Pydantic v2 and later. For older versions (such as Pydantic v1), the structure of field_info may be slightly different. Be sure to refer to the documentation for the Pydantic version you are using to ensure the correctness of the code. In Pydantic v1, max_length may be in field_info.extra , but this is not a reliable way to do it. It is recommended to upgrade to the latest version of Pydantic for better compatibility and a clearer API.

Summarize:

The key to getting the max_length value of the Pydantic model field is to directly access model_fields property and then get the required value through field_info.max_length . Make sure your code is compatible with the version of Pydantic you are using. Always refer to official documentation for the latest best practices and API information.

The above is the detailed content of How to get the max_length value of a field in a Pydantic model through code?. For more information, please follow other related articles on the PHP Chinese website!