pydantic.md

Pydantic Tricks

Constrained Lists

• Add type checking with additional constraints
• Enforce min or max number of items in list
• validate list is not empty and list are of a specific type

from typing import Annotated
from pydantic import BaseModel, Field

class MyModel(BaseModel):
    numbers: Annotated[list[int], Field(min_length=1, max_length=5)]

Custom Validator

• API or Data Processing needs to validate if incoming data meets certain criteria

Mode	When to Use
"after"	When you want to validate or modify data after Pydantic has done its initial parsing and type conversion.
"before"	When you need to preprocess raw input data before Pydantic's validation.
"wrap"	When you need the most control over the validation process, allowing you to intervene before and after Pydantic's validation.
"plain"	When you want to completely replace Pydantic's validation with your own logic.

    @model_validator(mode="after")
    def validate_input_fields(self) -> Self:
        if not any([self.resolution, self.root_cause, self.diagnostic_steps]):
            raise ValueError("Field 'resolution', 'root_cause' or 'diagnostic_steps' is required.")
        return self