py-pydantic-patterns

Pydantic v2 Patterns

Problem Statement

Pydantic v2 has significant API changes from v1. This codebase uses v2. Wrong patterns cause validation failures, serialization bugs, and frontend integration issues.

---

Pattern: v1 to v2 Migration

Critical changes to know:

# ❌ v1 (OLD - don't use)
from pydantic import validator
class Model(BaseModel):
    class Config:
        orm_mode = True
    
    @validator("email")
    def validate_email(cls, v):
        return v.lower()
    
    def dict(self):
        ...

# ✅ v2 (CURRENT)
from pydantic import field_validator, ConfigDict
class Model(BaseModel):
    model_config = ConfigDict(from_attributes=True)
    
    @field_validator("email")
    @classmethod
    def validate_email(cls, v: str) -> str:
        return v.lower()
    
    def model_dump(self):
        ...

Quick reference:

v1	v2
class Config	model_config = ConfigDict(...)
orm_mode = True	from_attributes=True
.dict()	.model_dump()
.json()	.model_dump_json()
@validator	@field_validator
@root_validator	@model_validator
parse_obj()	model_validate()
update_forward_refs()	model_rebuild()

---

Pattern: Field Validators

from pydantic import BaseModel, field_validator, ValidationInfo

class AssessmentCreate(BaseModel):
    title: str
    skill_areas: list[str]
    max_score: int
    
    # Single field validator
    @field_validator("title")
    @classmethod
    def title_not_empty(cls, v: str) -> str:
        if not v.strip():
            raise ValueError("Title cannot be empty")
        return v.strip()
    
    # Validator with access to other fields
    @field_validator("max_score")
    @classmethod
    def validate_max_score(cls, v: int, info: ValidationInfo) -> int:
        if v < 1:
            raise ValueError("Max score must be positive")
        return v
    
    # Multiple fields
    @field_validator("skill_areas")
    @classmethod
    def validate_skill_areas(cls, v: list[str]) -> list[str]:
        valid = {"fundamentals", "advanced", "strategy"}
        for area in v:
            if area not in valid:
                raise ValueError(f"Invalid skill area: {area}")
        return v

---

Pattern: Model Validators