pydantic

Pydantic Validation Skill

Summary

Python data validation using type hints and runtime type checking with Pydantic v2's Rust-powered core for high-performance validation.

When to Use

• API request/response validation (FastAPI, Django)
• Settings and configuration management (env variables, config files)
• ORM model validation (SQLAlchemy integration)
• Data parsing and serialization (JSON, dict, custom formats)
• Type-safe data classes with automatic validation
• CLI argument parsing with type safety

Quick Start

from pydantic import BaseModel, Field, EmailStr
from datetime import datetime

class User(BaseModel):
    id: int
    name: str = Field(..., min_length=1, max_length=100)
    email: EmailStr
    created_at: datetime = Field(default_factory=datetime.now)
    is_active: bool = True

# Validate data
user = User(id=1, name="Alice", email="[email protected]")
print(user.model_dump())  # {'id': 1, 'name': 'Alice', ...}

# Automatic type coercion
user2 = User(id="2", name="Bob", email="[email protected]")
assert user2.id == 2  # String "2" coerced to int

# Validation error
try:
    User(id=3, name="", email="invalid")
except ValidationError as e:
    print(e.errors())

---

Core Concepts

BaseModel Foundation

from pydantic import BaseModel, ConfigDict

class Product(BaseModel):
    model_config = ConfigDict(
        str_strip_whitespace=True,
        validate_assignment=True,
        use_enum_values=True,
        arbitrary_types_allowed=False
    )

    name: str
    price: float
    quantity: int = 0

# Usage
product = Product(name="  Widget  ", price=19.99)
assert product.name == "Widget"  # Whitespace stripped

# Validate on assignment
product.price = "29.99"  # Auto-converts to float

Field Configuration

from pydantic import Field, field_validator
from typing import Annotated

class Item(BaseModel):
    # Field constraints
    sku: str = Field(pattern=r'^[A-Z]{3}-\d{4}$')
    price: float = Field(gt=0, le=10000)
    stock: int = Field(ge=0, default=0)

    # Annotated types (Pydantic v2)
    quantity: Annotated[int, Field(ge=1, le=100)]

    # Descriptions and examples
    description: str = Field(
        ...,
        description="Product description",
        examples=["High-quality widget"]
    )

    # Deprecated fields
    old_field: str | None = Field(None, deprecated=True)

    @field_validator('sku')
    @classmethod
    def validate_sku(cls, v: str) -> str:
        if not v.startswith('ABC'):
            raise ValueError('SKU must start with ABC')
        return v