pydantic

Pydantic v2 Patterns

Validation, serialization, and settings management with Pydantic v2.

When to Activate

• Defining request/response schemas or domain models
• Writing @field_validator or @model_validator for custom validation
• Using Annotated to build reusable constrained types
• Controlling serialization with model_dump() / model_dump_json()
• Building generic models or discriminated unions
• Validating arbitrary data (not a model) with TypeAdapter
• Configuring app settings from environment variables with pydantic-settings

---

BaseModel Basics

from pydantic import BaseModel, Field
from datetime import datetime
from uuid import UUID

class User(BaseModel):
    id: UUID
    name: str
    email: str
    age: int = Field(ge=0, le=150)
    role: str = "user"          # default value
    created_at: datetime | None = None

# Instantiate
user = User(id="a1b2...", name="Alice", email="[email protected]", age=30)

# Access
user.name        # "Alice"
user.model_fields  # dict of FieldInfo

# Validate from dict / JSON
user = User.model_validate({"id": "...", "name": "Alice", ...})
user = User.model_validate_json('{"id": "...", "name": "Alice", ...}')

---

Field Constraints

from pydantic import BaseModel, Field
from typing import Annotated

class Product(BaseModel):
    name: str = Field(min_length=1, max_length=200, strip_whitespace=True)
    price: float = Field(gt=0, description="Price in USD")
    discount: float = Field(ge=0, le=1, default=0.0)    # 0–100%
    tags: list[str] = Field(default_factory=list, max_length=10)
    sku: str = Field(pattern=r"^[A-Z]{3}-\d{6}$")
    metadata: dict = Field(default_factory=dict)

    # Alias — accept "product_name" in input, use "name" in Python
    name: str = Field(alias="product_name")

Reusable constrained types with Annotated

from typing import Annotated
from pydantic import Field

# Define once, reuse everywhere
PositiveInt   = Annotated[int,   Field(gt=0)]
Percentage    = Annotated[float, Field(ge=0.0, le=1.0)]
NonEmptyStr   = Annotated[str,   Field(min_length=1, strip_whitespace=True)]
EmailStr      = Annotated[str,   Field(pattern=r"^[^@]+@[^@]+\.[^@]+$")]
UserId        = Annotated[str,   Field(min_length=36, max_length=36)]

class CreateUserRequest(BaseModel):
    name: NonEmptyStr
    email: EmailStr
    age: PositiveInt
    discount: Percentage = 0.0