Field Constraints with Field()

Field() lets you add constraints beyond just the type. Set minimum values, maximum lengths, patterns, and more.

from pydantic import BaseModel, Field

class User(BaseModel):
    name: str = Field(min_length=1, max_length=50)
    age: int = Field(ge=0, le=150)  # ge = greater or equal

Now validation enforces these rules:

User(name="", age=30)  # Error: name too short
User(name="Alice", age=-5)  # Error: age below 0

Common constraints:

• min_length, max_length - for strings
• ge, gt, le, lt - for numbers (greater/less than or equal)
• pattern - regex pattern for strings

You can also add descriptions and examples for documentation:

email: str = Field(description="User's email address", examples=["user@example.com"])

These show up in auto-generated API docs.

I cover Field constraints in depth in my Pydantic course.