What is Pydantic and why is it used with FastAPI?

Question

Accepted Answer

**Pydantic** is a **data validation** library using Python type hints.

```python
from pydantic import BaseModel, EmailStr, Field, validator
from datetime import datetime
from typing import Optional

class User(BaseModel):
    name: str = Field(..., min_length=1, max_length=100)
    email: EmailStr
    age: int = Field(ge=0, le=150)
    created_at: datetime = Field(default_factory=datetime.now)
    bio: Optional[str] = None

@validator("name")
    def name_must_be_title_case(cls, v):
        return v.title()

class Config:
        orm_mode = True  # Works with ORMs

# Validation happens automatically
user = User(name="alice", email="alice@example.com", age=25)
print(user.dict())  # Serialized to dict
print(user.json())  # Serialized to JSON

# Error handling
from pydantic import ValidationError
try:
    User(name="", email="invalid")
except ValidationError as e:
    print(e.errors())  # List of validation errors
```

**Benefits:**
- Automatic validation from type hints.
- Serialization (dict, JSON).
- IDE autocompletion.
- Fast (written in Rust).

Python: What is Pydantic and why is it used with FastAPI?