Stop Writing Manual Validation: Pydantic + FastAPI

One of the biggest time-sinks in web development is writing if/else statements to check if a request body is valid.

In the Node.js post I shared earlier, we talked about structure. Today, let’s talk about Safety. In Python, we use Pydantic models to handle the "boring stuff" automatically.

The Manual Way (Avoid this)

@app.post("/login")
def login(data: dict):
    if "email" not in data:
        return {"error": "Email is required"}
    # ... more messy checks

The FastAPI Way (Do this)
By defining a class, FastAPI will automatically return a 422 Unprocessable Entity error if the user sends bad data. You don't have to write a single if statement.

from pydantic import BaseModel, EmailStr

class LoginSchema(BaseModel):
    email: EmailStr
    password: str
    remember_me: bool = False  # Default value

@app.post("/login")
def login(data: LoginSchema):
    # data is already validated and typed here!
    return {"message": f"Welcome {data.email}"}

Why this wins:

• Type Safety: Your IDE knows exactly what data.email is.

• Auto Docs: Your Swagger UI updates instantly to show the required fields.

• Cleaner Code: Your business logic stays separate from your validation logic.

If you’re moving from Express to FastAPI, Pydantic is the "killer feature" you’ll wish you had sooner.