Module 5 Exercises: Pydantic Proficiency

Mastering Pydantic is the difference between a "Script" and a "System." These exercises will challenge your ability to handle complex validation rules.

---

Exercise 1: The Account Number Constraint

Design a Pydantic model called WireTransfer. It must have:

• sender_id (Integer)
• receiver_id (Integer)
• amount (Float, must be greater than zero)
• currency (String, must be exactly 3 uppercase characters, e.g., "USD")

---

Exercise 2: Implementing the Date Validator

Building on Exercise 1, add a Model Validator to ensure that sender_id and receiver_id are not the same. You shouldn't be able to wire money to yourself!

---

Exercise 3: The Secure Output

You are building a Patient Portal for a hospital. You have a database object for a patient that looks like this:

patient_record = {
    "id": 1,
    "name": "Jane Doe",
    "ssn": "123-44-5678", # SOCIAL SECURITY NUMBER (CRITICAL SECRET)
    "blood_type": "O+",
    "insurance_id": "INS-99"
}

Design a PatientOut model and a FastAPI function signature that ensures the ssn is never sent to the client.

---

Self-Correction / Discussion

Exercise 1 Answer:

from pydantic import BaseModel, Field

class WireTransfer(BaseModel):
    sender_id: int
    receiver_id: int
    amount: float = Field(..., gt=0)
    currency: str = Field(..., min_length=3, max_length=3, pattern="^[A-Z]{3}$")

Exercise 2 Answer:

from pydantic import model_validator

class WireTransfer(BaseModel):
    # ... previous fields ...
    @model_validator(mode='after')
    def check_different_parties(self) -> 'WireTransfer':
        if self.sender_id == self.receiver_id:
            raise ValueError('Sender and Receiver must be different')
        return self

Exercise 3 Answer:

class PatientOut(BaseModel):
    id: int
    name: str
    blood_type: str
    insurance_id: str

@app.get("/patients/{id}", response_model=PatientOut)
async def get_patient(id: int):
    return patient_record # Pydantic will drop the 'ssn' automatically!

---

Summary of Module 5

You now have a "Golden Layer" between your data and the world.

• Models: You can define strict schemas.
• Validation: You can write deep logic to prevent bad data.
• Shaping: You can protect secrets with output filters.

In Module 6: Dependency Injection, we will learn about FastAPI's most powerful architectural feature—the system that manages your database connections and security checks.