Create the Expense Model

Define a Pydantic model with field-level validation for expenses

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BaseModel: the foundation for your data

Every Pydantic model inherits from BaseModel. This base class gives your class automatic validation, type conversion, and JSON serialization. You define fields as type-annotated class attributes, and BaseModel handles the rest.

Why field constraints matter

A basic Pydantic model checks data types, but it does not check values. Without constraints, your API will accept an expense with amount: -50 or description: "". Both pass type validation — one is a valid float, the other is a valid string — but neither makes sense as real data.

Pydantic's Field() function adds constraints directly to each field. The validation runs automatically when a request arrives. No extra code needed in your endpoint.

The constraints you will use

Field(min_length=1): rejects empty strings. A description of "" fails validation.
Field(gt=0): rejects zero and negative numbers. An amount of -5.0 or 0 fails validation.
Literal: restricts a field to a fixed set of values. A category of "food" passes; "snacks" fails.
Optional with a default: makes a field non-required. If the client omits date, Pydantic sets it to None instead of raising an error.

The Expense model

Your model will have four fields:

Field	Type	Constraint
`description`	`str`	At least 1 character
`amount`	`float`	Greater than 0
`category`	`Literal`	One of: food, transport, entertainment, utilities, other
`date`	`Optional[str]`	Defaults to `None`

Instructions

Build the Expense model below the app = FastAPI() line.

Create a class named Expense that inherits from BaseModel.
Add description as str with Field(min_length=1).
Add amount as float with Field(gt=0).
Add category as Literal["food", "transport", "entertainment", "utilities", "other"].
Add date as Optional[str] with a default of None.

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from fastapi import FastAPI
from pydantic import BaseModel, Field
from typing import Literal, Optional

app = FastAPI()

# Step 1: Create a class named Expense that inherits from BaseModel
# Step 2: Add description as str with Field(min_length=1)
# Step 3: Add amount as float with Field(gt=0)
# Step 4: Add category as Literal["food", "transport", "entertainment", "utilities", "other"]
# Step 5: Add date as Optional[str] with a default of None