Write the PATCH Endpoint

Add a PATCH route that applies partial updates to a stored expense

💻

Writing code and entering commands is only available on desktop. Open this page on a larger screen to complete this chapter.

Merging changes into the stored expense

The PATCH endpoint follows the same pattern as GET and DELETE: extract the identifier from the URL, check that the expense exists, and raise a 404 if it does not.

The new part is applying the changes. The model_dump(exclude_unset=True) method returns a dictionary with only the fields the client sent. Python's dict.update() merges those key-value pairs into the stored expense:

# Client sends {"amount": 15.0}
changes = updates.model_dump(exclude_unset=True)  # {"amount": 15.0}
expenses[expense_id].update(changes)               # Only amount changes

Fields the client did not send remain untouched. The description, category, and date keep their original values.

Instructions

Add the PATCH endpoint at the bottom of the file.

Add the decorator @app.patch("/expenses/{expense_id}").
Define a function named update_expense with expense_id: int and updates: ExpenseUpdate as parameters.
If expense_id is not in expenses, raise HTTPException(status_code=404, detail="Expense not found").
Call expenses[expense_id].update(updates.model_dump(exclude_unset=True)) and return expenses[expense_id].

← Previous Chapter Next Chapter →

from datetime import datetime
from fastapi import FastAPI, HTTPException
from pydantic import BaseModel, Field, field_validator
from typing import Literal, Optional

app = FastAPI()

class Expense(BaseModel):
    description: str = Field(min_length=1)
    amount: float = Field(gt=0)
    category: Literal["food", "transport", "entertainment", "utilities", "other"]
    date: Optional[str] = None

    @field_validator("date")
    @classmethod
    def validate_date_format(cls, v):
        if v is not None:
            try:
                datetime.strptime(v, "%Y-%m-%d")
            except ValueError:
                raise ValueError("Date must be in YYYY-MM-DD format")
        return v

class ExpenseUpdate(BaseModel):
    description: Optional[str] = Field(default=None, min_length=1)
    amount: Optional[float] = Field(default=None, gt=0)
    category: Optional[Literal["food", "transport", "entertainment", "utilities", "other"]] = None

expenses = {}
counter = 0

@app.post("/expenses", status_code=201)
def create_expense(expense: Expense):
    global counter
    counter += 1
    expenses[counter] = {"id": counter, **expense.model_dump()}
    return expenses[counter]

@app.get("/expenses")
def list_expenses(category: str = None):
    if category:
        return [e for e in expenses.values() if e["category"] == category]
    return list(expenses.values())

@app.get("/expenses/{expense_id}")
def get_expense(expense_id: int):
    if expense_id not in expenses:
        raise HTTPException(status_code=404, detail="Expense not found")
    return expenses[expense_id]

@app.delete("/expenses/{expense_id}")
def delete_expense(expense_id: int):
    if expense_id not in expenses:
        raise HTTPException(status_code=404, detail="Expense not found")
    return expenses.pop(expense_id)

# Step 1: Add @app.patch("/expenses/{expense_id}")
# Step 2: Define update_expense with expense_id: int and updates: ExpenseUpdate
# Step 3: If expense_id not in expenses, raise HTTPException(status_code=404, detail="Expense not found")
# Step 4: Apply changes and return expenses[expense_id]

Write the PATCH Endpoint

Merging changes into the stored expense

Instructions

Interactive Code Editor