main.py

from fastapi import FastAPI,HTTPException
from pydantic import BaseModel,Field
from typing import Optional,List

app=FastAPI()

#pydantic model (data schema)
class Todo(BaseModel):
    id: int
    title:str
    description: Optional[str]=None
    completed: bool=False #added completed field to compare with incoming request having query paramater 'completed'

class TodoCreate(BaseModel):
    title: str = Field(...,min_length=1,description="Title cannot be empty") #Validation enhance using pydantic models. 
    description: Optional[str]=None

class TodoUpdate(BaseModel):
    title: Optional[str]=None
    description: Optional[str]=None

#In-Memory data store
todos=[] #acts like a mock database

#CRUD Routes
#Read all
@app.get("/todos",response_model=List[Todo])
def get_todos(completed:Optional[bool]=None, limit:Optional[int]=None,skip:Optional[int]=None):
    #Temporary dummy filter since we dont have 'completed' field yet
    results=todos
    #Simulate skip and limit(like pagination)
    if skip:
        results=results[skip:]
    if limit:
        results=results[:limit]
    if completed is not None:
        results=[todo for todo in results if todo.completed==completed]
    return results

#Read one by ID
@app.get("/todos/{item_id}",response_model=Todo)
def get_todo(item_id: int):
    for todo in todos:
        if todo.id==item_id:
            return todo
    raise HTTPException(status_code=404,detail="To-do not found")

#Read by adding path parameter
@app.get("/users/{user_id}/items/{item_id}")
def get_user_item(user_id: int,item_id:int):
    return {"user_id":user_id,
            "item_id":item_id,
            "message":f"User {user_id} requested item {item_id}"}
#create
@app.post("/todos",response_model=Todo)
def create_todo(todo_data: TodoCreate):
    new_id=len(todos)+1
    new_todo=Todo(id=new_id,**todo_data.model_dump())
    todos.append(new_todo)
    return new_todo

#Update (PUT-full replacement)

@app.put("/todos/{item_id}",response_model=Todo)
def update_todo(item_id:int,updated_todo:TodoCreate):
    for i,todo in enumerate(todos):
        if todo.id==item_id:
            todos[i]=Todo(id=item_id,**updated_todo.model_dump())
            return todos[i]
    raise HTTPException(status_code=404,detail="To-do not found")

#Partial update (Patch)
@app.patch("/todos/{item_id}",response_model=Todo)
def patch_todo(item_id: int,patch_data:TodoUpdate):
    for todo in todos:
        if todo.id==item_id:
            if patch_data.title is not None:
                todo.title=patch_data.title
            if patch_data.description is not None:
                todo.description=patch_data.description
            return todo
    raise HTTPException(status_code=404,detail="To-do not found")

#Delete 
@app.delete("/todos/{item_id}")
def delete_todo(item_id:int):
    for i,todo in enumerate(todos):
        if todo.id==item_id:
            del todos[i]
            return {"message":"To-do deleted successfully"}
    raise HTTPException(status_code=404,detail="To-do not found")