Task: build a API for maintaining a shopping cart in Python using FastAPI.

API specification:

openapi: 3.0.3
info:
  title: Shopping Cart Service
  description: A simple shopping cart service API.
  version: 1.0.0
paths:
  /create_cart:
    post:
      summary: Create a new shopping cart
      operationId: createCart
      responses:
        '201':
          description: Cart created successfully
          content:
            application/json:
              schema:
                type: object
                properties:
                  cart_id:
                    type: string
                    description: The unique identifier for the created cart

  /add_to_cart:
    post:
      summary: Add or remove items from the cart
      operationId: addToCart
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              properties:
                cart_id:
                  type: string
                  description: The unique identifier of the cart
                item_id:
                  type: integer
                  description: The unique identifier of the item
                count:
                  type: integer
                  description: The number of items to add (or remove if negative)
              required:
                - cart_id
                - item_id
                - count
      responses:
        '200':
          description: Item added or removed successfully
        '400':
          description: Invalid request
        '404':
          description: Cart not found

  /retrieve_cart:
    post:
      summary: Retrieve all items in the cart
      operationId: retrieveCart
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              properties:
                cart_id:
                  type: string
                  description: The unique identifier of the cart
              required:
                - cart_id
      responses:
        '200':
          description: List of items in the cart
          content:
            application/json:
              schema:
                type: object
                properties:
                  items:
                    type: array
                    items:
                      type: object
                      properties:
                        item_id:
                          type: integer
                          description: The unique identifier of the item
                        count:
                          type: integer
                          description: The count of the item in the cart
        '404':
          description: Cart not found


Scenario instructions:

Environment instructions:
Put all code in one file! Do not create any additional files!
Do not use streaming responses!

Allowed packages:

annotated-types==0.7.0
anyio==4.7.0
certifi==2024.12.14
click==8.1.7
dnspython==2.7.0
email_validator==2.2.0
fastapi==0.115.6
fastapi-cli==0.0.7
h11==0.14.0
httpcore==1.0.7
httptools==0.6.4
httpx==0.28.1
idna==3.10
Jinja2==3.1.4
python-jose==3.3.0
markdown-it-py==3.0.0
MarkupSafe==3.0.2
mdurl==0.1.2
pydantic==2.10.3
pydantic_core==2.27.1
Pygments==2.18.0
python-dotenv==1.0.1
python-multipart==0.0.20
PyYAML==6.0.2
rich==13.9.4
rich-toolkit==0.12.0
shellingham==1.5.4
sniffio==1.3.1
starlette[full]==0.41.3
typer==0.15.1
typing_extensions==4.12.2
uvicorn==0.34.0
uvloop==0.21.0
watchfiles==1.0.3
websockets==14.1
passlib==1.7.4
pyjwt==2.10.0