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1from fastapi import FastAPI, HTTPException, Header
2from pydantic import BaseModel
3from typing import Optional, Dict, List
4import hashlib
5import secrets
6 
7app = FastAPI()
8 
9users = {}
10tokens = {}
11groups = {}
12group_members = {}
13group_expenses = {}
14group_balances = {}
15next_user_id = 1
16next_group_id = 1
17next_expense_id = 1
18 
19class SignupRequest(BaseModel):
20    name: str
21    password: str
22 
23class LoginRequest(BaseModel):
24    name: str
25    password: str
26 
27class CreateGroupRequest(BaseModel):
28    name: str
29    member_ids: List[int]
30 
31class AddExpenseRequest(BaseModel):
32    group_id: int
33    amount: float
34    description: str = ""
35    paid_by: int
36 
37def get_user_from_token(authorization: str = Header(None)):
38    if not authorization:
39        raise HTTPException(status_code=401, detail="Missing auth header")
40    token = authorization.replace("Bearer ", "")
41    user_id = tokens.get(token)
42    if not user_id:
43        raise HTTPException(status_code=401, detail="Invalid token")
44    return user_id
45 
46@app.post("/signup")
47def signup(req: SignupRequest):
48    global next_user_id
49    for uid, u in users.items():
50        if u["name"] == req.name:
51            raise HTTPException(status_code=400, detail="User already exists")
52    user_id = next_user_id
53    next_user_id += 1
54    users[user_id] = {"id": user_id, "name": req.name, "password": req.password}
55    return {"id": user_id, "name": req.name}
56 
57@app.post("/login")
58def login(req: LoginRequest):
59    for uid, u in users.items():
60        if u["name"] == req.name and u["password"] == req.password:
61            token = secrets.token_hex(16)
62            tokens[token] = uid
63            return {"token": token}
64    raise HTTPException(status_code=401, detail="Invalid credentials")
65 
66@app.get("/users/{user_id}")
67def get_user(user_id: int, authorization: str = Header(None)):
68    get_user_from_token(authorization)
69    if user_id not in users:
70        raise HTTPException(status_code=404, detail="User not found")
71    return users[user_id]
72 
73@app.post("/groups")
74def create_group(req: CreateGroupRequest, authorization: str = Header(None)):
75    get_user_from_token(authorization)
76    global next_group_id
77    group_id = next_group_id
78    next_group_id += 1
79    groups[group_id] = {"id": group_id, "name": req.name}
80    group_members[group_id] = req.member_ids
81    group_expenses[group_id] = []
82    group_balances[group_id] = {}
83    for member_id in req.member_ids:
84        group_balances[group_id][member_id] = 0.0
85    return {"id": group_id, "name": req.name}
86 
87@app.get("/groups/{group_id}")
88def get_group(group_id: int, authorization: str = Header(None)):
89    get_user_from_token(authorization)
90    if group_id not in groups:
91        raise HTTPException(status_code=404, detail="Group not found")
92    group = groups[group_id].copy()
93    member_ids = group_members[group_id]
94    members = []
95    for mid in member_ids:
96        if mid in users:
97            members.append(users[mid]["name"])
98        else:
99            members.append(f"User {mid}")
100    group["members"] = members
101    total_expenses = sum(e["amount"] for e in group_expenses[group_id])
102    group["total_expenses"] = total_expenses
103    balances = group_balances[group_id]
104    balance_list = []
105    for mid, bal in balances.items():
106        name = users[mid]["name"] if mid in users else f"User {mid}"
107        balance_list.append({"user_id": mid, "name": name, "balance": bal})
108    group["balances"] = balance_list
109    return group
110 
111@app.get("/groups")
112def list_groups(authorization: str = Header(None)):
113    get_user_from_token(authorization)
114    result = []
115    for gid, group in groups.items():
116        g = group.copy()
117        member_ids = group_members[gid]
118        members = []
119        for mid in member_ids:
120            if mid in users:
121                members.append(users[mid]["name"])
122            else:
123                members.append(f"User {mid}")
124        g["members"] = members
125        total_expenses = sum(e["amount"] for e in group_expenses[gid])
126        g["total_expenses"] = total_expenses
127        balances = group_balances[gid]
128        balance_list = []
129        for mid, bal in balances.items():
130            name = users[mid]["name"] if mid in users else f"User {mid}"
131            balance_list.append({"user_id": mid, "name": name, "balance": bal})
132        g["balances"] = balance_list
133        result.append(g)
134    return result
135 
136@app.post("/expense")
137def add_expense(req: AddExpenseRequest, authorization: str = Header(None)):
138    user_id = get_user_from_token(authorization)
139    if req.group_id not in groups:
140        raise HTTPException(status_code=404, detail="Group not found")
141    if req.paid_by not in group_members[req.group_id]:
142        raise HTTPException(status_code=400, detail="Paid by user not in group")
143    global next_expense_id
144    expense_id = next_expense_id
145    next_expense_id += 1
146    expense = {
147        "id": expense_id,
148        "group_id": req.group_id,
149        "amount": req.amount,
150        "description": req.description,
151        "paid_by": req.paid_by
152    }
153    group_expenses[req.group_id].append(expense)
154    member_ids = group_members[req.group_id]
155    share = req.amount / len(member_ids)
156    for mid in member_ids:
157        if mid == req.paid_by:
158            group_balances[req.group_id][mid] += (req.amount - share)
159        else:
160            group_balances[req.group_id][mid] -= share
161    return expense
162 
163@app.get("/expense/{expense_id}")
164def get_expense(expense_id: int, authorization: str = Header(None)):
165    get_user_from_token(authorization)
166    for gid, expenses in group_expenses.items():
167        for e in expenses:
168            if e["id"] == expense_id:
169                return e
170    raise HTTPException(status_code=404, detail="Expense not found")

1fastapi
2uvicorn