app.py

from fastapi import FastAPI, Request, Form
from fastapi.responses import HTMLResponse
from fastapi.templating import Jinja2Templates
import sys
import io

app = FastAPI()
templates = Jinja2Templates(directory="templates")

# --- Core tracer logic ---
def run_code_with_trace(code: str):
    trace_data = []
    stdout_capture = io.StringIO()

    def tracer(frame, event, arg):
        if event == "line":
            trace_data.append({
                "line": frame.f_lineno,
                "locals": frame.f_locals.copy()
            })
        return tracer

    sys.settrace(tracer)
    try:
        exec(code, {}, {})
    except Exception as e:
        trace_data.append({"error": str(e)})
    finally:
        sys.settrace(None)

    output = stdout_capture.getvalue()
    stdout_capture.close()

    return trace_data, output


@app.get("/", response_class=HTMLResponse)
async def index(request: Request):
    return templates.TemplateResponse("index.html", {"request": request})


@app.post("/run", response_class=HTMLResponse)
async def run_code(request: Request, code: str = Form(...)):
    trace, output = run_code_with_trace(code)
    return templates.TemplateResponse(
        "index.html",
        {
            "request": request,
            "trace": trace,
            "output": output,
            "code": code
        }
    )