Update app.py

app.py

@@ -7,37 +7,56 @@ import io
 import json
 
 app = FastAPI()
+
+# Mount static files (for CSS, JS)
 app.mount("/static", StaticFiles(directory="static"), name="static")
 templates = Jinja2Templates(directory="templates")
 
+
 def run_code_with_trace(code: str):
     trace_data = []
     stdout_capture = io.StringIO()
 
+    # ✅ Safely serialize objects (avoid JSON TypeError)
+    def safe_serialize(obj):
+        try:
+            json.dumps(obj)
+            return obj
+        except (TypeError, OverflowError):
+            return str(obj)
+
+    # ✅ Tracer function
     def tracer(frame, event, arg):
         if event == "line":
+            locals_serialized = {k: safe_serialize(v) for k, v in frame.f_locals.items()}
             trace_data.append({
                 "line": frame.f_lineno,
-                "locals": frame.f_locals.copy()
+                "locals": locals_serialized
             })
         return tracer
 
+    # Redirect stdout temporarily (if user uses print)
     sys.settrace(tracer)
+    sys.stdout = stdout_capture
     try:
-        exec(code, {})
+        safe_globals = {"__builtins__": {"print": print, "range": range, "len": len}}
+        exec(code, safe_globals)
     except Exception as e:
         trace_data.append({"error": str(e)})
     finally:
         sys.settrace(None)
+        sys.stdout = sys.__stdout__  # reset stdout
 
     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)