Python: Remove workerd runtime frames from the top of stack trace

Python tracebacks include some frames that are not related to user code
but are in the workerd runtime. This trims them off.

Author: hoodmane

src/pyodide/internal/introspection.py

@@ -1,4 +1,6 @@
+import sys
 from inspect import isawaitable, isclass
+from traceback import TracebackException
 from types import FunctionType
 
 import js
@@ -86,3 +88,20 @@ async def wrapper_func(relaxed, inst, prop, *args, **kwargs):
         return python_to_rpc(await result)
     else:
         return python_to_rpc(result)
+
+
+def excepthook(_type, exc, _tb):
+    """Remove workerd runtime frames from the top of the stack trace."""
+    tb_exc = TracebackException.from_exception(exc)
+    for idx, frame in enumerate(tb_exc.stack):
+        fn = frame.filename
+        # We're trying to remove frames named "introspection.py" and
+        # "/lib/python313.zip/pyodide/code.py".
+        if fn.startswith("/") and not fn.startswith("/lib/python313.zip/pyodide/"):
+            first_user_frame = idx
+            break
+    del tb_exc.stack[:first_user_frame]
+    tb_exc.print()
+
+
+sys.excepthook = excepthook