proper handling of false positives

Author: Archmonger

src/reactpy/core/events.py

@@ -1,8 +1,7 @@
 from __future__ import annotations
 
 import asyncio
-import contextlib
-import inspect
+import dis
 from collections.abc import Sequence
 from typing import Any, Callable, Literal, cast, overload
 
@@ -116,23 +115,37 @@ def __init__(
         target: str | None = None,
     ) -> None:
         self.function = to_event_handler_function(function, positional_args=False)
-
-        if not (stop_propagation and prevent_default):
-            with contextlib.suppress(Exception):
-                func_to_inspect = cast(Any, function)
-                while hasattr(func_to_inspect, "__wrapped__"):
-                    func_to_inspect = func_to_inspect.__wrapped__
-
-                source = inspect.getsource(func_to_inspect)
-                if not stop_propagation and ".stopPropagation()" in source:
-                    stop_propagation = True
-                if not prevent_default and ".preventDefault()" in source:
-                    prevent_default = True
-
         self.prevent_default = prevent_default
         self.stop_propagation = stop_propagation
         self.target = target
 
+        # Check if our `preventDefault` or `stopPropagation` methods were called
+        # by inspecting the function's bytecode
+        func_to_inspect = cast(Any, function)
+        while hasattr(func_to_inspect, "__wrapped__"):
+            func_to_inspect = func_to_inspect.__wrapped__
+
+        code = func_to_inspect.__code__
+        if code.co_argcount > 0:
+            event_arg_name = code.co_varnames[0]
+            last_was_event = False
+
+            for instr in dis.get_instructions(func_to_inspect):
+                if instr.opname == "LOAD_FAST" and instr.argval == event_arg_name:
+                    last_was_event = True
+                    continue
+
+                if last_was_event and instr.opname in (
+                    "LOAD_METHOD",
+                    "LOAD_ATTR",
+                ):
+                    if instr.argval == "preventDefault":
+                        self.prevent_default = True
+                    elif instr.argval == "stopPropagation":
+                        self.stop_propagation = True
+
+                last_was_event = False
+
     __hash__ = None  # type: ignore
 
     def __eq__(self, other: object) -> bool:

tests/test_core/test_events.py

@@ -384,3 +384,25 @@ def test_event_export():
     from reactpy import Event
 
     assert Event is not None
+
+
+def test_detect_false_positive():
+    def handler(event: Event):
+        # This should not trigger detection
+        other = Event()
+        other.preventDefault()
+        other.stopPropagation()
+
+    eh = EventHandler(handler)
+    assert eh.prevent_default is False
+    assert eh.stop_propagation is False
+
+
+def test_detect_renamed_argument():
+    def handler(e: Event):
+        e.preventDefault()
+        e.stopPropagation()
+
+    eh = EventHandler(handler)
+    assert eh.prevent_default is True
+    assert eh.stop_propagation is True