fix: python results with a KeyError-raising __getattr__ fail the job (#11517)

* fix: python results with a KeyError-raising __getattr__ no longer fail

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* fix: keep the stream probe on the instance so enum results still serialize

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* fix: probe python results through class dicts, never their __getattr__

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

backend/tests/python_jobs.rs

@@ -496,6 +496,28 @@ async def main(x: int):
         );
     }
 
+    // `hasattr` does not swallow the KeyError this `__getattr__` raises; released
+    // `wmill` clients ship an `S3Object` built this way.
+    #[test]
+    fn test_python_execd_result_with_keyerror_getattr() {
+        let script = r#"
+class AttrDict(dict):
+    def __getattr__(self, attr):
+        return self[attr]
+
+def main(x: str):
+    return AttrDict(s3=x)
+"#;
+        let results = run_py_raw_protocol_test(
+            &[("f/test/attr_dict", script)],
+            vec![ProtocolCmd::Execd { args: serde_json::json!({"x": "a/b.txt"}) }],
+        );
+        assert_eq!(
+            results,
+            vec![DedicatedWorkerResult::Success(serde_json::json!({"s3": "a/b.txt"}))]
+        );
+    }
+
     // ==================== Argument Transformation Tests ====================
 
     #[test]

backend/windmill-worker/src/python_executor.rs

@@ -860,7 +860,7 @@ pub async fn handle_python_job(
         if v == '<function call>':
             del pre_args[k]
     _pre_result = inner_script.preprocessor(**pre_args)
-    if hasattr(_pre_result, '__await__'):
+    if any('__await__' in vars(c) for c in type(_pre_result).__mro__):
         import asyncio
         _pre_result = asyncio.run(_pre_result)
     kwargs = _pre_result if _pre_result is not None else {{}}
@@ -981,7 +981,7 @@ try:
         raise ValueError("{main_override} function is missing")
     res = _await(inner_script.{main_override}(**args))
     typ = type(res)
-    if hasattr(res, '__iter__') and not isinstance(res, (str, dict, list, bytes, tuple, set, frozenset, range, memoryview, bytearray)) and typ.__name__ != 'DataFrame':
+    if _defines(res, '__iter__') and not isinstance(res, (str, dict, list, bytes, tuple, set, frozenset, range, memoryview, bytearray)) and typ.__name__ != 'DataFrame':
         for chunk in res:
             print("WM_STREAM: " + chunk.replace('\n', '\\n'))
         res = None
@@ -3340,10 +3340,16 @@ This is not normal behavior, please make sure all workers have enough memory.\n
 /// loop-per-call would break them from the second job on. A loop the script
 /// set while importing (`asyncio.set_event_loop`) is reused. asyncio is
 /// imported lazily so sync jobs don't pay its import time.
+/// `_defines` reads the class dicts instead of calling `hasattr`: a result is
+/// arbitrary user data, and `hasattr` runs its `__getattr__` (or its
+/// metaclass's), which may raise something other than AttributeError or
+/// answer every name.
 const PY_AWAIT_HELPER: &str = r#"_loop = None
+def _defines(r, name):
+    return any(name in vars(c) for c in type(r).__mro__)
 def _await(r):
     global _loop
-    if not hasattr(r, '__await__'):
+    if not _defines(r, '__await__'):
         return r
     import asyncio
     if _loop is None or _loop.is_closed():

python-client/wmill/wmill/s3_types.py

@@ -1,6 +1,15 @@
 from typing import Optional
 
 
+def _item_as_attr(d: dict, attr: str):
+    # AttributeError, not KeyError: hasattr, copy and pickle probe optional
+    # attributes and only treat AttributeError as "absent".
+    try:
+        return d[attr]
+    except KeyError:
+        raise AttributeError(attr) from None
+
+
 class S3Object(dict):
     """S3 file reference with file key, optional storage identifier, and presigned token."""
     s3: str
@@ -8,7 +17,7 @@ class S3Object(dict):
     presigned: Optional[str]
 
     def __getattr__(self, attr):
-        return self[attr]
+        return _item_as_attr(self, attr)
 
 
 class S3FsClientKwargs(dict):
@@ -16,7 +25,7 @@ class S3FsClientKwargs(dict):
     region_name: str
 
     def __getattr__(self, attr):
-        return self[attr]
+        return _item_as_attr(self, attr)
 
 
 class S3FsArgs(dict):
@@ -29,7 +38,7 @@ class S3FsArgs(dict):
     client_kwargs: S3FsClientKwargs
 
     def __getattr__(self, attr):
-        return self[attr]
+        return _item_as_attr(self, attr)
 
 
 class StorageOptions(dict):
@@ -41,7 +50,7 @@ class StorageOptions(dict):
     aws_allow_http: str
 
     def __getattr__(self, attr):
-        return self[attr]
+        return _item_as_attr(self, attr)
 
 
 class PolarsConnectionSettings(dict):
@@ -50,7 +59,7 @@ class PolarsConnectionSettings(dict):
     storage_options: StorageOptions
 
     def __getattr__(self, attr):
-        return self[attr]
+        return _item_as_attr(self, attr)
 
 
 class Boto3ConnectionSettings(dict):
@@ -63,7 +72,7 @@ class Boto3ConnectionSettings(dict):
     aws_session_token: Optional[str]
 
     def __getattr__(self, attr):
-        return self[attr]
+        return _item_as_attr(self, attr)
 
 
 class DuckDbConnectionSettings(dict):
@@ -71,4 +80,4 @@ class DuckDbConnectionSettings(dict):
     connection_settings_str: str
 
     def __getattr__(self, attr):
-        return self[attr]
+        return _item_as_attr(self, attr)