From d918a7e20da6f718f3189f445c4f1cdaf3dfbfa3 Mon Sep 17 00:00:00 2001
From: "widehyo@gmail.com" <widehyo@gmail.com>
Date: Sun, 9 Aug 2026 14:43:25 +0900
Subject: [PATCH] itertools: defer dropwhile predicate validation

Store the `dropwhile` predicate as a Python object and call it while
advancing the iterator. This defers callable validation until the predicate
is first needed, matching CPython for empty input while preserving exception
propagation during iteration.

Remove the now-passing `test_dropwhile` expected-failure marker.

Assisted-by: Codex:gpt-5.6-sol
---
 Lib/test/test_itertools.py        | 1 -
 crates/vm/src/stdlib/itertools.rs | 9 ++++-----
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
index 585f6611ade..b91e3735d94 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -1260,7 +1260,6 @@ def test_takewhile(self):
         self.assertEqual(list(t), [1, 1, 1])
         self.assertRaises(StopIteration, next, t)
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_dropwhile(self):
         data = [1, 3, 5, 20, 2, 4, 6, 8]
         self.assertEqual(list(dropwhile(underten, data)), [20, 2, 4, 6, 8])
diff --git a/crates/vm/src/stdlib/itertools.rs b/crates/vm/src/stdlib/itertools.rs
index 041620298e7..30a4d8773be 100644
--- a/crates/vm/src/stdlib/itertools.rs
+++ b/crates/vm/src/stdlib/itertools.rs
@@ -10,7 +10,7 @@ mod decl {
             rc::PyRc,
         },
         convert::ToPyObject,
-        function::{ArgCallable, FuncArgs, OptionalArg, OptionalOption, PosArgs},
+        function::{FuncArgs, OptionalArg, OptionalOption, PosArgs},
         protocol::{PyIter, PyIterReturn, PyNumber},
         raise_if_stop,
         stdlib::sys,
@@ -477,7 +477,7 @@ mod decl {
     #[pyclass(name = "dropwhile")]
     #[derive(Debug, PyPayload)]
     struct PyItertoolsDropwhile {
-        predicate: ArgCallable,
+        predicate: PyObjectRef,
         iterable: PyIter,
         start_flag: AtomicCell<bool>,
     }
@@ -485,7 +485,7 @@ mod decl {
     #[derive(FromArgs)]
     struct DropwhileNewArgs {
         #[pyarg(positional)]
-        predicate: ArgCallable,
+        predicate: PyObjectRef,
         #[pyarg(positional)]
         iterable: PyIter,
     }
@@ -522,8 +522,7 @@ mod decl {
             if !zelf.start_flag.load() {
                 loop {
                     let obj = raise_if_stop!(iterable.next(vm)?);
-                    let pred = predicate.clone();
-                    let pred_value = pred.invoke((obj.clone(),), vm)?;
+                    let pred_value = predicate.call((obj.clone(),), vm)?;
                     if !pred_value.try_to_bool(vm)? {
                         zelf.start_flag.store(true);
                         return Ok(PyIterReturn::Return(obj));