From 1178841eabdbfa5f2bba776c3c6be9757fb627b7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=EC=9D=B4=EA=B4=91=ED=9A=A8?=
 <87641474+widehyo1@users.noreply.github.com>
Date: Thu, 13 Aug 2026 04:32:35 +0900
Subject: [PATCH] itertools: defer dropwhile predicate validation (#8504)

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 585f6611a..b91e3735d 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -1260,7 +1260,6 @@ class TestBasicOps(unittest.TestCase):
         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 041620298..30a4d8773 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));