From 4e733c450b0cb188ea99b8053cf2add70a9910c1 Mon Sep 17 00:00:00 2001
From: Alexander Stromberger <A@ST.fyi>
Date: Mon, 23 Aug 2021 23:45:57 +0200
Subject: [PATCH] added __add__ method to collections.deque

---
 Lib/test/test_deque.py       |  2 --
 vm/src/stdlib/collections.rs | 26 ++++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/Lib/test/test_deque.py b/Lib/test/test_deque.py
index 006d3d6cc..e8102378b 100644
--- a/Lib/test/test_deque.py
+++ b/Lib/test/test_deque.py
@@ -204,8 +204,6 @@ class TestBasic(unittest.TestCase):
         d.extend(d)
         self.assertEqual(list(d), list('abcdabcd'))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_add(self):
         d = deque()
         e = deque('abc')
diff --git a/vm/src/stdlib/collections.rs b/vm/src/stdlib/collections.rs
index 210015934..80e1ef988 100644
--- a/vm/src/stdlib/collections.rs
+++ b/vm/src/stdlib/collections.rs
@@ -480,6 +480,32 @@ mod _collections {
             !self.borrow_deque().is_empty()
         }
 
+        #[pymethod(magic)]
+        fn add(&self, other: PyObjectRef, vm: &VirtualMachine) -> PyResult<Self> {
+            if let Some(o) = other.payload_if_subclass::<PyDeque>(vm) {
+                let mut deque = self.borrow_deque().clone();
+                let elements = o.borrow_deque().clone();
+                deque.extend(elements);
+
+                let skipped = self
+                    .maxlen
+                    .and_then(|maxlen| deque.len().checked_sub(maxlen))
+                    .unwrap_or(0);
+                deque.drain(0..skipped);
+
+                Ok(PyDeque {
+                    deque: PyRwLock::new(deque),
+                    maxlen: self.maxlen,
+                    state: AtomicCell::new(0),
+                })
+            } else {
+                Err(vm.new_type_error(format!(
+                    "can only concatenate deque (not \"{}\") to deque",
+                    other.class().name
+                )))
+            }
+        }
+
         #[pymethod(magic)]
         fn iadd(
             zelf: PyRef<Self>,