add filter.__reduce__ (RustPython#3732)

Author: HyunP-dev

Lib/test/test_builtin.py

@@ -658,8 +658,6 @@ def badfunc():
         self.assertEqual(list(filter(lambda x: x>=3, (1, 2, 3, 4))), [3, 4])
         self.assertRaises(TypeError, list, filter(42, (1, 2)))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_filter_pickle(self):
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):
             f1 = filter(filter_char, "abcdeabcde")

Lib/test/test_itertools.py

@@ -856,8 +856,6 @@ def keyfunc(obj):
         keyfunc.skip = 1
         self.assertRaises(ExpectedError, gulp, [None, None], keyfunc)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_filter(self):
         self.assertEqual(list(filter(isEven, range(6))), [0,2,4])
         self.assertEqual(list(filter(None, [0,1,0,2,0])), [1,2])

vm/src/builtins/filter.rs

@@ -37,7 +37,15 @@ impl Constructor for PyFilter {
 }
 
 #[pyimpl(with(IterNext, Constructor), flags(BASETYPE))]
-impl PyFilter {}
+impl PyFilter {
+    #[pymethod(magic)]
+    fn reduce(&self, vm: &VirtualMachine) -> (PyTypeRef, (PyObjectRef, PyIter)) {
+        (
+            vm.ctx.types.filter_type.clone(),
+            (self.predicate.clone(), self.iterator.clone()),
+        )
+    }
+}
 
 impl IterNextIterable for PyFilter {}
 impl IterNext for PyFilter {