py: Implement __contains__ special method.

py/objtype.c

@@ -384,7 +384,9 @@ STATIC const qstr binary_op_method_name[] = {
     MP_BINARY_OP_LESS_EQUAL,
     MP_BINARY_OP_MORE_EQUAL,
     MP_BINARY_OP_NOT_EQUAL,
-    MP_BINARY_OP_IN,
+    */
+    [MP_BINARY_OP_IN] = MP_QSTR___contains__,
+    /*
     MP_BINARY_OP_IS,
     */
     [MP_BINARY_OP_EXCEPTION_MATCH] = MP_QSTR_, // not implemented, used to make sure array has full size

tests/basics/class-contains.py

@@ -0,0 +1,23 @@
+# A contains everything
+class A:
+    def __contains__(self, key):
+        return True
+
+a = A()
+print(True in a)
+print(1 in a)
+print(() in a)
+
+# B contains given things
+class B:
+    def __init__(self, items):
+        self.items = items
+    def __contains__(self, key):
+        return key in self.items
+
+b = B([])
+print(1 in b)
+b = B([1, 2])
+print(1 in b)
+print(2 in b)
+print(3 in b)