diff --git a/tests/testcases/common.py b/tests/testcases/common.py
index 15d9cbe..86ea483 100644
--- a/tests/testcases/common.py
+++ b/tests/testcases/common.py
@@ -15,6 +15,16 @@ def set_window_name(conn, wid, name):
     str_type = conn.core.InternAtom(True, len(str_type), str_type).reply().atom
     conn.core.ChangePropertyChecked(xproto.PropMode.Replace, wid, prop_name, str_type, 8, len(name), name).check()
 
+def set_window_class(conn, wid, name):
+    if not isinstance(name, bytearray):
+        name = name.encode()
+    name = name+b"\0"+name+b"\0"
+    prop_name = "WM_CLASS"
+    prop_name = conn.core.InternAtom(False, len(prop_name), prop_name).reply().atom
+    str_type = "STRING"
+    str_type = conn.core.InternAtom(True, len(str_type), str_type).reply().atom
+    conn.core.ChangePropertyChecked(xproto.PropMode.Replace, wid, prop_name, str_type, 8, len(name), name).check()
+
 def find_picom_window(conn):
     prop_name = "WM_NAME"
     prop_name = conn.core.InternAtom(True, len(prop_name), prop_name).reply().atom