diff --git a/mercurial/testing/__init__.py b/mercurial/testing/__init__.py
index 99c629101b738ae88dc5c7ac1b74c60dd75d9efd_bWVyY3VyaWFsL3Rlc3RpbmcvX19pbml0X18ucHk=..52cee44aa1a073fa34b79c7b03eea53e2a803169_bWVyY3VyaWFsL3Rlc3RpbmcvX19pbml0X18ucHk= 100644
--- a/mercurial/testing/__init__.py
+++ b/mercurial/testing/__init__.py
@@ -28,3 +28,8 @@
         if time.time() - start > timeout:
             raise RuntimeError(b"timed out waiting for file: %s" % path)
         time.sleep(0.01)
+
+
+def write_file(path, content=b''):
+    with open(path, 'wb') as f:
+        f.write(content)