diff --git a/mercurial/formatter.py b/mercurial/formatter.py
--- a/mercurial/formatter.py
+++ b/mercurial/formatter.py
@@ -48,7 +48,8 @@ Doctest helper:
 ...     ui.verbose = verbose
 ...     ui.pushbuffer()
 ...     try:
-...         return fn(ui, ui.formatter(fn.__name__, opts))
+...         return fn(ui, ui.formatter(pycompat.sysbytes(fn.__name__),
+...                   pycompat.byteskwargs(opts)))
 ...     finally:
 ...         print(pycompat.sysstr(ui.popbuffer()), end='')
 
diff --git a/tests/test-doctest.py b/tests/test-doctest.py
--- a/tests/test-doctest.py
+++ b/tests/test-doctest.py
@@ -51,7 +51,7 @@ testmod('mercurial.context')
 testmod('mercurial.dagparser', optionflags=doctest.NORMALIZE_WHITESPACE)
 testmod('mercurial.dispatch')
 testmod('mercurial.encoding')
-testmod('mercurial.formatter', py3=False)  # py3: write bytes to stdout
+testmod('mercurial.formatter')
 testmod('mercurial.hg')
 testmod('mercurial.hgweb.hgwebdir_mod', py3=False)  # py3: repr(bytes) ?
 testmod('mercurial.match')