Issue #25660: Fix a unittest and rlcompleter when readline isn't available

Lib/rlcompleter.py

@@ -75,9 +75,12 @@ class Completer:
 
         if not text.strip():
             if state == 0:
+                if _readline_available:
                     readline.insert_text('\t')
                     readline.redisplay()
                     return ''
+                else:
+                    return '\t'
             else:
                 return None
 
@@ -170,10 +173,11 @@ def get_class_members(klass):
 try:
     import readline
 except ImportError:
-    pass
+    _readline_available = False
 else:
     readline.set_completer(Completer().complete)
     # Release references early at shutdown (the readline module's
     # contents are quasi-immortal, and the completer function holds a
     # reference to globals).
     atexit.register(lambda: readline.set_completer(None))
+    _readline_available = True

Lib/test/test_rlcompleter.py

 import unittest
+import unittest.mock
 import builtins
 import rlcompleter
 
@@ -77,6 +78,7 @@ class TestRlcompleter(unittest.TestCase):
         self.assertEqual(completer.complete('f.b', 0), 'f.bar')
         self.assertEqual(f.calls, 1)
 
+    @unittest.mock.patch('rlcompleter._readline_available', False)
     def test_complete(self):
         completer = rlcompleter.Completer()
         self.assertEqual(completer.complete('', 0), '\t')