bpo-29553: Fixed ArgumentParses format_usage for mutually exclusive groups

Lib/argparse.py

@@ -401,13 +401,19 @@ def _format_actions_usage(self, actions, groups):
                             inserts[start] += ' ['
                         else:
                             inserts[start] = '['
-                        inserts[end] = ']'
+                        if end in inserts:
+                            inserts[end] += ']'
+                        else:
+                            inserts[end] = ']'
                     else:
                         if start in inserts:
                             inserts[start] += ' ('
                         else:
                             inserts[start] = '('
-                        inserts[end] = ')'
+                        if end in inserts:
+                            inserts[end] += ')'
+                        else:
+                            inserts[end] = ')'
                     for i in range(start + 1, end):
                         inserts[i] = '|'
 

Lib/test/test_argparse.py

@@ -2725,6 +2725,43 @@ def get_parser(self, required):
           -c          c help
         '''
 
+class TestMutuallyExclusiveNested(MEMixin, TestCase):
+    def get_parser(self, required):
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        group = parser.add_mutually_exclusive_group(required=required)
+        group.add_argument('-a')
+        group.add_argument('-b')
+        group2 = group.add_mutually_exclusive_group(required=required)
+        group2.add_argument('-c')
+        group2.add_argument('-d')
+        group3 = group2.add_mutually_exclusive_group(required=required)
+        group3.add_argument('-e')
+        group3.add_argument('-f')
+        return parser
+
+    failures = []
+    successes = []
+    successes_when_not_required = []
+
+    usage_when_not_required = '''\
+        usage: PROG [-h] [-a A | -b B | [-c C | -d D | [-e E | -f F]]]
+        '''
+    usage_when_required = '''\
+        usage: PROG [-h] (-a A | -b B | (-c C | -d D | (-e E | -f F)))
+        '''
+
+    help = '''\
+
+        optional arguments:
+          -h, --help  show this help message and exit
+          -a A
+          -b B
+          -c C
+          -d D
+          -e E
+          -f F
+        '''
+
 # =================================================
 # Mutually exclusive group in parent parser tests
 # =================================================

Misc/NEWS

@@ -228,6 +228,7 @@ Extension Modules
 
 Library
 -------
+- bpo-29553: Fixed ArgumentParses format_usage for mutually exclusive groups.
 
 - bpo-29576: Improve some deprecations in importlib. Some deprecated methods
   now emit DeprecationWarnings and have better descriptive messages.