diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -1048,9 +1048,9 @@
    usage: doors.py [-h] {1,2,3}
    doors.py: error: argument door: invalid choice: 4 (choose from 1, 2, 3)
 
-Any object that supports the ``in`` operator can be passed as the *choices*
-value, so :class:`dict` objects, :class:`set` objects, custom containers,
-etc. are all supported.
+Any :term:`iterable` can be passed as the *choices* value, so :class:`dict`
+objects, :class:`set` objects, custom :term:`sequences <sequence>`, etc.
+are all supported.
 
 
 required
diff --git a/Lib/argparse.py b/Lib/argparse.py
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -567,23 +567,30 @@
                 return (result, ) * tuple_size
         return format
 
+    def _format_nargs(self, nargs, get_metavar):
+        if nargs is None:
+            result = '%s' % get_metavar(1)
+        elif nargs == OPTIONAL:
+            result = '[%s]' % get_metavar(1)
+        elif nargs == ZERO_OR_MORE:
+            result = '[%s [%s ...]]' % get_metavar(2)
+        elif nargs == ONE_OR_MORE:
+            result = '%s [%s ...]' % get_metavar(2)
+        elif nargs == REMAINDER:
+            result = '...'
+        elif nargs == PARSER:
+            result = '%s ...' % get_metavar(1)
+        else:
+            formats = ['%s' for _ in range(nargs)]
+            result = ' '.join(formats) % get_metavar(nargs)
+        return result
+
     def _format_args(self, action, default_metavar):
         get_metavar = self._metavar_formatter(action, default_metavar)
-        if action.nargs is None:
-            result = '%s' % get_metavar(1)
-        elif action.nargs == OPTIONAL:
-            result = '[%s]' % get_metavar(1)
-        elif action.nargs == ZERO_OR_MORE:
-            result = '[%s [%s ...]]' % get_metavar(2)
-        elif action.nargs == ONE_OR_MORE:
-            result = '%s [%s ...]' % get_metavar(2)
-        elif action.nargs == REMAINDER:
-            result = '...'
-        elif action.nargs == PARSER:
-            result = '%s ...' % get_metavar(1)
-        else:
-            formats = ['%s' for _ in range(action.nargs)]
-            result = ' '.join(formats) % get_metavar(action.nargs)
+        try:
+            result = self._format_nargs(action.nargs, get_metavar)
+        except TypeError:
+            raise ValueError("length of metavar tuple does not match nargs")
         return result
 
     def _expand_help(self, action):
@@ -1325,12 +1332,10 @@
         if not callable(type_func):
             raise ValueError('%r is not callable' % (type_func,))
 
-        # raise an error if the metavar does not match the type
+        # raise an error if the metavar length is not consistent with nargs
+        # or if the choices is not iterable, for example.
         if hasattr(self, "_get_formatter"):
-            try:
-                self._get_formatter()._format_args(action, None)
-            except TypeError:
-                raise ValueError("length of metavar tuple does not match nargs")
+            self._get_formatter()._format_args(action, None)
 
         return self._add_action(action)
 
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -4885,6 +4885,24 @@
     def test_nargs_3_metavar_length3(self):
         self.do_test_no_exception(nargs=3, metavar=("1", "2", "3"))
 
+# ==========================
+# add_argument choices tests
+# ==========================
+
+class TestAddArgumentChoices(TestCase):
+
+    def test_non_iterable_choices(self):
+        """Check that adding a non-iterable choices argument raises TypeError.
+        This also checks that it is not sufficient to support just the "in"
+        operator (e.g. by implementing __contains__).  See issue #16468."""
+        class NonIterableChoices(object):
+            def __contains__(self, item):
+                return True
+        parser = argparse.ArgumentParser()
+        self.assertRaises(TypeError, parser.add_argument, "foo",
+                          choices=NonIterableChoices())
+
+
 # ============================
 # from argparse import * tests
 # ============================