Add special handling for optional boolean flags

defopt.py

@@ -131,6 +131,18 @@ def _populate_parser(func, parser, parsers=None):
         type_ = _get_type(func, name, doc, hints)
         if param.kind == param.VAR_KEYWORD:
             raise ValueError('**kwargs not supported')
+        if type_.type == bool and param.default != param.empty:
+            # Special case: just add parameterless --name and --no-name flags.
+            parser.add_argument('--' + name,
+                                action='store_true',
+                                default=param.default,
+                                # Add help if available.
+                                **kwargs)
+            parser.add_argument('--no-' + name,
+                                action='store_false',
+                                default=param.default,
+                                dest=name)
+            continue
         if type_.container:
             assert type_.container == list
             name_or_flag = '--' + name

examples/booleans.py

@@ -0,0 +1,32 @@
+"""Example showing boolean flags in defopt.
+
+Optional boolean parameters are automatically converted to
+``--name`` and ``--no-name`` flags which take no arguments
+and store `True` and `False` respectively.
+
+Code usage::
+
+    >>> main('hello!', upper=False, repeat=True)
+
+Command line usage::
+
+    $ python booleans.py 'hello!' --no-upper --repeat
+"""
+import defopt
+
+
+def main(message, upper=True, repeat=False):
+    """Example function with boolean flags.
+
+    :param str message: Message to display
+    :param bool upper: Display the message in upper case
+    :param bool repeat: Display the message twice
+    """
+    if upper:
+        message = message.upper()
+    for _ in range(1 + repeat):
+        print(message)
+
+
+if __name__ == '__main__':
+    defopt.run(main)

test_defopt.py

@@ -156,6 +156,25 @@ def test_list_bare(self):
         with self.assertRaises(ValueError):
             defopt._get_parser(list)
 
+    def test_bool(self):
+        def main(foo):
+            """:type foo: bool"""
+            return foo
+        self.assertIs(defopt.run(main, argv=['1']), True)
+        self.assertIs(defopt.run(main, argv=['0']), False)
+        with self.assertRaises(SystemExit):
+            defopt.run(main, argv=[])
+
+    def test_bool_kwarg(self):
+        default = object()
+
+        def main(foo=default):
+            """:type foo: bool"""
+            return foo
+        self.assertIs(defopt.run(main, argv=['--foo']), True)
+        self.assertIs(defopt.run(main, argv=['--no-foo']), False)
+        self.assertIs(defopt.run(main, argv=[]), default)
+
 
 class TestParsersDeprecated(unittest.TestCase):
     def setUp(self):