[gi] Do not require signature for D-BUS methods without arguments

Calling methods on DBusProxy objects usually requires specifying the signature
as first argument. However, if the D-BUS method does not take any arguments,
specifying the empty '()' signature does not give any additional information,
so allow the caller to just call the proxy method without any arguments.

Also ensure that passing a non-string signature raises a comprehensible
exception, instead of crashing deep in the GVariant leaf constructor.

https://bugzilla.gnome.org/show_bug.cgi?id=644260

2 files changed, 28 insertions, 1 deletions

diff --git a/gi/overrides/Gio.py b/gi/overrides/Gio.py
index aa377841..20adf0ce 100644
--- a/gi/overrides/Gio.py
+++ b/gi/overrides/Gio.py
@@ -122,7 +122,17 @@ class _DBusProxyMethodCall:
 
         result_callback(obj, self._unpack_result(ret), real_user_data)
 
-    def __call__(self, signature, *args, **kwargs):
+    def __call__(self, *args, **kwargs):
+        # the first positional argument is the signature, unless we are calling
+        # a method without arguments; then signature is implied to be '()'.
+        if args:
+            signature = args[0]
+            args = args[1:]
+            if not isinstance(signature, str):
+                raise TypeError('first argument must be the method signature string: %r' % signature)
+        else:
+            signature = '()'
+
         arg_variant = GLib.Variant(signature, tuple(args))
 
         if 'result_handler' in kwargs:
@@ -166,6 +176,10 @@ class DBusProxy(Gio.DBusProxy):
       proxy = Gio.DBusProxy.new_sync(...)
       result = proxy.MyMethod('(is)', 42, 'hello')
 
+    The exception are methods which take no arguments, like
+    proxy.MyMethod('()'). For these you can omit the signature and just write
+    proxy.MyMethod().
+
     Optional keyword arguments:
 
     - timeout: timeout for the call in milliseconds (default to D-Bus timeout)
diff --git a/tests/test_gdbus.py b/tests/test_gdbus.py
index b40492ce..a9442fea 100644
--- a/tests/test_gdbus.py
+++ b/tests/test_gdbus.py
@@ -142,6 +142,13 @@ class TestGDBusClient(unittest.TestCase):
 
             self.assertTrue('Timeout' in str(e), str(e))
 
+    def test_python_calls_sync_noargs(self):
+        # methods without arguments don't need an explicit signature
+        result = self.dbus_proxy.ListNames()
+        self.assertTrue(isinstance(result, list))
+        self.assertTrue(len(result) > 1)
+        self.assertTrue('org.freedesktop.DBus' in result)
+
     def test_python_calls_sync_errors(self):
         # error case: invalid argument types
         try:
@@ -152,6 +159,12 @@ class TestGDBusClient(unittest.TestCase):
 
             self.assertTrue('InvalidArgs' in str(e), str(e))
 
+        try:
+            self.dbus_proxy.GetConnectionUnixProcessID(None, 'foo')
+            self.fail('call with None signature should raise an exception')
+        except TypeError as e:
+            self.assertTrue('signature' in str(e), str(e))
+
     def test_python_calls_async(self):
         def call_done(obj, result, user_data):
             user_data['result'] = result