Added write only property functions for issue #1142 (#1144)

py::class_<T>'s `def_property` and `def_property_static` can now take a
`nullptr` as the getter to allow a write-only property to be established
(mirroring Python's `property()` built-in when `None` is given for the
getter).

This also updates properties to use the new nullptr constructor internally.

tests/test_methods_and_attributes.py

@@ -98,6 +98,21 @@ def test_properties():
     instance.def_property = 3
     assert instance.def_property == 3
 
+    with pytest.raises(AttributeError) as excinfo:
+        dummy = instance.def_property_writeonly  # noqa: F841 unused var
+    assert "unreadable attribute" in str(excinfo)
+
+    instance.def_property_writeonly = 4
+    assert instance.def_property_readonly == 4
+
+    with pytest.raises(AttributeError) as excinfo:
+        dummy = instance.def_property_impossible  # noqa: F841 unused var
+    assert "unreadable attribute" in str(excinfo)
+
+    with pytest.raises(AttributeError) as excinfo:
+        instance.def_property_impossible = 5
+    assert "can't set attribute" in str(excinfo)
+
 
 def test_static_properties():
     assert m.TestProperties.def_readonly_static == 1
@@ -108,13 +123,27 @@ def test_static_properties():
     m.TestProperties.def_readwrite_static = 2
     assert m.TestProperties.def_readwrite_static == 2
 
-    assert m.TestProperties.def_property_readonly_static == 2
     with pytest.raises(AttributeError) as excinfo:
-        m.TestProperties.def_property_readonly_static = 3
+        dummy = m.TestProperties.def_writeonly_static  # noqa: F841 unused var
+    assert "unreadable attribute" in str(excinfo)
+
+    m.TestProperties.def_writeonly_static = 3
+    assert m.TestProperties.def_readonly_static == 3
+
+    assert m.TestProperties.def_property_readonly_static == 3
+    with pytest.raises(AttributeError) as excinfo:
+        m.TestProperties.def_property_readonly_static = 99
     assert "can't set attribute" in str(excinfo)
 
-    m.TestProperties.def_property_static = 3
-    assert m.TestProperties.def_property_static == 3
+    m.TestProperties.def_property_static = 4
+    assert m.TestProperties.def_property_static == 4
+
+    with pytest.raises(AttributeError) as excinfo:
+        dummy = m.TestProperties.def_property_writeonly_static
+    assert "unreadable attribute" in str(excinfo)
+
+    m.TestProperties.def_property_writeonly_static = 5
+    assert m.TestProperties.def_property_static == 5
 
     # Static property read and write via instance
     instance = m.TestProperties()
@@ -127,6 +156,13 @@ def test_static_properties():
     assert m.TestProperties.def_readwrite_static == 2
     assert instance.def_readwrite_static == 2
 
+    with pytest.raises(AttributeError) as excinfo:
+        dummy = instance.def_property_writeonly_static  # noqa: F841 unused var
+    assert "unreadable attribute" in str(excinfo)
+
+    instance.def_property_writeonly_static = 4
+    assert instance.def_property_static == 4
+
     # It should be possible to override properties in derived classes
     assert m.TestPropertiesOverride().def_readonly == 99
     assert m.TestPropertiesOverride.def_readonly_static == 99