pgraph: fix has_tags() after clearing Python tags

Fixes #936

panda/src/pgraph/pandaNode_ext.cxx

@@ -165,6 +165,12 @@ clear_python_tag(PyObject *key) {
   if (PyDict_GetItem(dict, key) != nullptr) {
     PyDict_DelItem(dict, key);
   }
+
+  if (PyDict_Size(dict) == 0 && Py_REFCNT(dict) == 1) {
+    // This was the last tag, and do_get_python_tags() made sure we have a
+    // unique reference to the tags, so clear the tag object.
+    _this->_python_tag_data.clear();
+  }
 }
 
 /**

tests/pgraph/test_nodepath.py

@@ -151,6 +151,35 @@ def test_nodepath_python_tags():
     assert rc1 == rc2
 
 
+def test_nodepath_clear_python_tag():
+    from panda3d.core import NodePath
+
+    path = NodePath("node")
+    assert not path.has_python_tag("a")
+    assert not path.has_python_tag("b")
+    assert not path.node().has_tags()
+
+    path.set_python_tag("a", "value")
+    assert path.has_python_tag("a")
+    assert not path.has_python_tag("b")
+    assert path.node().has_tags()
+
+    path.set_python_tag("b", "value")
+    assert path.has_python_tag("a")
+    assert path.has_python_tag("b")
+    assert path.node().has_tags()
+
+    path.clear_python_tag("a")
+    assert not path.has_python_tag("a")
+    assert path.has_python_tag("b")
+    assert path.node().has_tags()
+
+    path.clear_python_tag("b")
+    assert not path.has_python_tag("a")
+    assert not path.has_python_tag("b")
+    assert not path.node().has_tags()
+
+
 def test_nodepath_replace_texture():
     from panda3d.core import NodePath, Texture