merge from master artifact

Author: rocky

.isort.cfg

@@ -1,4 +1,4 @@
 [settings]
 # sort and black fight over the formatting of this file.
 # Remove when this issue is resolved.
-skip=pymathics/graph/base.py
+skip=pymathics/graph/base.py,pymathics/graph/properties.py

pymathics/graph/properties.py

@@ -91,6 +91,11 @@ def eval(self, graph, expression, evaluation, options):
 
 class DirectedGraphQ(_NetworkXBuiltin):
     """
+    <dl>
+      <dt>'DirectedGraphQ'[$graph$]
+      <dd>True if $graph$ is a 'Graph' and all the edges are directed.
+    </dl>
+
     >> g = Graph[{1 -> 2, 2 -> 3}]; DirectedGraphQ[g]
      = True
 
@@ -108,8 +113,7 @@ def eval(self, graph, expression, evaluation, options):
         "%(name)s[graph_, OptionsPattern[%(name)s]]"
         graph = self._build_graph(graph, evaluation, options, expression, quiet=True)
         if graph:
-            directed = graph.G.is_directed() and not graph.is_mixed_graph()
-            return from_python(directed)
+            return from_python(graph.is_directed())
         else:
             return SymbolFalse