More informative GraphML exceptions (#5058)

* Add tests that fail on uninformative exception.

* Raise informative exception when non-supported type used.

* Add importorskip to new tests.

* Change naming scheme for backward compatibility.

2 files changed, 53 insertions, 6 deletions

diff --git a/networkx/readwrite/graphml.py b/networkx/readwrite/graphml.py
index bb70a0d7..589173dc 100644
--- a/networkx/readwrite/graphml.py
+++ b/networkx/readwrite/graphml.py
@@ -447,6 +447,17 @@ class GraphML:
         1: True,
     }
 
+    def get_xml_type(self, key):
+        """Wrapper around the xml_type dict that raises a more informative
+        exception message when a user attempts to use data of a type not
+        supported by GraphML."""
+        try:
+            return self.xml_type[key]
+        except KeyError as e:
+            raise TypeError(
+                f"GraphML does not support type {type(key)} as data values."
+            ) from e
+
 
 class GraphMLWriter(GraphML):
     def __init__(
@@ -504,7 +515,7 @@ class GraphMLWriter(GraphML):
             types = self.attribute_types[(name, scope)]
 
             if len(types) > 1:
-                types = {self.xml_type[t] for t in types}
+                types = {self.get_xml_type(t) for t in types}
                 if "string" in types:
                     return str
                 elif "float" in types or "double" in types:
@@ -551,7 +562,7 @@ class GraphMLWriter(GraphML):
             raise nx.NetworkXError(
                 f"GraphML writer does not support {element_type} as data values."
             )
-        keyid = self.get_key(name, self.xml_type[element_type], scope, default)
+        keyid = self.get_key(name, self.get_xml_type(element_type), scope, default)
         data_element = self.myElement("data", key=keyid)
         data_element.text = str(value)
         return data_element
@@ -765,7 +776,7 @@ class GraphMLWriterLxml(GraphMLWriter):
         for k, v in graphdata.items():
             self.attribute_types[(str(k), "graph")].add(type(v))
         for k, v in graphdata.items():
-            element_type = self.xml_type[self.attr_type(k, "graph", v)]
+            element_type = self.get_xml_type(self.attr_type(k, "graph", v))
             self.get_key(str(k), element_type, "graph", None)
         # Nodes and data
         for node, d in G.nodes(data=True):
@@ -773,7 +784,7 @@ class GraphMLWriterLxml(GraphMLWriter):
                 self.attribute_types[(str(k), "node")].add(type(v))
         for node, d in G.nodes(data=True):
             for k, v in d.items():
-                T = self.xml_type[self.attr_type(k, "node", v)]
+                T = self.get_xml_type(self.attr_type(k, "node", v))
                 self.get_key(str(k), T, "node", node_default.get(k))
         # Edges and data
         if G.is_multigraph():
@@ -782,7 +793,7 @@ class GraphMLWriterLxml(GraphMLWriter):
                     self.attribute_types[(str(k), "edge")].add(type(v))
             for u, v, ekey, d in G.edges(keys=True, data=True):
                 for k, v in d.items():
-                    T = self.xml_type[self.attr_type(k, "edge", v)]
+                    T = self.get_xml_type(self.attr_type(k, "edge", v))
                     self.get_key(str(k), T, "edge", edge_default.get(k))
         else:
             for u, v, d in G.edges(data=True):
@@ -790,7 +801,7 @@ class GraphMLWriterLxml(GraphMLWriter):
                     self.attribute_types[(str(k), "edge")].add(type(v))
             for u, v, d in G.edges(data=True):
                 for k, v in d.items():
-                    T = self.xml_type[self.attr_type(k, "edge", v)]
+                    T = self.get_xml_type(self.attr_type(k, "edge", v))
                     self.get_key(str(k), T, "edge", edge_default.get(k))
 
         # Now add attribute keys to the xml file
diff --git a/networkx/readwrite/tests/test_graphml.py b/networkx/readwrite/tests/test_graphml.py
index 4e6064a2..c97d07cc 100644
--- a/networkx/readwrite/tests/test_graphml.py
+++ b/networkx/readwrite/tests/test_graphml.py
@@ -1500,3 +1500,39 @@ class TestXMLGraphML(TestWriteGraphML):
     @classmethod
     def setup_class(cls):
         TestWriteGraphML.setup_class()
+
+
+def test_exception_for_unsupported_datatype_node_attr():
+    """Test that a detailed exception is raised when an attribute is of a type
+    not supported by GraphML, e.g. a list"""
+    pytest.importorskip("lxml.etree")
+    # node attribute
+    G = nx.Graph()
+    G.add_node(0, my_list_attribute=[0, 1, 2])
+    fh = io.BytesIO()
+    with pytest.raises(TypeError, match="GraphML does not support"):
+        nx.write_graphml(G, fh)
+
+
+def test_exception_for_unsupported_datatype_edge_attr():
+    """Test that a detailed exception is raised when an attribute is of a type
+    not supported by GraphML, e.g. a list"""
+    pytest.importorskip("lxml.etree")
+    # edge attribute
+    G = nx.Graph()
+    G.add_edge(0, 1, my_list_attribute=[0, 1, 2])
+    fh = io.BytesIO()
+    with pytest.raises(TypeError, match="GraphML does not support"):
+        nx.write_graphml(G, fh)
+
+
+def test_exception_for_unsupported_datatype_graph_attr():
+    """Test that a detailed exception is raised when an attribute is of a type
+    not supported by GraphML, e.g. a list"""
+    pytest.importorskip("lxml.etree")
+    # graph attribute
+    G = nx.Graph()
+    G.graph["my_list_attribute"] = [0, 1, 2]
+    fh = io.BytesIO()
+    with pytest.raises(TypeError, match="GraphML does not support"):
+        nx.write_graphml(G, fh)