[nx_pylab] fix StopIteration, if empty dict is passed for labels. (#5372)

* [nx_pylab] fix StopIteration, if empty dict is passed for labels.

The new detection for multi-edge labels raises, if an empty dict
was passed. Before this detection that was fine, because the loop
iterating over the passed labels just passed.

Added small test as well.

* incorporate suggestions [ci skip]

* Add regression test for empty edge label dict.

* Simplify dict lookup (keys only).

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

2 files changed, 13 insertions, 2 deletions

diff --git a/networkx/drawing/nx_pylab.py b/networkx/drawing/nx_pylab.py
index 26c76134..0e392e2b 100644
--- a/networkx/drawing/nx_pylab.py
+++ b/networkx/drawing/nx_pylab.py
@@ -1058,7 +1058,7 @@ def draw_networkx_edge_labels(
         A dictionary with nodes as keys and positions as values.
         Positions should be sequences of length 2.
 
-    edge_labels : dictionary (default={})
+    edge_labels : dictionary (default=None)
         Edge labels in a dictionary of labels keyed by edge two-tuple.
         Only labels for the keys in the dictionary are drawn.
 
@@ -1131,11 +1131,14 @@ def draw_networkx_edge_labels(
         labels = edge_labels
         # Informative exception for multiedges
         try:
-            (u, v), d = next(iter(labels.items()))
+            (u, v) = next(iter(labels))  # ensures no edge key provided
         except ValueError as err:
             raise nx.NetworkXError(
                 "draw_networkx_edge_labels does not support multiedges."
             ) from err
+        except StopIteration:
+            pass
+
     text_items = {}
     for (n1, n2), label in labels.items():
         (x1, y1) = pos[n1]
diff --git a/networkx/drawing/tests/test_pylab.py b/networkx/drawing/tests/test_pylab.py
index b8ea9423..54ea6e0c 100644
--- a/networkx/drawing/tests/test_pylab.py
+++ b/networkx/drawing/tests/test_pylab.py
@@ -716,3 +716,11 @@ def test_draw_networkx_edge_label_multiedge_exception():
     pos = {n: (n, n) for n in G}
     with pytest.raises(nx.NetworkXError, match=exception_msg):
         nx.draw_networkx_edge_labels(G, pos, edge_labels=edge_labels)
+
+
+def test_draw_networkx_edge_label_empty_dict():
+    """Regression test for draw_networkx_edge_labels with empty dict. See
+    gh-5372."""
+    G = nx.path_graph(3)
+    pos = {n: (n, n) for n in G.nodes}
+    assert nx.draw_networkx_edge_labels(G, pos, edge_labels={}) == {}