Warn on unused visualization kwargs that only apply to FancyArrowPatch edges (#6098)

* Add test for UserWarning with ignored vizopts.

* Implement UserWarning for bad vizopts combos.

* Fix instance in test suite caught by new warnings.

* Update test to check warnings are *not* raised when using FAPs.

2 files changed, 73 insertions, 4 deletions

diff --git a/networkx/drawing/nx_pylab.py b/networkx/drawing/nx_pylab.py
index 09e5ed3a..674c28a4 100644
--- a/networkx/drawing/nx_pylab.py
+++ b/networkx/drawing/nx_pylab.py
@@ -650,6 +650,42 @@ def draw_networkx_edges(
     # undirected graphs (for performance reasons) and use FancyArrowPatches
     # for directed graphs.
     # The `arrows` keyword can be used to override the default behavior
+    use_linecollection = not G.is_directed()
+    if arrows in (True, False):
+        use_linecollection = not arrows
+
+    # Some kwargs only apply to FancyArrowPatches. Warn users when they use
+    # non-default values for these kwargs when LineCollection is being used
+    # instead of silently ignoring the specified option
+    if use_linecollection and any(
+        [
+            arrowstyle is not None,
+            arrowsize != 10,
+            connectionstyle != "arc3",
+            min_source_margin != 0,
+            min_target_margin != 0,
+        ]
+    ):
+        import warnings
+
+        msg = (
+            "\n\nThe {0} keyword argument is not applicable when drawing edges\n"
+            "with LineCollection.\n\n"
+            "To make this warning go away, either specify `arrows=True` to\n"
+            "force FancyArrowPatches or use the default value for {0}.\n"
+            "Note that using FancyArrowPatches may be slow for large graphs.\n"
+        )
+        if arrowstyle is not None:
+            msg = msg.format("arrowstyle")
+        if arrowsize != 10:
+            msg = msg.format("arrowsize")
+        if connectionstyle != "arc3":
+            msg = msg.format("connectionstyle")
+        if min_source_margin != 0:
+            msg = msg.format("min_source_margin")
+        if min_target_margin != 0:
+            msg = msg.format("min_target_margin")
+        warnings.warn(msg, category=UserWarning, stacklevel=2)
 
     if arrowstyle == None:
         if G.is_directed():
@@ -657,10 +693,6 @@ def draw_networkx_edges(
         else:
             arrowstyle = "-"
 
-    use_linecollection = not G.is_directed()
-    if arrows in (True, False):
-        use_linecollection = not arrows
-
     if ax is None:
         ax = plt.gca()
 
diff --git a/networkx/drawing/tests/test_pylab.py b/networkx/drawing/tests/test_pylab.py
index f642dcc4..cef2702d 100644
--- a/networkx/drawing/tests/test_pylab.py
+++ b/networkx/drawing/tests/test_pylab.py
@@ -1,6 +1,7 @@
 """Unit tests for matplotlib drawing functions."""
 import itertools
 import os
+import warnings
 
 import pytest
 
@@ -396,6 +397,7 @@ def test_labels_and_colors():
         G,
         pos,
         edgelist=[(4, 5), (5, 6), (6, 7), (7, 4)],
+        arrows=True,
         min_source_margin=0.5,
         min_target_margin=0.75,
         width=8,
@@ -752,3 +754,38 @@ def test_draw_networkx_edges_undirected_selfloop_colors():
     for fap, clr, slp in zip(ax.patches, edge_colors[-3:], sl_points):
         assert fap.get_path().contains_point(slp)
         assert mpl.colors.same_color(fap.get_edgecolor(), clr)
+    plt.delaxes(ax)
+
+
+@pytest.mark.parametrize(
+    "fap_only_kwarg",  # Non-default values for kwargs that only apply to FAPs
+    (
+        {"arrowstyle": "-"},
+        {"arrowsize": 20},
+        {"connectionstyle": "arc3,rad=0.2"},
+        {"min_source_margin": 10},
+        {"min_target_margin": 10},
+    ),
+)
+def test_user_warnings_for_unused_edge_drawing_kwargs(fap_only_kwarg):
+    """Users should get a warning when they specify a non-default value for
+    one of the kwargs that applies only to edges drawn with FancyArrowPatches,
+    but FancyArrowPatches aren't being used under the hood."""
+    G = nx.path_graph(3)
+    pos = {n: (n, n) for n in G}
+    fig, ax = plt.subplots()
+    # By default, an undirected graph will use LineCollection to represent
+    # the edges
+    kwarg_name = list(fap_only_kwarg.keys())[0]
+    with pytest.warns(
+        UserWarning, match=f"\n\nThe {kwarg_name} keyword argument is not applicable"
+    ):
+        nx.draw_networkx_edges(G, pos, ax=ax, **fap_only_kwarg)
+    # FancyArrowPatches are always used when `arrows=True` is specified.
+    # Check that warnings are *not* raised in this case
+    with warnings.catch_warnings():
+        # Escalate warnings -> errors so tests fail if warnings are raised
+        warnings.simplefilter("error")
+        nx.draw_networkx_edges(G, pos, ax=ax, arrows=True, **fap_only_kwarg)
+
+    plt.delaxes(ax)