Add check before plotting wind barbs

src/metpy/plots/declarative.py

@@ -1660,9 +1660,19 @@ def _build(self):
 
         wind_slice = (slice(None, None, self.skip[0]), slice(None, None, self.skip[1]))
 
+        # If too many of the winds/vectors to plot have excessively high magnitudes,
+        # the call to ax.barbs will cause matplotlib to draw so many pennants that all
+        # memory will be consumed, possibly hanging the system.  Thus, here we check
+        # what we are about to plot to avoid the hang.
+        u_vals = u.values[wind_slice]
+        v_vals = v.values[wind_slice]
+        speeds = np.hypot(u_vals, v_vals)
+        if np.median(speeds) > 50000:
+            msg = "Too many large wind barbs to plot.  Perhaps 'scale' is too high?"
+            raise ValueError(msg)
+
         self.handle = self.parent.ax.barbs(
-            x_like[wind_slice], y_like[wind_slice],
-            u.values[wind_slice], v.values[wind_slice],
+            x_like[wind_slice], y_like[wind_slice], u_vals, v_vals,
             color=self.color, pivot=self.pivot, length=self.barblength, zorder=2, **kwargs)
 
 

tests/plots/test_declarative.py

@@ -565,6 +565,31 @@ def test_no_field_error_barbs():
         barbs.draw()
 
 
+@needs_cartopy
+def test_too_big_error_barbs():
+    """Make sure we get an error when we are about to draw too many pennants."""
+    data = xr.open_dataset(get_test_data('narr_example.nc', as_file_obj=False)).squeeze()
+
+    bp = BarbPlot()
+    bp.data = data
+    bp.field = ['u_wind', 'v_wind']
+    bp.level = 500
+    bp.scale = 1e4
+    bp.skip = [8, 8]
+
+    mp = MapPanel()
+    mp.layout = (1, 1, 1)
+    mp.area = (-100, -70, 30, 45)
+    mp.plots = [bp]
+
+    pc = PanelContainer()
+    pc.size = (10, 8)
+    pc.panels = [mp]
+
+    with pytest.raises(ValueError):
+        pc.draw()
+
+
 @pytest.mark.mpl_image_compare(remove_text=True, tolerance=0.377)
 def test_projection_object(ccrs, cfeature):
     """Test that we can pass a custom map projection."""