Add copy method for declarative interface (Fixes Unidata#1719)

MapPanel needed an explicity written __copy__() to make sure the copy process worked correctly. All other classes in declarative.py did not need it. A .copy() method is also provided for ease of use.

src/metpy/plots/declarative.py

@@ -4,6 +4,7 @@
 """Declarative plotting tools."""
 
 import contextlib
+import copy
 from datetime import datetime, timedelta
 
 import matplotlib.pyplot as plt
@@ -568,6 +569,10 @@ def show(self):
         self.draw()
         plt.show()
 
+    def copy(self):
+        """Return a copy of the panel container."""
+        return copy.copy(self)
+
 
 @exporter.export
 class MapPanel(Panel):
@@ -579,7 +584,7 @@ class MapPanel(Panel):
     projection, graphics area, and title.
     """
 
-    parent = Instance(PanelContainer)
+    parent = Instance(PanelContainer, allow_none=True)
 
     layout = Tuple(Int(), Int(), Int(), default_value=(1, 1, 1))
     layout.__doc__ = """A tuple that contains the description (nrows, ncols, index) of the
@@ -754,6 +759,27 @@ def draw(self):
             self.ax.set_title(title)
             self._need_redraw = False
 
+    def __copy__(self):
+        """Return a copy of this MapPanel."""
+        # Create new, blank instance of MapPanel
+        cls = self.__class__
+        obj = cls.__new__(cls)
+
+        # Copy each attribute from current MapPanel to new MapPanel
+        for name in self.trait_names():
+            # The 'plots' attribute is a list.
+            # A copy must be made for each plot in the list.
+            if name == 'plots':
+                obj.plots = [copy.copy(plot) for plot in self.plots]
+            else:
+                setattr(obj, name, getattr(self, name))
+
+        return obj
+
+    def copy(self):
+        """Return a copy of the panel."""
+        return copy.copy(self)
+
 
 @exporter.export
 class Plots2D(HasTraits):
@@ -880,6 +906,10 @@ def name(self):
             ret += f'@{self.level:d}'
         return ret
 
+    def copy(self):
+        """Return a copy of the plot."""
+        return copy.copy(self)
+
 
 @exporter.export
 class PlotScalar(Plots2D):
@@ -1007,6 +1037,9 @@ class ColorfillTraits(HasTraits):
     ``None``.
     """
 
+    def copy(self):
+        return copy.copy(self)
+
 
 @exporter.export
 class ImagePlot(PlotScalar, ColorfillTraits):
@@ -1604,3 +1637,7 @@ def _build(self):
             if self.vector_field_length is not None:
                 vector_kwargs['length'] = self.vector_field_length
             self.handle.plot_barb(u, v, **vector_kwargs)
+
+    def copy(self):
+        """Return a copy of the plot."""
+        return copy.copy(self)

tests/plots/test_declarative.py

@@ -1184,3 +1184,37 @@ def test_panel():
 
     pc.panel = panel
     assert pc.panel is panel
+
+
+@needs_cartopy
+def test_copy():
+    """Test that the copy method works for all classes in `declarative.py`."""
+    # Copies of plot objects
+    objects = [ImagePlot(), ContourPlot(), FilledContourPlot(), BarbPlot(), PlotObs()]
+
+    for obj in objects:
+        obj.time = datetime.now()
+        copied_obj = obj.copy()
+        assert obj is not copied_obj
+        assert obj.time == copied_obj.time
+
+    # Copies of MapPanel and PanelContainer
+    obj = MapPanel()
+    obj.title = "Sample Text"
+    copied_obj = obj.copy()
+    assert obj is not copied_obj
+    assert obj.title == copied_obj.title
+
+    obj = PanelContainer()
+    obj.size = (10, 10)
+    copied_obj = obj.copy()
+    assert obj is not copied_obj
+    assert obj.size == copied_obj.size
+
+    # Copies of plots in MapPanels should not point to same location in memory
+    obj = MapPanel()
+    obj.plots = [PlotObs(), BarbPlot(), FilledContourPlot(), ContourPlot(), ImagePlot()]
+    copied_obj = obj.copy()
+
+    for i in range(len(obj.plots)):
+        assert obj.plots[i] is not copied_obj.plots[i]