Traitlets validation of MapPanel's 'area' trait

TraitError thrown when bad inputs are provided to 'area' trait. Additionally, code to parse the 'area' string is removed from draw() and put in a new method _parse_area_string().

src/metpy/plots/declarative.py

@@ -11,8 +11,8 @@
 import matplotlib.pyplot as plt
 import numpy as np
 import pandas as pd
-from traitlets import (Any, Bool, Float, HasTraits, Instance, Int, List, observe, Tuple,
-                       Unicode, Union)
+from traitlets import (Any, Bool, Float, HasTraits, Instance, Int, List, observe, TraitError,
+                       Tuple, Unicode, Union, validate)
 
 from . import ctables
 from . import wx_symbols
@@ -651,6 +651,28 @@ class MapPanel(Panel):
     This trait sets a user-defined title that will plot at the top center of the figure.
     """
 
+    @validate('area')
+    def _valid_area(self, proposal):
+        """Check that the proposed area string or tuple is a valid value."""
+        area = proposal['value']
+
+        # If string, check that the string exists in list of areas ('global' is special case)
+        if isinstance(area, str):
+            region = self._parse_area_string(area)[0]
+            if region.lower() not in _areas.keys() and region != 'global':
+                raise TraitError('"' + area + '" is not a valid string area.')
+        # If tuple, check that the latitudes and longitudes make sense
+        elif isinstance(area, tuple):
+            west_lon, east_lon, south_lat, north_lat = area
+            valid_west = -180 <= west_lon <= 180
+            valid_east = -180 <= east_lon <= 180
+            valid_south = -90 <= south_lat <= 90
+            valid_north = -90 <= north_lat <= 90
+            if not valid_west or not valid_east or not valid_south or not valid_north:
+                raise TraitError(str(area) + ' is not a valid latitude/longitude extent.')
+
+        return area
+
     @observe('plots')
     def _plots_changed(self, change):
         """Handle when our collection of plots changes."""
@@ -736,6 +758,27 @@ def _zoom_extent(extent, zoom):
 
         return (new_west_lon, new_east_lon, new_south_lat, new_north_lat)
 
+    def _parse_area_string(self, area=None):
+        """Split the area string into the 'region' and the 'modifier'.
+
+        For example, if area is 'co++', this will return 'co' and '++'.
+
+        """
+        # If an area is not provided, assume we want to parse with self.area
+        if area is None:
+            area = self.area
+
+        # Find where to split area string so that we are left with 'region' and 'modifier'
+        first_pos_found = [area.find('+'), area.find('-')]
+        has_modifier = first_pos_found != [-1, -1]
+        if has_modifier:
+            split_pos = min([i for i in first_pos_found if i >= 0])
+            region = area[:split_pos]
+            modifier = area[split_pos:]
+            return region, modifier
+        else:
+            return area, ''
+
     @property
     def ax(self):
         """Get the :class:`matplotlib.axes.Axes` to draw on.
@@ -775,17 +818,11 @@ def draw(self):
                 self.ax.set_global()
             elif self.area is not None:
                 # Get extent from specified area and zoom in/out with '+' or '-' suffix
-                if isinstance(self.area, str) and ('+' in self.area or '-' in self.area):
-                    pos = [self.area.find('+'), self.area.find('-')]
-                    split_pos = min([i for i in pos if i > 0])
-                    area = self.area[:split_pos]
-                    modifier = self.area[split_pos:]
-                    extent = _areas[area.lower()]
+                if isinstance(self.area, str):
+                    region, modifier = self._parse_area_string()
+                    extent = _areas[region.lower()]
                     zoom = Counter(modifier)['+'] - Counter(modifier)['-']
                     extent = self._zoom_extent(extent, zoom)
-                # Get extent from specified area
-                elif isinstance(self.area, str):
-                    extent = _areas[self.area.lower()]
                 # Otherwise, assume we have a tuple to use as the extent
                 else:
                     extent = self.area