Traitlets validation of MapPanel's 'area' trait

TraitError thrown when bad inputs are provided to 'area' trait. Code to determine map extent using 'area' string is removed from draw() and included in the validation method, _valid_area().

src/metpy/plots/declarative.py

@@ -3,16 +3,16 @@
 #  SPDX-License-Identifier: BSD-3-Clause
 """Declarative plotting tools."""
 
-from collections import Counter
 import contextlib
 import copy
 from datetime import datetime, timedelta
 
 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)
+import re
+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 +651,39 @@ 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 proposed string or tuple is valid and turn string into a tuple extent."""
+        area = proposal['value']
+
+        # Parse string, check that string is valid, and determine extent based on string
+        if isinstance(area, str):
+            try:
+                region, modifier = re.match(r'(\w+)([-+]*)$', area).groups()
+            except AttributeError:
+                raise TraitError('"' + area + '" is not a valid string area.')
+
+            if region.lower() not in _areas.keys() and region != 'global':
+                raise TraitError('"' + area + '" is not a valid string area.')
+            elif region == 'global':
+                extent = 'global'
+            else:
+                extent = _areas[region.lower()]
+                zoom = modifier.count('+') - modifier.count('-')
+                extent = self._zoom_extent(extent, zoom)
+        # Otherwise, assume area is a tuple and check that latitudes/longitudes are valid
+        else:
+            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.')
+            extent = area
+
+        return extent
+
     @observe('plots')
     def _plots_changed(self, change):
         """Handle when our collection of plots changes."""
@@ -715,24 +748,17 @@ def _zoom_extent(extent, zoom):
         If ``zoom`` < 0, the returned extent will be larger (zoomed out)
 
         """
-        # Measure the current extent
-        center_lon = (extent[0] + extent[1]) / 2
-        center_lat = (extent[2] + extent[3]) / 2
-        lon_range = extent[1] - extent[0]
-        lat_range = extent[3] - extent[2]
-
-        # Transforming zoom by e^-x prevents multiplication by zero or a negative number below
-        zoom_multiplier = np.exp(-0.5 * zoom)
+        west_lon, east_lon, south_lat, north_lat = extent
 
-        # Calculate "width" and "height" of new, zoomed extent
-        new_lon_range = lon_range * zoom_multiplier
-        new_lat_range = lat_range * zoom_multiplier
+        # Turn number of pluses and minuses into a number than can scale the latitudes and
+        # longitudes of our extent
+        zoom_multiplier = (1 - 2**-zoom) / 2
 
-        # Calculate bounds for new, zoomed extent with new "width" and "height"
-        new_west_lon = center_lon - 0.5 * new_lon_range
-        new_east_lon = center_lon + 0.5 * new_lon_range
-        new_south_lat = center_lat - 0.5 * new_lat_range
-        new_north_lat = center_lat + 0.5 * new_lat_range
+        # Calculate bounds for new, zoomed extent
+        new_north_lat = north_lat + (south_lat - north_lat) * zoom_multiplier
+        new_south_lat = south_lat - (south_lat - north_lat) * zoom_multiplier
+        new_east_lon = east_lon + (west_lon - east_lon) * zoom_multiplier
+        new_west_lon = west_lon - (west_lon - east_lon) * zoom_multiplier
 
         return (new_west_lon, new_east_lon, new_south_lat, new_north_lat)
 
@@ -770,26 +796,11 @@ def draw(self):
         # Only need to run if we've actually changed.
         if self._need_redraw:
 
-            # Set the extent as appropriate based on the area. One special case for 'global'
+            # Set the extent as appropriate based on the area. One special case for 'global'.
             if self.area == 'global':
                 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()]
-                    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
-                self.ax.set_extent(extent, ccrs.PlateCarree())
+                self.ax.set_extent(self.area, ccrs.PlateCarree())
 
             # Draw all of the plots.
             for p in self.plots:

tests/plots/test_declarative.py

@@ -1198,6 +1198,23 @@ def test_declarative_region_modifier_zoom_out():
     return pc.figure
 
 
+def test_declarative_bad_area():
+    """Test that a invalid string or tuple provided to the area trait raises an error."""
+    panel = MapPanel()
+
+    # Test for string that cannot be grouped into a region and a modifier by regex
+    with pytest.raises(TraitError):
+        panel.area = 'a$z+'
+
+    # Test for string that is not in our list of areas
+    with pytest.raises(TraitError):
+        panel.area = 'PS'
+
+    # Test for nonsense coordinates
+    with pytest.raises(TraitError):
+        panel.area = (136, -452, -65, -88)
+
+
 def test_save():
     """Test that our saving function works."""
     pc = PanelContainer()