METAR/Station Plot fixes

src/metpy/io/metar.py

@@ -477,7 +477,7 @@ def _metars_to_dataframe(metar_iter, *, year=None, month=None):
     # Set the units for the dataframe--filter out warning from Pandas
     with warnings.catch_warnings():
         warnings.simplefilter('ignore', UserWarning)
-        df.units = col_units
+        df.units = col_units.copy()
 
     return df
 

src/metpy/plots/station_plot.py

@@ -163,7 +163,7 @@ def plot_symbols(mapper, name, nwrap=10, figsize=(10, 1.4)):
         kwargs['fontproperties'] = wx_symbol_font.copy()
         return self.plot_parameter(location, codes, symbol_mapper, **kwargs)
 
-    def plot_parameter(self, location, parameter, formatter='.0f', **kwargs):
+    def plot_parameter(self, location, parameter, formatter=None, **kwargs):
         """At the specified location in the station model plot a set of values.
 
         This specifies that at the offset `location`, the data in `parameter` should be
@@ -186,7 +186,7 @@ def plot_parameter(self, location, parameter, formatter='.0f', **kwargs):
         formatter : str or Callable, optional
             How to format the data as a string for plotting. If a string, it should be
             compatible with the :func:`format` builtin. If a callable, this should take a
-            value and return a string. Defaults to '0.f'.
+            value and return a string. Defaults to 'z0.f'.
         plot_units: `pint.unit`
             Units to plot in (performing conversion if necessary). Defaults to given units.
         kwargs
@@ -372,6 +372,14 @@ def _make_kwargs(self, kwargs):
     @staticmethod
     def _to_string_list(vals, fmt):
         """Convert a sequence of values to a list of strings."""
+        if fmt is None:
+            import sys
+            if sys.version_info >= (3, 11):
+                fmt = 'z.0f'
+            else:
+                def fmt(s):
+                    """Perform default formatting with no decimal places and no negative 0."""
+                    return format(round(s, 0) + 0., '.0f')
         if not callable(fmt):
             def formatter(s):
                 """Turn a format string into a callable."""

tests/io/test_metar.py

@@ -399,3 +399,12 @@ def test_repr():
                    "dewp=TreeNode(text='M10', offset=38)), "
                    "altim=TreeNode(text=' A3026', offset=41), "
                    "remarks=TreeNode(text='', offset=47), end=TreeNode(text='', offset=47))")
+
+
+def test_metar_units_in_place():
+    """Test that parsing a METAR yields units that can be changed safely in-place."""
+    df = parse_metar_to_dataframe('KDEN 012153Z 09010KT 10SM FEW060 BKN110 BKN220 27/13 A3010')
+    df.units['air_temperature'] = 'degF'
+
+    df = parse_metar_to_dataframe('KDEN 012153Z 09010KT 10SM FEW060 BKN110 BKN220 27/13 A3010')
+    assert df.units['air_temperature'] == 'degC'

tests/plots/test_station_plot.py

@@ -101,6 +101,18 @@ def test_station_plot_locations():
     return fig
 
 
+def test_station_plot_negative_zero():
+    """Test that we avoid formatting a negative 0 by default."""
+    fig = plt.figure()
+    ax = fig.add_subplot(1, 1, 1)
+    sp = StationPlot(ax, [0], [0])
+    text = sp.plot_parameter('C', [-0.04])
+
+    assert text.text[0] == '0'
+
+    plt.close(fig)
+
+
 @pytest.mark.mpl_image_compare(tolerance=0.00413, savefig_kwargs={'dpi': 300},
                                remove_text=True)
 def test_stationlayout_api():