Allow for plotting dummy netCDF4.datetime objects. (#1261)

* Allow for plotting dummy netCDF4.datetime objects.

* Add decorator to skip if netCDF4 is missing.

* Test line plot with netCDF4.datetime coordinate.

* Rewrite checks for plottable data.

Allow for numeric and date-like numpy data types and for datetime
objects.

* Address review by @shoyer

- Use datetime instead of netCDF4.datetime.
- Do not warn but assert in private function.

* Line continuation after parenthesis.

* Check for generic dtype to avoid expensive loop

Author: willirath

xarray/plot/plot.py

@@ -13,32 +13,48 @@
 
 import numpy as np
 import pandas as pd
+from datetime import datetime
 
 from .utils import _determine_cmap_params, _infer_xy_labels, get_axis
 from .facetgrid import FacetGrid
 from xarray.core.pycompat import basestring
 
 
-# Maybe more appropriate to keep this in .utils
-def _right_dtype(arr, types):
+def _valid_numpy_subdtype(x, numpy_types):
     """
-    Is the numpy array a sub dtype of anything in types?
+    Is any dtype from numpy_types superior to the dtype of x?
     """
-    return any(np.issubdtype(arr.dtype, t) for t in types)
+    # If any of the types given in numpy_types is understood as numpy.generic,
+    # all possible x will be considered valid.  This is probably unwanted.
+    for t in numpy_types:
+        assert not np.issubdtype(np.generic, t)
 
+    return any(np.issubdtype(x.dtype, t) for t in numpy_types)
 
-def _ensure_plottable(*args):
+
+def _valid_other_type(x, types):
     """
-    Raise exception if there is anything in args that can't be plotted on
-    an axis.
+    Do all elements of x have a type from types?
     """
-    plottypes = [np.floating, np.integer, np.timedelta64, np.datetime64]
+    if not np.issubdtype(np.generic, x.dtype):
+        return False
+    else:
+        return all(any(isinstance(el, t) for t in types) for el in np.ravel(x))
 
-    # Lists need to be converted to np.arrays here.
-    if not any(_right_dtype(np.array(x), plottypes) for x in args):
-        raise TypeError('Plotting requires coordinates to be numeric '
-                        'or dates.')
 
+def _ensure_plottable(*args):
+    """
+    Raise exception if there is anything in args that can't be plotted on an
+    axis.
+    """
+    numpy_types = [np.floating, np.integer, np.timedelta64, np.datetime64]
+    other_types = [datetime]
+
+    for x in args:
+        if not (_valid_numpy_subdtype(np.array(x), numpy_types)
+                or _valid_other_type(np.array(x), other_types)):
+            raise TypeError('Plotting requires coordinates to be numeric '
+                            'or dates.')
 
 def _easy_facetgrid(darray, plotfunc, x, y, row=None, col=None,
                     col_wrap=None, sharex=True, sharey=True, aspect=None,

xarray/tests/test_plot.py

@@ -16,6 +16,7 @@
 
 import numpy as np
 import pandas as pd
+from datetime import datetime
 
 from xarray import DataArray
 
@@ -1192,7 +1193,7 @@ def test_facetgrid_polar(self):
                                     subplot_kws=dict(projection='polar'),
                                     sharex=False, sharey=False)
 
-        
+
 class TestFacetGrid4d(PlotTestCase):
 
     def setUp(self):
@@ -1218,3 +1219,22 @@ def test_default_labels(self):
         # Top row should be labeled
         for label, ax in zip(self.darray.coords['col'].values, g.axes[0, :]):
             self.assertTrue(substring_in_axes(label, ax))
+
+
+class TestDatetimePlot(PlotTestCase):
+
+    def setUp(self):
+        '''
+        Create a DataArray with a time-axis that contains datetime objects.
+        '''
+        month = np.arange(1, 13, 1)
+        data = np.sin(2 * np.pi * month / 12.0)
+
+        darray = DataArray(data, dims=['time'])
+        darray.coords['time'] = np.array([datetime(2017, m, 1) for m in month])
+
+        self.darray = darray
+
+    def test_datetime_line_plot(self):
+        # test if line plot raises no Exception
+        self.darray.plot.line()