MNT: fix flake8 errors

docs/references.rst

@@ -45,8 +45,7 @@ References
 
 .. [Esterheld2008] Esterheld, J., and Giuliano, D., 2008: Discriminating between Tornadic and
            Non-Tornadic Supercells: A New Hodograph Technique. *E-Journal Of Severe Storms
-           Meteorology*, **3(2)**, Retrieved May 10, 2018, from
-           http://www.ejssm.org/ojs/index.php/ejssm/article/view/33
+           Meteorology*, **3(2)**, Retrieved May 10, 2018.
 
 .. [Hobbs1977] Hobbs, P. V., and J. M. Wallace, 1977: *Atmospheric Science: An
            Introductory Survey*. Academic Press, 350 pp.

metpy/plots/tests/test_station_plot.py

@@ -255,7 +255,7 @@ def test_layout_str():
     layout.add_value('W', 'temp')
     layout.add_symbol('C', 'cover', lambda x: x)
     assert str(layout) == ('{C: (symbol, cover, ...), E: (text, stid, ...), '
-                           'W: (value, temp, ...), barb: (barb, (\'u\', \'v\'), ...)}')
+                           "W: (value, temp, ...), barb: (barb, ('u', 'v'), ...)}")
 
 
 @pytest.mark.mpl_image_compare(tolerance={'1.4': 0.08}.get(MPL_VERSION, 0.00145),

metpy/tests/test_units.py

@@ -144,7 +144,7 @@ def test_pandas_units_simple():
     assert_array_equal(res['colb'], colb_truth)
 
 
-@pytest.mark.filterwarnings('ignore:Pandas doesn\'t allow columns to be created')
+@pytest.mark.filterwarnings("ignore:Pandas doesn't allow columns to be created")
 def test_pandas_units_on_dataframe():
     """Unit attachment based on a units attribute to a dataframe."""
     df = pd.DataFrame(data=[[1, 4], [2, 5], [3, 6]], columns=['cola', 'colb'])
@@ -156,7 +156,7 @@ def test_pandas_units_on_dataframe():
     assert_array_equal(res['colb'], colb_truth)
 
 
-@pytest.mark.filterwarnings('ignore:Pandas doesn\'t allow columns to be created')
+@pytest.mark.filterwarnings("ignore:Pandas doesn't allow columns to be created")
 def test_pandas_units_on_dataframe_not_all_united():
     """Unit attachment with units attribute with a column with no units."""
     df = pd.DataFrame(data=[[1, 4], [2, 5], [3, 6]], columns=['cola', 'colb'])

metpy/xarray.py

@@ -76,7 +76,7 @@ def _axis(self, axis):
                     return coord_var
             raise AttributeError(axis + ' attribute is not available.')
         else:
-            raise AttributeError('\'' + axis + '\' is not an interpretable axis.')
+            raise AttributeError("'" + axis + "' is not an interpretable axis.")
 
     def coordinates(self, *args):
         """Return the coordinate variables corresponding to the given axes types."""