Prevent creation of invalid CF variable names. (SciTools#3009)

Prevent creation of invalid CF variable names.

docs/iris/src/whatsnew/contributions_2.1/bugfix_2018-May-03_var_name_constraint.txt

@@ -0,0 +1,2 @@
+* All var names being written to NetCDF are now CF compliant. Non alpha-numeric characters are replaced with '_', and must always have a leading letter.
+  Ref: https://github.com/SciTools/iris/pull/2930

lib/iris/fileformats/netcdf.py

@@ -1339,6 +1339,27 @@ def _get_dim_names(self, cube):
                 dimension_names.append(dim_name)
         return dimension_names
 
+    @staticmethod
+    def cf_valid_var_name(var_name):
+        """
+        Return a valid CF var_name given a potentially invalid name.
+
+        Args:
+
+        * var_name (str):
+            The var_name to normalise
+
+        Returns:
+            A var_name suitable for passing through for variable creation.
+
+        """
+        # Replace invalid charaters with an underscore ("_").
+        var_name = re.sub(r'[^a-zA-Z0-9]', "_", var_name)
+        # Ensure the variable name starts with a letter.
+        if re.match(r'^[^a-zA-Z]', var_name):
+            var_name = 'var_{}'.format(var_name)
+        return var_name
+
     @staticmethod
     def _cf_coord_identity(coord):
         """
@@ -1448,6 +1469,7 @@ def _get_cube_variable_name(self, cube):
             # Convert to lower case and replace whitespace by underscores.
             cf_name = '_'.join(cube.name().lower().split())
 
+        cf_name = self.cf_valid_var_name(cf_name)
         return cf_name
 
     def _get_coord_variable_name(self, cube, coord):
@@ -1480,6 +1502,8 @@ def _get_coord_variable_name(self, cube, coord):
                     name = 'unknown_scalar'
             # Convert to lower case and replace whitespace by underscores.
             cf_name = '_'.join(name.lower().split())
+
+        cf_name = self.cf_valid_var_name(cf_name)
         return cf_name
 
     def _create_cf_cell_measure_variable(self, cube, dimension_names,

lib/iris/tests/unit/fileformats/netcdf/test_Saver.py

@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2013 - 2017, Met Office
+# (C) British Crown Copyright 2013 - 2018, Met Office
 #
 # This file is part of Iris.
 #
@@ -472,6 +472,33 @@ def test_masked_byte_fill_value_passed(self):
                 pass
 
 
+class Test_cf_valid_var_name(tests.IrisTest):
+    def test_no_replacement(self):
+        self.assertEqual(Saver.cf_valid_var_name('valid_Nam3'),
+                         'valid_Nam3')
+
+    def test_special_chars(self):
+        self.assertEqual(Saver.cf_valid_var_name('inv?alid'),
+                         'inv_alid')
+
+    def test_leading_underscore(self):
+        self.assertEqual(Saver.cf_valid_var_name('_invalid'),
+                         'var__invalid')
+
+    def test_leading_number(self):
+        self.assertEqual(Saver.cf_valid_var_name('2invalid'),
+                         'var_2invalid')
+
+    def test_leading_invalid(self):
+        self.assertEqual(Saver.cf_valid_var_name('?invalid'),
+                         'var__invalid')
+
+    def test_no_hyphen(self):
+        # CF explicitly prohibits hyphen, even though it is fine in NetCDF.
+        self.assertEqual(Saver.cf_valid_var_name('valid-netcdf'),
+                         'valid_netcdf')
+
+
 class _Common__check_attribute_compliance(object):
     def setUp(self):
         self.container = mock.Mock(name='container', attributes={})