Ordered the inputs so that filenames are loaded in the given order.

lib/iris/io/__init__.py

@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2016, Met Office
+# (C) British Crown Copyright 2010 - 2017, Met Office
 #
 # This file is part of Iris.
 #
@@ -23,6 +23,7 @@
 from six.moves import (filter, input, map, range, zip)  # noqa
 import six
 
+from collections import OrderedDict
 import glob
 import os.path
 import types
@@ -148,28 +149,34 @@ def expand_filespecs(file_specs):
         File paths which may contain '~' elements or wildcards.
 
     Returns:
-        A list of matching absolute file paths.  If any of the file-specs matches no
-        existing files, an exception is raised.
+        A well-ordered list of matching absolute file paths.
+        If any of the file-specs match no existing files, an
+        exception is raised.
 
     """
-    # Remove any hostname component - currently unused - expand paths as absolutes
-    filenames = [os.path.abspath(os.path.expanduser(fn[2:] if fn.startswith('//') else fn))
+    # Remove any hostname component - currently unused
+    filenames = [os.path.abspath(os.path.expanduser(
+                    fn[2:] if fn.startswith('//') else fn))
                  for fn in file_specs]
 
     # Try to expand all filenames as globs
-    glob_expanded = {fn : sorted(glob.glob(fn)) for fn in filenames}
+    glob_expanded = OrderedDict([[fn, sorted(glob.glob(fn))]
+                                 for fn in filenames])
 
     # If any of the specs expanded to an empty list then raise an error
-    value_list_raw = glob_expanded.values()
-
-    # Here is a term to sort value_lists alphabetically, for consistency
-    value_lists = sorted(value_list_raw)
-    if not all(value_lists):
-        raise IOError("One or more of the files specified did not exist %s." %
-        ["%s expanded to %s" % (pattern, expanded if expanded else "empty")
-         for pattern, expanded in sorted(six.iteritems(glob_expanded))])
-
-    return sum(value_lists, [])
+    all_expanded = glob_expanded.values()
+
+    if not all(all_expanded):
+        msg = "One or more of the files specified did not exist:"
+        for pattern, expanded in six.iteritems(glob_expanded):
+            if expanded:
+                file_list = '\n       - {}'.format(', '.join(expanded))
+            else:
+                file_list = ''
+            msg += '\n    - "{}" matched {} file(s){}'.format(pattern, len(expanded), file_list)
+        raise IOError(msg)
+
+    return [fname for fnames in all_expanded for fname in fnames]
 
 
 def load_files(filenames, callback, constraints=None):

lib/iris/tests/__init__.py

@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2016, Met Office
+# (C) British Crown Copyright 2010 - 2017, Met Office
 #
 # This file is part of Iris.
 #
@@ -210,6 +210,16 @@ def _assert_str_same(self, reference_str, test_str, reference_filename, type_com
                                                  'Reference', 'Test result', '', '', 0))
             self.fail("%s do not match: %s\n%s" % (type_comparison_name, reference_filename, diff))
 
+    def assertStringEqual(self, reference_str, test_str,
+                          type_comparison_name='strings'):
+        if reference_str != test_str:
+            diff = '\n'.join(difflib.unified_diff(reference_str.splitlines(),
+                                                test_str.splitlines(),
+                                                'Reference', 'Test result',
+                                                '', '', 0))
+            self.fail("{} do not match:\n{}".format(type_comparison_name,
+                                                    diff))
+
     def result_path(self, basename=None, ext=''):
         """
         Return the full path to a test result, generated from the \

lib/iris/tests/unit/io/test_expand_filespecs.py

@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2016, Met Office
+# (C) British Crown Copyright 2017, Met Office
 #
 # This file is part of Iris.
 #
@@ -26,12 +26,12 @@
 import os
 import tempfile
 import shutil
+import textwrap
 
 import iris.io as iio
 
 
 class TestExpandFilespecs(tests.IrisTest):
-
     def setUp(self):
         tests.IrisTest.setUp(self)
         self.tmpdir = tempfile.mkdtemp()
@@ -59,16 +59,38 @@ def test_relative_path(self):
         item_in = [os.path.join(self.tmpdir, fname) for fname in self.fnames]
         self.assertEqual(item_out, item_in)
 
+    def test_return_order(self):
+        # It is really quite important what order we return the
+        # files. They should be in the order that was provided,
+        # so that we can control the order of load (for instance,
+        # this can be used with PP files to ensure that there is
+        # a surface reference).
+        patterns = [os.path.join(self.tmpdir, 'a.*'),
+                    os.path.join(self.tmpdir, 'b.*')]
+        expected = [os.path.join(self.tmpdir, fname)
+                    for fname in ['a.foo', 'b.txt']]
+        result = iio.expand_filespecs(patterns)
+        self.assertEqual(result, expected)
+        result = iio.expand_filespecs(patterns[::-1])
+        self.assertEqual(result, expected[::-1])
+
     def test_no_files_found(self):
-        msg = 'b expanded to empty'
+        msg = r'_b\" matched 0 file\(s\)'
         with self.assertRaisesRegexp(IOError, msg):
             iio.expand_filespecs([self.tmpdir + '_b'])
 
     def test_files_and_none(self):
-        msg = 'b expanded to empty.*expanded to .*b.txt'
-        with self.assertRaisesRegexp(IOError, msg):
+        with self.assertRaises(IOError) as err:
             iio.expand_filespecs([self.tmpdir + '_b',
-                                 os.path.join(self.tmpdir, '*')])
+                                  os.path.join(self.tmpdir, '*')])
+        expected = textwrap.dedent("""
+            One or more of the files specified did not exist:
+                - "{0}_b" matched 0 file(s)
+                - "{0}/*" matched 2 file(s)
+                   - {0}/a.foo, {0}/b.txt
+            """).strip().format(self.tmpdir)
+
+        self.assertStringEqual(str(err.exception), expected)
 
 
 if __name__ == "__main__":