Add a (hopefully correct) Tag=Value list parser, with tests.

dkim/tests/test_dkim.py

@@ -2,6 +2,10 @@ import os.path
 import unittest
 
 import dkim
+from dkim.util import (
+    InvalidTagValueList,
+    parse_tag_value,
+    )
 
 
 def read_test_data(filename):
@@ -50,5 +54,33 @@ class TestSignAndVerify(unittest.TestCase):
         self.assertFalse(res)
 
 
+class TestParseTagValue(unittest.TestCase):
+    """Tag=Value parsing tests."""
+
+    def test_single(self):
+        self.assertEqual(
+            {'foo': 'bar'},
+            parse_tag_value('foo=bar'))
+
+    def test_trailing_separator_ignored(self):
+        self.assertEqual(
+            {'foo': 'bar'},
+            parse_tag_value('foo=bar;'))
+
+    def test_multiple(self):
+        self.assertEqual(
+            {'foo': 'bar', 'baz': 'foo'},
+            parse_tag_value('foo=bar;baz=foo'))
+
+    def test_value_with_equals(self):
+        self.assertEqual(
+            {'foo': 'bar', 'baz': 'foo=bar'},
+            parse_tag_value('foo=bar;baz=foo=bar'))
+
+    def test_no_value(self):
+        self.assertRaises(
+            InvalidTagValueList,
+            parse_tag_value, 'foo=bar;baz')
+
 if __name__ == '__main__':
     unittest.main()

dkim/util.py

@@ -0,0 +1,46 @@
+# This software is provided 'as-is', without any express or implied
+# warranty.  In no event will the author be held liable for any damages
+# arising from the use of this software.
+#
+# Permission is granted to anyone to use this software for any purpose,
+# including commercial applications, and to alter it and redistribute it
+# freely, subject to the following restrictions:
+#
+# 1. The origin of this software must not be misrepresented; you must not
+#    claim that you wrote the original software. If you use this software
+#    in a product, an acknowledgment in the product documentation would be
+#    appreciated but is not required.
+# 2. Altered source versions must be plainly marked as such, and must not be
+#    misrepresented as being the original software.
+# 3. This notice may not be removed or altered from any source distribution.
+#
+# Copyright (c) 2011 William Grant <me@williamgrant.id.au>
+
+__all__ = [
+    'InvalidTagValueList',
+    'parse_tag_value',
+    ]
+
+
+class InvalidTagValueList(Exception):
+    pass
+
+
+def parse_tag_value(tag_list):
+    """Parse a DKIM Tag=Value list.
+
+    Interprets the syntax specified by RFC4871 section 3.2.
+    Assumes that folding whitespace is already unfolded.
+    """
+    tags = {}
+    tag_specs = tag_list.split(';')
+    # Trailing semicolons are valid.
+    if not tag_specs[-1]:
+        tag_specs.pop()
+    for tag_spec in tag_specs:
+        try:
+            key, value = tag_spec.split('=', 1)
+        except ValueError:
+            raise InvalidTagValueList()
+        tags[key.strip()] = value.strip()
+    return tags