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

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