add util.to_iso8601_duration()

for snarfed/granary#169

tests/test_util.py

@@ -543,6 +543,17 @@ def test_parse_iso8601_duration(self):
         self.assertEqual(datetime.timedelta(*expected),
                          util.parse_iso8601_duration(input))
 
+  def test_to_iso8601_duration(self):
+    for bad in (None, 3, 4.5, '', 'bad'):
+        self.assertRaises(TypeError, util.to_iso8601_duration, bad)
+
+    for input, expected in (
+        ((0, 0), 'P0DT0S'),
+        ((1, 2), 'P1DT2S'),
+        ((3, 4.5), 'P3DT4S'),
+    ):
+        self.assertEqual(expected, util.to_iso8601_duration(datetime.timedelta(*input)))
+
   def test_maybe_iso8601_to_rfc3339(self):
     for input, expected in (
       (None, None),

util.py

@@ -747,7 +747,8 @@ def parse_iso8601_duration(input):
   https://en.wikipedia.org/wiki/ISO_8601#Durations
 
   Returns:
-    timedelta, or None if input cannot be parsed as an ISO 8601 duration
+    :class:`datetime.timedelta`, or None if input cannot be parsed as an ISO
+      8601 duration
   """
   if not input:
     return None
@@ -765,6 +766,28 @@ def g(i):
                             hours=g(6), minutes=g(7), seconds=g(8))
 
 
+def to_iso8601_duration(input):
+  """Converts a timedelta to an ISO 8601 duration.
+
+  Returns a fairly strict format: 'PnMTnS'. Fractional seconds are silently
+  dropped.
+
+  Args:
+    input: :class:`datetime.timedelta`
+
+  https://en.wikipedia.org/wiki/ISO_8601#Durations
+
+  Returns:
+    string ISO 8601 duration, e.g. 'P3DT4S'
+
+  Raises: :class:`TypeError` if delta is not a :class:`datetime.timedelta`
+  """
+  if not isinstance(input, datetime.timedelta):
+    raise TypeError('Expected datetime.timedelta, got %s' % input.__class__)
+
+  return 'P%sDT%sS' % (input.days, input.seconds)
+
+
 def maybe_iso8601_to_rfc3339(input):
   """Tries to convert an ISO 8601 date/time string to RFC 3339.