utils: iso8601: strip fractional seconds

According to ISO8601 any number of digits is permitted for fractional
seconds, the issue is that Python assumes these are microsecond, and
will fail if we have more than 6 digits.

In the event that we detect more that 6 digits, truncate the fractional
part and pass it to datetime.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>

sigmf/utils.py

@@ -10,6 +10,7 @@
 from datetime import datetime
 import sys
 import numpy as np
+import re
 
 from . import error
 
@@ -30,6 +31,12 @@ def parse_iso8601_datetime(datestr: str) -> datetime:
     >>> parse_iso8601_datetime("1955-11-05T06:15:00Z")
     datetime.datetime(1955, 11, 5, 6, 15)
     """
+    m = re.match(r'^(?P<dt>.*)(?P<frac>\.[0-9]{7,})Z$', datestr)
+    if m:
+        d =  m.groupdict()
+        l = min(7, len(d['frac']))
+        datestr = ''.join([d['dt'], d['frac'][:l], 'Z'])
+
     try:
         timestamp = datetime.strptime(datestr, '%Y-%m-%dT%H:%M:%S.%fZ')
     except ValueError:

tests/test_utils.py

@@ -0,0 +1,17 @@
+import datetime
+
+import pytest
+
+from sigmf import utils
+
+@pytest.mark.parametrize('ts,expected', [
+    ('1955-08-05T06:15:00Z',                datetime.datetime(year=1955, month=8 , day=5, hour=6 , minute=15, second=00, microsecond=0)),
+    ('1956-09-06T07:15:12Z',                datetime.datetime(year=1956, month=9 , day=6, hour=7 , minute=15, second=12, microsecond=0)),
+    ('1957-10-07T08:15:12.345Z',            datetime.datetime(year=1957, month=10, day=7, hour=8 , minute=15, second=12, microsecond=345000)),
+    ('1958-11-08T09:15:12.0345Z',           datetime.datetime(year=1958, month=11, day=8, hour=9 , minute=15, second=12, microsecond=34500)),
+    ('1959-12-09T10:15:12.000000Z',         datetime.datetime(year=1959, month=12, day=9, hour=10, minute=15, second=12, microsecond=0)),
+    ('1959-12-09T10:15:12.123456789123Z',   datetime.datetime(year=1959, month=12, day=9, hour=10, minute=15, second=12, microsecond=123456)),
+])
+def test_parse_simple_iso8601(ts, expected):
+    dt = utils.parse_iso8601_datetime(ts)
+    assert dt == expected