Add helper methods for RFC 3339 datetime handling

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Added
 
+- Additional util methods `now_in_utc` and `now_to_rfc3339_str` ([#760](https://github.com/stac-utils/pystac/pull/760))
 - Add `media_type` and `role` filtering to Item and Collection `get_assets()` method ([#936](https://github.com/stac-utils/pystac/pull/936))
 - `Asset.has_role` ([#936](https://github.com/stac-utils/pystac/pull/936))
 - Enum MediaType entry for flatgeobuf ([discussion](https://github.com/flatgeobuf/flatgeobuf/discussions/112#discussioncomment-4606721)) ([#938](https://github.com/stac-utils/pystac/pull/938))

pystac/utils.py

@@ -329,14 +329,28 @@ def datetime_to_str(dt: datetime, timespec: str = "auto") -> str:
 def str_to_datetime(s: str) -> datetime:
     """Converts a string timestamp to a :class:`datetime.datetime` instance using
     :meth:`dateutil.parser.parse` under the hood. The input string may be in any
-    format :std:doc:`supported by the parser <parser>`.
+    format :std:doc:`supported by the parser <parser>`. This includes many formats
+    including ISO 8601 and RFC 3339.
 
     Args:
         s (str) : The string to convert to :class:`datetime.datetime`.
+
+    Returns:
+        str: The :class:`datetime.datetime` represented the by the string.
     """
     return dateutil.parser.isoparse(s)
 
 
+def now_in_utc() -> datetime:
+    """Returns a datetime value of now with the UTC timezone applied"""
+    return datetime.now(timezone.utc)
+
+
+def now_to_rfc3339_str() -> str:
+    """Returns an RFC 3339 string representing now"""
+    return datetime_to_str(now_in_utc())
+
+
 def geometry_to_bbox(geometry: Dict[str, Any]) -> List[float]:
     """Extract the bounding box from a geojson geometry
 

tests/test_utils.py

@@ -7,13 +7,16 @@
 from datetime import datetime, timedelta, timezone
 from typing import Optional
 
+import pytest
 from dateutil import tz
 
 from pystac import utils
 from pystac.utils import (
     is_absolute_href,
     make_absolute_href,
     make_relative_href,
+    now_in_utc,
+    now_to_rfc3339_str,
     str_to_datetime,
 )
 from tests.utils import TestCases
@@ -349,3 +352,79 @@ def test_geojson_bbox(self) -> None:
             for geom in geom_dicts:
                 got = utils.geometry_to_bbox(geom)
                 self.assertNotEqual(got, None)
+
+
+@pytest.mark.parametrize(
+    "datetime",
+    [
+        "37-01-01T12:00:27.87Z",  # invalid year, must be 4 digits
+        "21985-12-12T23:20:50.52Z",  # year must be 4 digits
+        "1985-13-12T23:20:50.52Z",  # month > 12
+        "1985-12-32T23:20:50.52Z",  # day > 31
+        "1985-12-01T25:20:50.52Z",  # hour > 24
+        "1985-12-01T00:60:50.52Z",  # minute > 59
+        "1985-12-01T00:06:61.52Z",  # second > 60
+        "1985-04-12T23:20:50.Z",  # fractional sec . but no frac secs
+        "1985-04-12T23:20:50,Z",  # fractional sec , but no frac secs
+        "1990-12-31T23:59:61Z",  # second > 60 w/o fractional seconds
+    ],
+)
+def test_parse_invalid_rfc3339_str_to_datetime(datetime: str) -> None:
+    with pytest.raises(ValueError):
+        str_to_datetime(datetime)
+
+
+@pytest.mark.parametrize(
+    "datetime",
+    [
+        "1985-04-12",  # date only
+        "1937-01-01T12:00:27.87+0100",  # invalid TZ format, no sep :
+        "1985-12-12T23:20:50.52",  # no TZ
+        "1985-04-12T23:20:50,52Z",  # comma as frac sec sep disallowed in RFC3339
+    ],
+)
+def test_parse_invalid_rfc3339_but_valid_iso8601_str_to_datetime(datetime: str) -> None:
+    assert str_to_datetime(datetime)
+
+
+@pytest.mark.parametrize(
+    "datetime",
+    [
+        "1985-04-12T23:20:50.52Z",
+        "1996-12-19T16:39:57-00:00",
+        "1996-12-19T16:39:57+00:00",
+        "1996-12-19T16:39:57-08:00",
+        "1996-12-19T16:39:57+08:00",
+        "1937-01-01T12:00:27.87+01:00",
+        "1985-04-12T23:20:50.52Z",
+        "1937-01-01T12:00:27.8710+01:00",
+        "1937-01-01T12:00:27.8+01:00",
+        "1937-01-01T12:00:27.8Z",
+        "2020-07-23T00:00:00.000+03:00",
+        "2020-07-23T00:00:00+03:00",
+        "1985-04-12t23:20:50.000z",
+        "2020-07-23T00:00:00Z",
+        "2020-07-23T00:00:00.0Z",
+        "2020-07-23T00:00:00.01Z",
+        "2020-07-23T00:00:00.012Z",
+        "2020-07-23T00:00:00.0123Z",
+        "2020-07-23T00:00:00.01234Z",
+        "2020-07-23T00:00:00.012345Z",
+        "2020-07-23T00:00:00.0123456Z",
+        "2020-07-23T00:00:00.01234567Z",
+        "2020-07-23T00:00:00.012345678Z",
+    ],
+)
+def test_parse_valid_iso8601_str_to_datetime(datetime: str) -> None:
+    assert str_to_datetime(datetime)
+
+
+def test_now_functions() -> None:
+    now1 = now_in_utc()
+    time.sleep(1)
+    now2 = now_in_utc()
+
+    assert now1 < now2
+    assert now1.tzinfo == timezone.utc
+
+    assert str_to_datetime(now_to_rfc3339_str())