sequence_join: Correctly join datetime-like values in pd.DataFrame/xr.DataArray objects (#4097)

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

Author: seisman

pygmt/alias.py

@@ -109,6 +109,21 @@ def _to_string(
 
     >>> _to_string([[1, 2], [3, 4]], sep="/", ndim=2)
     ['1/2', '3/4']
+
+    >>> import datetime
+    >>> import numpy as np
+    >>> import pandas as pd
+    >>> import xarray as xr
+    >>> _to_string(
+    ...     [
+    ...         datetime.date(2010, 1, 1),
+    ...         datetime.datetime(2010, 3, 1),
+    ...         pd.Timestamp("2015-01-01T12:00:00.123456789"),
+    ...         xr.DataArray(data=np.datetime64("2005-01-01T08:00:00", "ns")),
+    ...     ],
+    ...     sep="/",
+    ... )
+    '2010-01-01/2010-03-01T00:00:00.000000/2015-01-01T12:00:00.123456/2005-01-01T08:00:00.000000000'
     """
     # None and False are converted to None.
     if value is None or value is False:

pygmt/helpers/utils.py

@@ -16,6 +16,7 @@
 from pathlib import Path
 from typing import Any, Literal
 
+import numpy as np
 import xarray as xr
 from pygmt._typing import PathLike
 from pygmt.encodings import charset
@@ -816,6 +817,30 @@ def sequence_join(
     ['1/2', '3/4']
     >>> sequence_join([1, 2, 3, 4], separator=",")
     '1,2,3,4'
+
+    >>> # Join a sequence of datetime-like objects into a string.
+    >>> import datetime
+    >>> import numpy as np
+    >>> import pandas as pd
+    >>> import xarray as xr
+    >>> sequence_join(
+    ...     [
+    ...         datetime.date(2010, 1, 1),
+    ...         np.datetime64("2010-01-01T16:00:00"),
+    ...         np.array("2010-03-01T00:00:00", dtype=np.datetime64),
+    ...     ],
+    ...     sep="/",
+    ... )
+    '2010-01-01/2010-01-01T16:00:00/2010-03-01T00:00:00'
+    >>> sequence_join(
+    ...     [
+    ...         datetime.datetime(2010, 3, 1),
+    ...         pd.Timestamp("2015-01-01T12:00:00.123456789"),
+    ...         xr.DataArray(data=np.datetime64("2005-01-01T08:00:00", "ns")),
+    ...     ],
+    ...     sep="/",
+    ... )
+    '2010-03-01T00:00:00.000000/2015-01-01T12:00:00.123456/2005-01-01T08:00:00.000000000'
     """
     # Return the original value if it is not a sequence (e.g., None, bool, or str).
     if not is_nonstr_iter(value):
@@ -848,7 +873,17 @@ def sequence_join(
                 f"but got {len(value)} values."
             )
             raise GMTInvalidInput(msg)
-        return sep.join(str(v) for v in value)
+        # 'str(v)' produces a string like '2024-01-01 00:00:00' for some datetime-like
+        # objects (e.g., datetime.datetime, pandas.Timestamp), which contains a space.
+        # If so, use np.datetime_as_string to convert it to ISO 8601 string format
+        # YYYY-MM-DDThh:mm:ss.ffffff.
+        _values = [
+            np.datetime_as_string(np.asarray(item, dtype="datetime64"))
+            if " " in (s := str(item))
+            else s
+            for item in value
+        ]
+        return sep.join(_values)  # type: ignore[arg-type]
 
     # Now it must be a 2-D sequence.
     if ndim == 1: