Add type hints for registrations

pygmt/datasets/mars_relief.py

@@ -4,14 +4,20 @@
 
 The grids are available in various resolutions.
 """
+from typing import Literal
+
 from pygmt.datasets.load_remote_dataset import _load_remote_dataset
 from pygmt.helpers import kwargs_to_strings
 
 __doctest_skip__ = ["load_mars_relief"]
 
 
 @kwargs_to_strings(region="sequence")
-def load_mars_relief(resolution="01d", region=None, registration=None):
+def load_mars_relief(
+    resolution="01d",
+    region=None,
+    registration: Literal["gridline", "pixel", None] = None,
+):
     r"""
     Load the Mars relief dataset in various resolutions.
 
@@ -58,8 +64,9 @@ def load_mars_relief(resolution="01d", region=None, registration=None):
         (i.e., ``"05m"``).
     registration : str
         Grid registration type. Either ``"pixel"`` for pixel registration or
-        ``"gridline"`` for gridline registration. Default is ``"gridline"`` for
-        all resolutions except for ``"12s"`` which is ``"pixel"`` only.
+        ``"gridline"`` for gridline registration. Default is ``None``, means
+        ``"gridline"`` for all resolutions except for ``"12s"`` which is
+        ``"pixel"`` only.
 
     Returns
     -------