Figure.solar: Add type hints to the 'terminator' parameter and simplify codes

pygmt/src/solar.py

@@ -1,6 +1,10 @@
 """
 solar - Plot day-night terminators and twilight.
 """
+from __future__ import annotations
+
+from typing import Literal
+
 import pandas as pd
 from pygmt.clib import Session
 from pygmt.exceptions import GMTInvalidInput
@@ -22,32 +26,38 @@
     t="transparency",
 )
 @kwargs_to_strings(R="sequence", c="sequence_comma", p="sequence")
-def solar(self, terminator="d", terminator_datetime=None, **kwargs):
+def solar(
+    self,
+    terminator: Literal["astronomical", "civil", "day_night", "nautical"] = "day_night",
+    terminator_datetime=None,
+    **kwargs,
+):
     r"""
     Plot day-light terminators or twilights.
 
-    This function plots the day-night terminator. Alternatively, it can plot
-    the terminators for civil twilight, nautical twilight, or astronomical
-    twilight.
+    This function plots the day-night terminator. Alternatively, it can plot the
+    terminators for civil twilight, nautical twilight, or astronomical twilight.
 
     Full option list at :gmt-docs:`solar.html`
 
     {aliases}
 
     Parameters
     ----------
-    terminator : str
-        Set the type of terminator displayed. Valid arguments are
-        ``"day_night"``, ``"civil"``, ``"nautical"``, and ``"astronomical"``,
-        which can be set with either the full name or the first letter of the
-        name [Default is ``"day_night"``].
+    terminator
+        Set the type of terminator displayed, which can be set with either the full name
+        or the first letter of the name. Available options are:
 
-        Refer to https://en.wikipedia.org/wiki/Twilight for the definitions of
-        different types of twilight.
+        - ``"astronomical"``: Astronomical twilight
+        - ``"civil"``: Civil twilight
+        - ``"day_night"``: Day/night terminator
+        - ``"nautical"``: Nautical twilight
+
+        Refer to https://en.wikipedia.org/wiki/Twilight for the definitions of different
+        types of twilight.
     terminator_datetime : str or datetime object
-        Set the UTC date and time of the displayed terminator
-        [Default is the current UTC date and time]. It can be
-        passed as a string or Python datetime object.
+        Set the UTC date and time of the displayed terminator [Default is the current
+        UTC date and time]. It can be passed as a string or Python datetime object.
     {region}
     {projection}
     {frame}
@@ -86,25 +96,16 @@ def solar(self, terminator="d", terminator_datetime=None, **kwargs):
     >>> # show the plot
     >>> fig.show()
     """
-
     kwargs = self._preprocess(**kwargs)
     if kwargs.get("T") is not None:
+        msg = "Use 'terminator' and 'terminator_datetime' instead of 'T'."
+        raise GMTInvalidInput(msg)
+
+    valid_terminators = ["day_night", "civil", "nautical", "astronomical"]
+    if terminator not in valid_terminators and terminator not in "dcna":
         raise GMTInvalidInput(
-            "Use 'terminator' and 'terminator_datetime' instead of 'T'."
-        )
-    if terminator not in [
-        "day_night",
-        "civil",
-        "nautical",
-        "astronomical",
-        "d",
-        "c",
-        "n",
-        "a",
-    ]:
-        raise GMTInvalidInput(
-            f"Unrecognized solar terminator type '{terminator}'. Valid values "
-            "are 'day_night', 'civil', 'nautical', and 'astronomical'."
+            f"Unrecognized solar terminator type '{terminator}'. "
+            f"Valid values are {valid_terminators}."
         )
     kwargs["T"] = terminator[0]
     if terminator_datetime: