Check that inner_pen is set when inner_gap is set

Author: seisman

pygmt/params/box.py

@@ -6,7 +6,7 @@
 from collections.abc import Sequence
 
 from pygmt.alias import Alias
-from pygmt.exceptions import GMTValueError
+from pygmt.exceptions import GMTInvalidInput, GMTValueError
 from pygmt.params.base import BaseParam
 
 
@@ -98,6 +98,9 @@ def _innerborder(self) -> list[str | float] | None:
         """
         Inner border of the box, formatted as a list of 1-2 values, or None.
         """
+        if self.inner_gap is not None and self.inner_pen is None:
+            msg = "Parameter 'inner_pen' is required when 'inner_gap' is set."
+            raise GMTInvalidInput(msg)
         return [v for v in (self.inner_gap, self.inner_pen) if v is not None] or None
 
     @property