Validate anchor code

Author: seisman

pygmt/params/position.py

@@ -181,6 +181,13 @@ def _validate(self):
                         description="reference point",
                         reason="Expect a valid 2-character justification code.",
                     )
+        # Validate the anchor if specified.
+        if self.anchor is not None and self.anchor not in _valid_anchors:
+            raise GMTValueError(
+                self.anchor,
+                description="anchor point",
+                reason="Expect a valid 2-character justification code.",
+            )
 
     @property
     def _aliases(self):

pygmt/tests/test_params_position.py

@@ -38,6 +38,8 @@ def test_params_position_invalid_location():
     """
     with pytest.raises(GMTValueError):
         Position("invalid", type="mapcoords")
+    with pytest.raises(GMTValueError):
+        Position((1, 2, 3), type="mapcoords")
     with pytest.raises(GMTValueError):
         Position(5, type="plotcoords")
     with pytest.raises(GMTValueError):
@@ -46,3 +48,11 @@ def test_params_position_invalid_location():
         Position((10, 20), type="inside")
     with pytest.raises(GMTValueError):
         Position("TT", type="outside")
+
+
+def test_params_position_invalid_anchor():
+    """
+    Test that invalid anchor inputs raise GMTValueError.
+    """
+    with pytest.raises(GMTValueError):
+        Position((10, 20), type="mapcoords", anchor="XX")