Figure.set_panel: Fix the bug when panel is not set

Author: seisman

pygmt/src/subplot.py

@@ -419,13 +419,11 @@ def set_panel(
     aliasdict = AliasSystem(A=Alias(tag, name="tag")).add_common(V=verbose)
     aliasdict.merge(kwargs)
 
+    args = ["set"]
+    if panel is not None:
+        args.append(Alias(panel, name="panel", sep=",", size=2)._value)
+    args.extend(build_arg_list(aliasdict))
+
     with Session() as lib:
-        lib.call_module(
-            module="subplot",
-            args=[
-                "set",
-                Alias(panel, name="panel", sep=",", size=2)._value,
-                *build_arg_list(aliasdict),
-            ],
-        )
+        lib.call_module(module="subplot", args=args)
         yield

pygmt/tests/test_subplot.py

@@ -132,6 +132,25 @@ def test_subplot_outside_plotting_positioning():
     return fig
 
 
+@pytest.mark.mpl_image_compare(filename="test_subplot_basic_frame.png")
+def test_subplot_set_panel_without_panel():
+    """
+    Ensure Figure.set_panel works when panel is omitted.
+
+    Omitting ``panel`` should let GMT advance to the next panel according to the subplot
+    order instead of passing an invalid ``None`` argument to the C API.
+
+    Use the baseline image from test_subplot_basic_frame.
+    """
+    fig = Figure()
+    with fig.subplot(nrows=1, ncols=2, figsize=("6c", "3c"), frame=Frame(axes="WSne")):
+        with fig.set_panel():
+            fig.basemap(region=[0, 3, 0, 3], frame=Frame(title="plot0"))
+        with fig.set_panel():
+            fig.basemap(region=[0, 3, 0, 3], frame=Frame(title="plot1"))
+    return fig
+
+
 def test_subplot_deprecated_autolabel():
     """
     Test that using the deprecated autolabel parameter raises a warning when conflicted