Rewrite offset_copy for better error message

Author: oscargus

lib/matplotlib/tests/test_transforms.py

@@ -744,3 +744,14 @@ def test_scale_swapping(fig_test, fig_ref):
         ax.plot(x, np.exp(-(x**2) / 2) / np.sqrt(2 * np.pi))
         fig.canvas.draw()
         ax.set_yscale('linear')
+
+
+def test_offset_copy_errors():
+    with pytest.raises(ValueError,
+                       match="'fontsize' is not a valid value for units;"
+                             " supported values are 'dots', 'points', 'inches'"):
+        mtransforms.offset_copy(None, units='fontsize')
+
+    with pytest.raises(ValueError,
+                       match='For units of inches or points a fig kwarg is needed'):
+        mtransforms.offset_copy(None, units='inches')

lib/matplotlib/transforms.py

@@ -2952,15 +2952,13 @@ def offset_copy(trans, fig=None, x=0.0, y=0.0, units='inches'):
     `Transform` subclass
         Transform with applied offset.
     """
+    _api.check_in_list(['dots', 'points', 'inches'], units=units)
     if units == 'dots':
         return trans + Affine2D().translate(x, y)
     if fig is None:
         raise ValueError('For units of inches or points a fig kwarg is needed')
     if units == 'points':
         x /= 72.0
         y /= 72.0
-    elif units == 'inches':
-        pass
-    else:
-        _api.check_in_list(['dots', 'points', 'inches'], units=units)
+    # Default units are 'inches'
     return trans + ScaledTranslation(x, y, fig.dpi_scale_trans)