fix and add test

gwcs/tests/test_wcs.py

@@ -500,7 +500,17 @@ def test_to_fits_sip():
     fitsvalx, fitsvaly = fitssip.all_pix2world(xflat+1, yflat+1, 1)
     gwcsvalx, gwcsvaly = miriwcs(xflat, yflat)
     assert_allclose(gwcsvalx, fitsvalx, atol=1e-10, rtol=0)
-    assert_allclose(gwcsvaly, fitsvaly, atol=1e-10, rtol=0)    
+    assert_allclose(gwcsvaly, fitsvaly, atol=1e-10, rtol=0)
     fits_inverse_valx, fits_inverse_valy = fitssip.all_world2pix(fitsvalx, fitsvaly, 1)
     assert_allclose(xflat, fits_inverse_valx - 1, atol=0.1, rtol=0)
     assert_allclose(yflat, fits_inverse_valy - 1, atol=0.1, rtol=0)
+
+    mirisip = miriwcs.to_fits_sip(bounding_box=None, max_inv_pix_error=0.1)
+    fitssip = astwcs.WCS(mirisip)
+    fitsvalx, fitsvaly = fitssip.all_pix2world(xflat+1, yflat+1, 1)
+    assert_allclose(gwcsvalx, fitsvalx, atol=1e-10, rtol=0)
+    assert_allclose(gwcsvaly, fitsvaly, atol=1e-10, rtol=0)
+
+    with pytest.raises(ValueError):
+        miriwcs.bounding_box = None
+        mirisip = miriwcs.to_fits_sip(bounding_box=None, max_inv_pix_error=0.1)

gwcs/wcs.py

@@ -703,13 +703,12 @@ def to_fits_sip(self, bounding_box=None, max_pix_error=0.25, degree=None,
 
         transform = self.forward_transform
         # Determine reference points.
-        if bounding_box is not None:
-            (xmin, xmax), (ymin, ymax) = bounding_box
-        else:
-            try:
-                (xmin, xmax), (ymin, ymax) = self.bounding_box
-            except KeyError:
-                raise TypeError("A bounding_box is needed to proceed.")
+        if bounding_box is None and self.bounding_box is None:
+            raise ValueError("A bounding_box is needed to proceed.")
+        if bounding_box is None:
+            bounding_box = self.bounding_box
+
+        (xmin, xmax), (ymin, ymax) = bounding_box
         crpix1 = (xmax - xmin) // 2
         crpix2 = (ymax - ymin) // 2
         crval1, crval2 = transform(crpix1, crpix2)