Improve cf.Field.__getitem__ performance by not re-calculating axis cyclicity

Changelog.rst

@@ -11,6 +11,8 @@ version NEXT
   (https://github.com/NCAS-CMS/cf-python/issues/715)
 * Improve `cf.Field.collapse` performance by lazily computing reduced
   axis coordinates (https://github.com/NCAS-CMS/cf-python/issues/741)
+* Improve `cf.Field.__getitem__` performance by not re-calculating
+  axis cyclicity (https://github.com/NCAS-CMS/cf-python/issues/744)
 * Fix misleading error message when it is not possible to create area
   weights requested from `cf.Field.collapse`
   (https://github.com/NCAS-CMS/cf-python/issues/731)

cf/field.py

@@ -441,6 +441,22 @@ def __getitem__(self, indices):
         for axis, size in zip(data_axes, new_data.shape):
             domain_axes[axis].set_size(size)
 
+        # Record which axes were cyclic before the subspace
+        org_cyclic = [data_axes.index(axis) for axis in new.cyclic()]
+
+        # Set the subspaced data
+        new.set_data(new_data, axes=data_axes, copy=False)
+
+        # Update axis cylcicity. Note that this can only entail
+        # setting an originally cyclic axis to be non-cyclic. Doing
+        # this now enables us to disable the (possibly very slow)
+        # automatic check for cyclicity on the 'set_construct' calls
+        # below.
+        if org_cyclic:
+            new_cyclic = new_data.cyclic()
+            [new.cyclic(i, iscyclic=False) for i in org_cyclic if i not in new_cyclic]
+
+
         # ------------------------------------------------------------
         # Subspace constructs with data
         # ------------------------------------------------------------
@@ -507,6 +523,7 @@ def __getitem__(self, indices):
                         key=key,
                         axes=construct_axes,
                         copy=False,
+                        autocyclic={"no-op": True},
                     )
 
         new.set_data(new_data, axes=data_axes, copy=False)

cf/mixin/fielddomain.py

@@ -1109,10 +1109,15 @@ def autocyclic(self, key=None, coord=None, verbose=None, config={}):
 
         :Returns:
 
-           `bool`
+           `bool` or `None`
+               `True` if the dimension is cyclic, `False` if it isn't,
+               or `None` if no checks were done.
 
         """
         noop = config.get("no-op")
+        if noop:
+            # Don't do anything
+            return
 
         if "cyclic" in config:
             if not config["cyclic"]:

cf/test/test_Field.py

@@ -663,6 +663,14 @@ def test_Field__getitem__(self):
         self.assertTrue(np.allclose(f[:, -3:].array, g[:, :3].array))
         self.assertTrue(f[:, :4].equals(g[:, 3:]))
 
+        # Test setting of axis cyclicity
+        f.cyclic("grid_longitude", iscyclic=True)
+        self.assertEqual(f.data.cyclic(), {1})
+        g = f[0, :]
+        self.assertEqual(g.data.cyclic(), {1})
+        g = f[:, 0]
+        self.assertEqual(g.data.cyclic(), set())
+
     def test_Field__setitem__(self):
         f = self.f.copy().squeeze()