Fix bug that caused Data._axes to be incorrect after a call to cf.Field.collapse

Changelog.rst

@@ -13,6 +13,9 @@ version 3.17.0
   ``key`` (https://github.com/NCAS-CMS/cf-python/issues/802)
 * New keyword parameter to `cf.histogram`: ``density``
   (https://github.com/NCAS-CMS/cf-python/issues/794)
+* Fix bug that caused `Data._axes` to be incorrect after a call to
+  `cf.Field.collapse`
+  (https://github.com/NCAS-CMS/cf-python/issues/857)
 * Changed dependency: ``Python>=3.9.0``
 * Changed dependency: ``numpy>=2.0.0``
 * Changed dependency: ``cfdm>=1.12.0.0, <1.12.1.0``

cf/data/utils.py

@@ -402,8 +402,14 @@ def collapse(
             weights, axis)``.
 
     """
+    original_size = d.size
+    if axis is None:
+        axis = range(d.ndim)
+    else:
+        axis = d._parse_axes(axis)
+
     kwargs = {
-        "axis": axis,
+        "axis": tuple(axis),
         "keepdims": keepdims,
         "split_every": split_every,
         "mtol": mtol,
@@ -424,6 +430,14 @@ def collapse(
     dx = func(dx, **kwargs)
     d._set_dask(dx)
 
+    if not keepdims:
+        # Remove collapsed axis names
+        d._axes = [a for i, a in enumerate(d._axes) if i not in axis]
+
+    if d.size != original_size:
+        # Remove the out-dated HDF5 chunking strategy
+        d.nc_clear_hdf5_chunksizes()
+
     return d, weights
 
 

cf/test/test_Data.py

@@ -4657,6 +4657,33 @@ def test_Data_is_masked(self):
         self.assertTrue(d[0].is_masked)
         self.assertFalse(d[1].is_masked)
 
+    def test_Data_collapse_axes_hdf_chunks(self):
+        """Test that _axes and hdf_chunks are updated after a collapse."""
+        d = cf.Data([[1, 2, 3, 4]])
+        chunks = d.shape
+        d.nc_set_hdf5_chunksizes(chunks)
+        e = d.mean(axes=1)
+        self.assertEqual(d._axes, ("dim0", "dim1"))
+        self.assertEqual(d.nc_hdf5_chunksizes(), chunks)
+
+        e = d.mean(axes=1)
+        self.assertNotEqual(e.size, d.size)
+        self.assertEqual(e._axes, d._axes)
+        self.assertEqual(e.nc_hdf5_chunksizes(), None)
+
+        e = d.mean(axes=1, squeeze=True)
+        self.assertEqual(e._axes, d._axes[:1])
+        self.assertEqual(e.nc_hdf5_chunksizes(), None)
+
+        e = d.mean(axes=0)
+        self.assertEqual(e.size, d.size)
+        self.assertEqual(e._axes, d._axes)
+        self.assertEqual(e.nc_hdf5_chunksizes(), chunks)
+
+        e = d.mean(axes=0, squeeze=True)
+        self.assertEqual(e._axes, d._axes[1:])
+        self.assertEqual(e.nc_hdf5_chunksizes(), chunks)
+
 
 if __name__ == "__main__":
     print("Run date:", datetime.datetime.now())