From 5967991485014f4ca4d4133860e5a1451cb3e45a Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Thu, 30 May 2024 12:07:13 +0100
Subject: [PATCH] cf.field.cell_methods bug fix

---
 Changelog.rst           | 11 +++++++++++
 cfdm/field.py           | 19 +++++++++++++++++++
 cfdm/test/test_Field.py |  4 ++++
 3 files changed, 34 insertions(+)

diff --git a/Changelog.rst b/Changelog.rst
index 301219bae..7636bb5ae 100644
--- a/Changelog.rst
+++ b/Changelog.rst
@@ -1,3 +1,14 @@
+Version NEXTVERSION
+-------------------
+
+**2024-??-??**
+
+* Fix bug that returned incorrect results when an invalid identifer is
+  provided to `cf.Field.cell_methods`
+  (https://github.com/NCAS-CMS/cfdm/issues/2699)
+
+----
+
 Version 1.11.1.0
 ----------------
 
diff --git a/cfdm/field.py b/cfdm/field.py
index 90ba5a5f3..61b1d1f1d 100644
--- a/cfdm/field.py
+++ b/cfdm/field.py
@@ -662,6 +662,18 @@ def cell_methods(self, *identities, **filter_kwargs):
 
         **Examples**
 
+        >>> f = {{package}}.example_field(1)
+        >>> print(f.cell_methods())
+        Constructs:
+        {'cellmethod0': <CellMethod: domainaxis1: domainaxis2: mean where land (interval: 0.1 degrees)>,
+        'cellmethod1': <CellMethod: domainaxis3: maximum>}
+        >>> print(f.cell_methods('time'))
+        Constructs:
+        {'cellmethod1': <CellMethod: domainaxis3: maximum>}
+        >>> print(f.cell_methods('bad identifier'))
+        Constructs:
+        {}
+
         """
         cached = filter_kwargs.get("cached")
         if cached is not None:
@@ -702,6 +714,13 @@ def cell_methods(self, *identities, **filter_kwargs):
                         keys.add(cm_key)
 
             identities = ()
+            if not keys:
+                # Specify a key of None to ensure that no cell methods
+                # are selected. (If keys is an empty set then all cell
+                # methods are selected, which is not what we want,
+                # here.)
+                keys = (None,)
+
             filter_kwargs = {
                 "filter_by_key": keys,
                 "todict": filter_kwargs.pop("todict", False),
diff --git a/cfdm/test/test_Field.py b/cfdm/test/test_Field.py
index cc6ab7b22..a5a33e905 100644
--- a/cfdm/test/test_Field.py
+++ b/cfdm/test/test_Field.py
@@ -355,6 +355,10 @@ def test_Field_CONSTRUCTS(self):
         for key, value in constructs.items():
             self.assertIsInstance(value, cfdm.CellMethod)
 
+        # Check that no cell methods are returned when a bad
+        # identifier is provided
+        self.assertFalse(f.cell_methods("bad identifier"))
+
         constructs = f.coordinate_references()
         n = 2
         self.assertEqual(len(constructs), n)