Provide option to Field.dump to not display the first and last data values

Changelog.rst

@@ -1,8 +1,10 @@
 Version NEXTVERSION
---------------
+----------------
 
 **2025-??-??**
 
+* New keyword parameter to `cfdm.Field.dump`: ``data``
+  (https://github.com/NCAS-CMS/cfdm/issues/345)
 * New dependency: ``distributed>=2025.5.1``
 
 ----

cfdm/field.py

@@ -1787,7 +1787,7 @@ def creation_commands(
         return out
 
     @_display_or_return
-    def dump(self, display=True, _level=0, _title=None):
+    def dump(self, data=True, display=True, _level=0, _title=None):
         """A full description of the field construct.
 
         Returns a description of all properties, including those of
@@ -1798,6 +1798,22 @@ def dump(self, display=True, _level=0, _title=None):
 
         :Parameters:
 
+            data: `bool`, optional
+                If True (the default) then display the first and last
+                Field data values. This can take a long time if the
+                data needs an expensive computation (possibly
+                including a slow read from local or remote disk), in
+                which case setting *data* to False will not display
+                these values, thereby avoiding the computational
+                cost. This only applies to the Field's data - the
+                first and last values of data arrays stored in
+                metadata constructs are always displayed.
+
+                Note that when the first and last values are
+                displayed, they are cached for fast future retrieval.
+
+                .. versionadded:: NEXTVERSION
+
             display: `bool`, optional
                 If False then return the description as a string. By
                 default the description is printed.
@@ -1839,12 +1855,27 @@ def dump(self, display=True, _level=0, _title=None):
             string.append(self._dump_properties(_level=_level))
 
         # Data
-        data = self.get_data(None)
-        if data is not None:
+        d = self.get_data(None)
+        if d is not None:
             x = [axis_to_name[axis] for axis in self.get_data_axes(default=())]
+            x = f"{indent0}Data({', '.join(x)})"
+            if data:
+                # Show selected data values
+                x += f" = {d}"
+            else:
+                # Don't show any data values
+                units = d.Units
+                if units.isreftime:
+                    calendar = getattr(units, "calendar", None)
+                    if calendar is not None:
+                        x += f" {calendar}"
+                else:
+                    units = getattr(units, "units", None)
+                    if units is not None:
+                        x += f" {units}"
 
             string.append("")
-            string.append(f"{indent0}Data({', '.join(x)}) = {data}")
+            string.append(x)
             string.append("")
 
         # Quantization

cfdm/test/test_Field.py

@@ -58,6 +58,7 @@ def test_Field__repr__str__dump_construct_type(self):
         repr(f)
         str(f)
         self.assertIsInstance(f.dump(display=False), str)
+        self.assertIsInstance(f.dump(data=False, display=False), str)
         self.assertEqual(f.construct_type, "field")
 
         # Test when any construct which can have data in fact has no data.