Allow printing of a length-0 coordinate

Coord.summary() formats string and date-like values by first finding the
longest string with max(...), which raised ValueError on an empty array.
A length-0 coordinate is a valid object (e.g. a time coordinate for an
unfilled unlimited dimension), so its summary should not crash. Pass
default=0 to max() so an empty array formats as '[]'.

Fixes #6531.

Author: gaoflow

docs/src/whatsnew/latest.rst

@@ -56,6 +56,10 @@ This document explains the changes made to Iris for this release
    always promoting the result to ``float64``. Integer inputs are still returned
    as ``float64``. (:issue:`4119`)
 
+#. :user:`gaoflow` fixed printing of a length-0 coordinate (e.g. a time
+   coordinate for an as-yet-unfilled unlimited dimension), which previously
+   raised a ``ValueError`` instead of producing a summary. (:issue:`6531`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/coords.py

@@ -406,8 +406,9 @@ def array_summary(data, n_max, n_edge, linewidth, precision):
 
             if data.dtype.kind == "U":
                 # Strings : N.B. includes all missing data
-                # find the longest.
-                length = max(len(str(x)) for x in data.flatten())
+                # find the longest (an empty array, e.g. a length-0 time
+                # coordinate, has no elements, so fall back to 0). See #6531.
+                length = max((len(str(x)) for x in data.flatten()), default=0)
                 # Pre-apply a common formatting width.
                 formatter = {"all": lambda x: str(x).ljust(length)}
 

lib/iris/tests/unit/coords/test_Coord.py

@@ -1482,6 +1482,37 @@ def test_long_time_interval__bounded(self):
         result = coord.__str__()
         assert expected == result
 
+    def test_empty_time_coord(self):
+        # A length-0 time coordinate (an as-yet-unfilled unlimited dimension)
+        # should print rather than raising (#6531).
+        coord = DimCoord([], standard_name="time", units="days since 1970-01-01")
+        expected = "\n".join(
+            [
+                "DimCoord :  time / (days since 1970-01-01, standard calendar)",
+                "    points: []",
+                "    shape: (0,)",
+                "    dtype: float64",
+                "    standard_name: 'time'",
+            ]
+        )
+        result = coord.__str__()
+        assert expected == result
+
+    def test_empty_string_coord(self):
+        # A length-0 string coordinate also exercises the empty-array path.
+        coord = AuxCoord(np.array([], dtype="<U1"), long_name="label")
+        expected = "\n".join(
+            [
+                "AuxCoord :  label / (unknown)",
+                "    points: []",
+                "    shape: (0,)",
+                "    dtype: <U1",
+                "    long_name: 'label'",
+            ]
+        )
+        result = coord.__str__()
+        assert expected == result
+
     def test_non_time_unit(self):
         coord = DimCoord([1.0])
         expected = "\n".join(