Add support for saving string stash codes to PP (#6289)

docs/src/whatsnew/latest.rst

@@ -45,7 +45,8 @@ This document explains the changes made to Iris for this release
 🐛 Bugs Fixed
 =============
 
-#. N/A
+#. `@rcomer`_ added handling for string stash codes when saving pp files.
+   (:issue:`6239`, :pull:`6289`)
 
 
 💣 Incompatible Changes

lib/iris/fileformats/pp_save_rules.py

@@ -13,6 +13,7 @@
 from iris.aux_factory import HybridHeightFactory, HybridPressureFactory
 from iris.fileformats._ff_cross_references import STASH_TRANS
 from iris.fileformats._pp_lbproc_pairs import LBPROC_MAP
+import iris.fileformats.pp
 from iris.fileformats.rules import (
     aux_factory,
     has_aux_factory,
@@ -96,6 +97,8 @@ def _stash_rules(cube, pp):
     """
     if "STASH" in cube.attributes:
         stash = cube.attributes["STASH"]
+        if isinstance(stash, str):
+            stash = iris.fileformats.pp.STASH.from_msi(stash)
         if isinstance(stash, iris.fileformats.pp.STASH):
             pp.lbuser[3] = 1000 * (stash.section or 0) + (stash.item or 0)
             pp.lbuser[6] = stash.model or 0

lib/iris/tests/unit/fileformats/pp/test_save.py

@@ -55,6 +55,27 @@ def test_realization():
     assert member_number == 42
 
 
+def test_stash_string():
+    cube = stock.lat_lon_cube()
+    cube.attributes["STASH"] = "m01s34i001"
+    with mock.patch("iris.fileformats.pp.PPField3", autospec=True) as pp_field:
+        pp_field.lbuser = list(range(7))
+        verify(cube, pp_field)
+        stash_num = pp_field.lbuser[3]
+
+    assert stash_num == 34001
+
+
+def test_bad_stash_string():
+    cube = stock.lat_lon_cube()
+    cube.attributes["STASH"] = "ooovarvoo"
+    with mock.patch("iris.fileformats.pp.PPField3", autospec=True) as pp_field:
+        with pytest.raises(
+            ValueError, match='Expected STASH code MSI string "mXXsXXiXXX"'
+        ):
+            verify(cube, pp_field)
+
+
 def _pp_save_ppfield_values(cube):
     """Emulate saving a cube as PP, and capture the resulting PP field values."""
     # Create a test object to stand in for a real PPField.