Fix regression in spec format string for indiviual variants (#46206)

Fix a regression in {variants.X} and {variants.X.value} spec format strings.

2 files changed, 19 insertions, 2 deletions

diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 0c869febb1..e420eaada9 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -3914,9 +3914,12 @@ class Spec:
                 if part.startswith("_"):
                     raise SpecFormatStringError("Attempted to format private attribute")
                 else:
-                    if part == "variants" and isinstance(current, VariantMap):
+                    if isinstance(current, VariantMap):
                         # subscript instead of getattr for variant names
-                        current = current[part]
+                        try:
+                            current = current[part]
+                        except KeyError:
+                            raise SpecFormatStringError(f"Variant '{part}' does not exist")
                     else:
                         # aliases
                         if part == "arch":
diff --git a/lib/spack/spack/test/spec_semantics.py b/lib/spack/spack/test/spec_semantics.py
index cd0f930acd..0f3b58e486 100644
--- a/lib/spack/spack/test/spec_semantics.py
+++ b/lib/spack/spack/test/spec_semantics.py
@@ -741,6 +741,13 @@ class TestSpecSemantics:
             ("{/hash}", "/", lambda s: "/" + s.dag_hash()),
         ]
 
+        variants_segments = [
+            ("{variants.debug}", spec, "debug"),
+            ("{variants.foo}", spec, "foo"),
+            ("{^pkg-a.variants.bvv}", spec["pkg-a"], "bvv"),
+            ("{^pkg-a.variants.foo}", spec["pkg-a"], "foo"),
+        ]
+
         other_segments = [
             ("{spack_root}", spack.paths.spack_root),
             ("{spack_install}", spack.store.STORE.layout.root),
@@ -768,6 +775,12 @@ class TestSpecSemantics:
             callpath, fmt_str = depify("callpath", named_str, sigil)
             assert spec.format(fmt_str) == getter(callpath)
 
+        for named_str, test_spec, variant_name in variants_segments:
+            assert test_spec.format(named_str) == str(test_spec.variants[variant_name])
+            assert test_spec.format(named_str[:-1] + ".value}") == str(
+                test_spec.variants[variant_name].value
+            )
+
         for named_str, expected in other_segments:
             actual = spec.format(named_str)
             assert expected == actual
@@ -827,6 +840,7 @@ class TestSpecSemantics:
             r"{dag_hash}",
             r"{foo}",
             r"{+variants.debug}",
+            r"{variants.this_variant_does_not_exist}",
         ],
     )
     def test_spec_formatting_bad_formats(self, default_mock_concretization, fmt_str):