imports: move `conditional` to `directives.py`

`conditional()`, which defines conditional variant values, and the other ways to declare
variant values should probably be in a layer above `spack.variant`. This does the simple
thing and moves *just* `conditional()` to `spack.directives` to avoid a circular import.

We can revisit the public variant interface later, when we split packages from core.

Co-authored-by: Harmen Stoppels <me@harmenstoppels.nl>
Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

5 files changed, 17 insertions, 19 deletions

diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
index 8f9e43bf8b..7a3657e222 100644
--- a/lib/spack/spack/directives.py
+++ b/lib/spack/spack/directives.py
@@ -64,6 +64,7 @@ __all__ = [
     "DirectiveMeta",
     "DisableRedistribute",
     "version",
+    "conditional",
     "conflicts",
     "depends_on",
     "extends",
@@ -577,6 +578,15 @@ def patch(
     return _execute_patch
 
 
+def conditional(*values: List[Any], when: Optional[WhenType] = None):
+    """Conditional values that can be used in variant declarations."""
+    # _make_when_spec returns None when the condition is statically false.
+    when = _make_when_spec(when)
+    return spack.variant.ConditionalVariantValues(
+        spack.variant.Value(x, when=when) for x in values
+    )
+
+
 @directive("variants")
 def variant(
     name: str,
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index bf8538032a..f8028d9eca 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -103,12 +103,7 @@ from spack.package_completions import *
 from spack.spec import InvalidSpecDetected, Spec
 from spack.util.executable import *
 from spack.util.filesystem import file_command, fix_darwin_install_name, mime_type
-from spack.variant import (
-    any_combination_of,
-    auto_or_any_combination_of,
-    conditional,
-    disjoint_sets,
-)
+from spack.variant import any_combination_of, auto_or_any_combination_of, disjoint_sets
 from spack.version import Version, ver
 
 # These are just here for editor support; they will be replaced when the build env
diff --git a/lib/spack/spack/variant.py b/lib/spack/spack/variant.py
index e5a5ddfa3c..e5dcf72f36 100644
--- a/lib/spack/spack/variant.py
+++ b/lib/spack/spack/variant.py
@@ -266,7 +266,7 @@ def _flatten(values) -> Collection:
 
     flattened: List = []
     for item in values:
-        if isinstance(item, _ConditionalVariantValues):
+        if isinstance(item, ConditionalVariantValues):
             flattened.extend(item)
         else:
             flattened.append(item)
@@ -884,17 +884,10 @@ def prevalidate_variant_value(
     )
 
 
-class _ConditionalVariantValues(lang.TypedMutableSequence):
+class ConditionalVariantValues(lang.TypedMutableSequence):
     """A list, just with a different type"""
 
 
-def conditional(*values: List[Any], when: Optional["spack.directives.WhenType"] = None):
-    """Conditional values that can be used in variant declarations."""
-    # _make_when_spec returns None when the condition is statically false.
-    when = spack.directives._make_when_spec(when)
-    return _ConditionalVariantValues([Value(x, when=when) for x in values])
-
-
 class DuplicateVariantError(error.SpecError):
     """Raised when the same variant occurs in a spec twice."""
 
diff --git a/var/spack/repos/builtin/packages/geant4/package.py b/var/spack/repos/builtin/packages/geant4/package.py
index 8dd3f18b5d..38ebb060fb 100644
--- a/var/spack/repos/builtin/packages/geant4/package.py
+++ b/var/spack/repos/builtin/packages/geant4/package.py
@@ -4,7 +4,7 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 from spack.package import *
-from spack.variant import _ConditionalVariantValues
+from spack.variant import ConditionalVariantValues
 
 
 class Geant4(CMakePackage):
@@ -180,7 +180,7 @@ class Geant4(CMakePackage):
 
     def std_when(values):
         for v in values:
-            if isinstance(v, _ConditionalVariantValues):
+            if isinstance(v, ConditionalVariantValues):
                 for c in v:
                     yield (c.value, c.when)
             else:
diff --git a/var/spack/repos/builtin/packages/vecgeom/package.py b/var/spack/repos/builtin/packages/vecgeom/package.py
index d48585ad13..ff0fdd145d 100644
--- a/var/spack/repos/builtin/packages/vecgeom/package.py
+++ b/var/spack/repos/builtin/packages/vecgeom/package.py
@@ -5,7 +5,7 @@
 
 
 from spack.package import *
-from spack.variant import _ConditionalVariantValues
+from spack.variant import ConditionalVariantValues
 
 
 class Vecgeom(CMakePackage, CudaPackage):
@@ -196,7 +196,7 @@ class Vecgeom(CMakePackage, CudaPackage):
 
     def std_when(values):
         for v in values:
-            if isinstance(v, _ConditionalVariantValues):
+            if isinstance(v, ConditionalVariantValues):
                 for c in v:
                     yield (c.value, c.when)
             else: