From aa01b953d6568c6ae0bd67a7ee6570df14085c05 Mon Sep 17 00:00:00 2001
From: David Feltell <david.feltell@foundry.com>
Date: Mon, 19 Aug 2024 14:06:21 +0100
Subject: [PATCH] [Core] Use frozenset for Python kTraitSet

Closes #55. Define the Python Specifications trait set using a
`frozenset` so it can be hashed. Simplifies switching on trait set in
manager implementations in Python, where a handler map is a nice way to
structure code. I.e. a `frozenset` can be used as a dictionary key.

Signed-off-by: David Feltell <david.feltell@foundry.com>
---
 RELEASE_NOTES.md                                 |  7 ++++++-
 .../templates/python/specifications.py.in        |  4 ++--
 tests/generators/test_python.py                  | 16 ++++++++++++++++
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 9415b0e..5d3d9e9 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -11,7 +11,12 @@ v1.0.0-alpha.x
   tested or published, whereas Python 3.11 is now published.
   [OpenAssetIO#1351](https://github.com/OpenAssetIO/OpenAssetIO/issues/1351)
 
-v1.0.0-alpha.9
+- Changed the `kTraitSet` member of Specification classes in Python to
+  use the `frozenset` type, rather than standard `set`. This allows
+  the `kTraitSet` to be used as a dictionary key.
+  [#55](https://github.com/OpenAssetIO/OpenAssetIO-TraitGen/issues/55)`
+
+`v1.0.0-alpha.9
 --------------
 
 ### Breaking Changes
diff --git a/python/openassetio_traitgen/templates/python/specifications.py.in b/python/openassetio_traitgen/templates/python/specifications.py.in
index 5fac579..b3defba 100644
--- a/python/openassetio_traitgen/templates/python/specifications.py.in
+++ b/python/openassetio_traitgen/templates/python/specifications.py.in
@@ -31,7 +31,7 @@ class {{ specification.id | to_py_class_name }}Specification:
     Usage: {{ specification.usage | join(', ') }}
     {% endif -%}
     """
-    kTraitSet = {
+    kTraitSet = frozenset({
         {% for trait in specification.trait_set -%}
         # '{{ trait.id }}'
         {% if trait.package == package.id -%}
@@ -40,7 +40,7 @@ class {{ specification.id | to_py_class_name }}Specification:
         {{ trait.package | to_py_module_name }}.traits.{{ trait.namespace | to_py_module_name }}.{{ trait.name | to_py_class_name }}Trait.kId,
         {% endif -%}
         {% endfor %}
-    }
+    })
 
     def __init__(self, traitsData):
         """
diff --git a/tests/generators/test_python.py b/tests/generators/test_python.py
index bec9c7a..58f29cd 100644
--- a/tests/generators/test_python.py
+++ b/tests/generators/test_python.py
@@ -216,6 +216,12 @@ def test_trait_set_composes_target_trait_kIds(self, module_all):
         }
         assert module_all.specifications.test.TwoLocalTraitsSpecification.kTraitSet == expected
 
+        # Ensure we can use the trait set as a dict key, i.e. it is a
+        # `frozenset`.
+        assert {module_all.specifications.test.TwoLocalTraitsSpecification.kTraitSet: True}[
+            module_all.specifications.test.TwoLocalTraitsSpecification.kTraitSet
+        ] is True
+
     def test_has_trait_getters_with_expected_docstring(self, module_all):
         trait_one = module_all.traits.aNamespace.NoPropertiesTrait
         trait_two = module_all.traits.anotherNamespace.NoPropertiesTrait
@@ -260,6 +266,11 @@ def test_trait_set_composes_target_trait_kIds(self, module_all, module_traits_on
             module_traits_only.traits.test.AnotherTrait.kId,
         }
         assert module_all.specifications.test.OneExternalTraitSpecification.kTraitSet == expected
+        # Ensure we can use the trait set as a dict key, i.e. it is a
+        # `frozenset`.
+        assert {module_all.specifications.test.OneExternalTraitSpecification.kTraitSet: True}[
+            module_all.specifications.test.OneExternalTraitSpecification.kTraitSet
+        ] is True
 
     def test_has_trait_getters_with_expected_docstring(self, module_all, module_traits_only):
         trait = module_traits_only.traits.test.AnotherTrait
@@ -294,6 +305,11 @@ def test_trait_set_composes_target_trait_kIds(self, module_all, module_traits_on
         assert (
             module_all.specifications.test.LocalAndExternalTraitSpecification.kTraitSet == expected
         )
+        # Ensure we can use the trait set as a dict key, i.e. it is a
+        # `frozenset`.
+        assert {module_all.specifications.test.LocalAndExternalTraitSpecification.kTraitSet: True}[
+            module_all.specifications.test.LocalAndExternalTraitSpecification.kTraitSet
+        ] is True
 
     def test_has_trait_getters_with_expected_docstring(self, module_all, module_traits_only):
         trait_one = module_all.traits.aNamespace.NoPropertiesTrait