Use set instead of list

Author: chrysle

src/pip/_internal/commands/show.py

@@ -100,11 +100,11 @@ def _get_requiring_packages(current_dist: BaseDistribution) -> Iterator[str]:
         except KeyError:
             continue
 
-        requires = []
+        requires = {}
         # Avoid duplicates in requirements due to environment markers
         for req in dist.iter_dependencies():
             if req.name not in requires:
-                requires.append(req.name)
+                requires.add(req.name)
         requires = sorted(requires, key=str.lower)
         required_by = sorted(_get_requiring_packages(dist), key=str.lower)
 

tests/functional/test_show.py

@@ -364,6 +364,7 @@ def test_show_deduplicate_requirements(script: PipTestEnvironment) -> None:
         name="simple",
         version="1.0",
         install_requires=[
+            "pip >= 19.0.1",
             'pip >= 19.3.1; python_version < "3.8"',
             'pip >= 23.0.1; python_version < "3.9"',
         ],