Calculate candidate string versions only once in `get_applicable_cand…

…idates`

src/pip/_internal/index/package_finder.py

@@ -452,6 +452,7 @@ def get_applicable_candidates(
         # Using None infers from the specifier instead.
         allow_prereleases = self._allow_all_prereleases or None
         specifier = self._specifier
+        candidates_and_versions = [(c, str(c.version)) for c in candidates]
         versions = {
             str(v)
             for v in specifier.filter(
@@ -462,13 +463,13 @@ def get_applicable_candidates(
                 # types. This way we'll use a str as a common data interchange
                 # format. If we stop using the pkg_resources provided specifier
                 # and start using our own, we can drop the cast to str().
-                (str(c.version) for c in candidates),
+                (v for _, v in candidates_and_versions),
                 prereleases=allow_prereleases,
             )
         }
 
         # Again, converting version to str to deal with debundling.
-        applicable_candidates = [c for c in candidates if str(c.version) in versions]
+        applicable_candidates = [c for c, v in candidates_and_versions if v in versions]
 
         filtered_applicable_candidates = filter_unallowed_hashes(
             candidates=applicable_candidates,