Support arbitrary equality on arbitrary strings for Specifier and SpecifierSet's filter and contains method.

src/packaging/specifiers.py

@@ -268,34 +268,53 @@ def __init__(self, spec: str = "", prereleases: bool | None = None) -> None:
         # Specifier version cache
         self._spec_version: tuple[str, Version] | None = None
 
-    def _get_spec_version(self, version: str) -> Version:
+    def _get_spec_version(self, version: str) -> Version | None:
         """One element cache, as only one spec Version is needed per Specifier."""
         if self._spec_version is not None and self._spec_version[0] == version:
             return self._spec_version[1]
 
-        version_specifier = Version(version)
+        version_specifier = _coerce_version(version)
+        if version_specifier is None:
+            return None
+
         self._spec_version = (version, version_specifier)
         return version_specifier
 
+    def _require_spec_version(self, version: str) -> Version:
+        """Get spec version, asserting it's valid (not for === operator).
+
+        This method should only be called for operators where version
+        strings are guaranteed to be valid PEP 440 versions (not ===).
+        """
+        spec_version = self._get_spec_version(version)
+        assert spec_version is not None
+        return spec_version
+
     @property
-    def prereleases(self) -> bool:
+    def prereleases(self) -> bool | None:
         # If there is an explicit prereleases set for this, then we'll just
         # blindly use that.
         if self._prereleases is not None:
             return self._prereleases
 
         # Only the "!=" operator does not imply prereleases when
         # the version in the specifier is a prerelease.
-        operator, version = self._spec
+        operator, item = self._spec
         if operator != "!=":
             # The == specifier with trailing .* cannot include prereleases
             # e.g. "==1.0a1.*" is not valid.
-            if operator == "==" and version.endswith(".*"):
+            if operator == "==" and item.endswith(".*"):
                 return False
 
+            # "===" can have arbitrary string versions, so we cannot
+            # parse those, we take prereleases as False for those.
+            version = self._get_spec_version(item)
+            if version is None:
+                return None
+
             # For all other operators, use the check if spec Version
             # object implies pre-releases.
-            if self._get_spec_version(version).is_prerelease:
+            if version.is_prerelease:
                 return True
 
         return False
@@ -356,9 +375,10 @@ def _canonical_spec(self) -> tuple[str, str]:
         if operator == "===" or version.endswith(".*"):
             return operator, version
 
+        spec_version = self._require_spec_version(version)
+
         canonical_version = canonicalize_version(
-            self._get_spec_version(version),
-            strip_trailing_zero=(operator != "~="),
+            spec_version, strip_trailing_zero=(operator != "~=")
         )
 
         return operator, canonical_version
@@ -451,7 +471,7 @@ def _compare_equal(self, prospective: Version, spec: str) -> bool:
             return shortened_prospective == split_spec
         else:
             # Convert our spec string into a Version
-            spec_version = self._get_spec_version(spec)
+            spec_version = self._require_spec_version(spec)
 
             # If the specifier does not have a local segment, then we want to
             # act as if the prospective version also does not have a local
@@ -468,18 +488,18 @@ def _compare_less_than_equal(self, prospective: Version, spec: str) -> bool:
         # NB: Local version identifiers are NOT permitted in the version
         # specifier, so local version labels can be universally removed from
         # the prospective version.
-        return _public_version(prospective) <= self._get_spec_version(spec)
+        return _public_version(prospective) <= self._require_spec_version(spec)
 
     def _compare_greater_than_equal(self, prospective: Version, spec: str) -> bool:
         # NB: Local version identifiers are NOT permitted in the version
         # specifier, so local version labels can be universally removed from
         # the prospective version.
-        return _public_version(prospective) >= self._get_spec_version(spec)
+        return _public_version(prospective) >= self._require_spec_version(spec)
 
     def _compare_less_than(self, prospective: Version, spec_str: str) -> bool:
         # Convert our spec to a Version instance, since we'll want to work with
         # it as a version.
-        spec = self._get_spec_version(spec_str)
+        spec = self._require_spec_version(spec_str)
 
         # Check to see if the prospective version is less than the spec
         # version. If it's not we can short circuit and just return False now
@@ -506,7 +526,7 @@ def _compare_less_than(self, prospective: Version, spec_str: str) -> bool:
     def _compare_greater_than(self, prospective: Version, spec_str: str) -> bool:
         # Convert our spec to a Version instance, since we'll want to work with
         # it as a version.
-        spec = self._get_spec_version(spec_str)
+        spec = self._require_spec_version(spec_str)
 
         # Check to see if the prospective version is greater than the spec
         # version. If it's not we can short circuit and just return False now
@@ -537,7 +557,7 @@ def _compare_greater_than(self, prospective: Version, spec_str: str) -> bool:
         # same version in the spec.
         return True
 
-    def _compare_arbitrary(self, prospective: Version, spec: str) -> bool:
+    def _compare_arbitrary(self, prospective: Version | str, spec: str) -> bool:
         return str(prospective).lower() == str(spec).lower()
 
     def __contains__(self, item: str | Version) -> bool:
@@ -627,9 +647,12 @@ def filter(
         for version in iterable:
             parsed_version = _coerce_version(version)
             if parsed_version is None:
-                continue
-
-            if operator_callable(parsed_version, self.version):
+                # === operator can match arbitrary (non-version) strings
+                if self.operator == "===" and self._compare_arbitrary(
+                    version, self.version
+                ):
+                    yield version
+            elif operator_callable(parsed_version, self.version):
                 # If it's not a prerelease or prereleases are allowed, yield it directly
                 if not parsed_version.is_prerelease or include_prereleases:
                     found_non_prereleases = True
@@ -944,13 +967,12 @@ def contains(
         True
         """
         version = _coerce_version(item)
-        if version is None:
-            return False
 
-        if installed and version.is_prerelease:
+        if version is not None and installed and version.is_prerelease:
             prereleases = True
 
-        return bool(list(self.filter([version], prereleases=prereleases)))
+        check_item = item if version is None else version
+        return bool(list(self.filter([check_item], prereleases=prereleases)))
 
     def filter(
         self, iterable: Iterable[UnparsedVersionVar], prereleases: bool | None = None
@@ -1019,22 +1041,28 @@ def filter(
                 return (
                     item
                     for item in iterable
-                    if (version := _coerce_version(item)) is not None
-                    and not version.is_prerelease
+                    if (version := _coerce_version(item)) is None
+                    or not version.is_prerelease
                 )
 
         # Finally if prereleases is None, apply PEP 440 logic:
         # exclude prereleases unless there are no final releases that matched.
-        filtered: list[UnparsedVersionVar] = []
+        filtered_items: list[UnparsedVersionVar] = []
         found_prereleases: list[UnparsedVersionVar] = []
+        found_final_release = False
 
         for item in iterable:
             parsed_version = _coerce_version(item)
+            # Arbitrary strings are always included as it is not
+            # possible to determine if they are prereleases,
+            # and they have already passed all specifiers.
             if parsed_version is None:
-                continue
-            if parsed_version.is_prerelease:
+                filtered_items.append(item)
+                found_prereleases.append(item)
+            elif parsed_version.is_prerelease:
                 found_prereleases.append(item)
             else:
-                filtered.append(item)
+                filtered_items.append(item)
+                found_final_release = True
 
-        return iter(filtered if filtered else found_prereleases)
+        return iter(filtered_items if found_final_release else found_prereleases)