Allow creating a SpecifierSet from a list of specifiers (#777)

* Allow creating a SpecifierSet from a list of specifiers

* Change from Union to | in type annotation

CHANGELOG.rst

@@ -24,6 +24,7 @@ No unreleased changes.
   have not been provided (:issue:`733`)
 * Fix a bug preventing the use of the built in ``ExceptionGroup`` on versions of
   Python that support it (:issue:`725`)
+* Support creating a ``SpecifierSet`` from an iterable of ``Specifier`` objects (:issue:`775`)
 
 23.2 - 2023-10-01
 ~~~~~~~~~~~~~~~~~

src/packaging/specifiers.py

@@ -694,12 +694,18 @@ class SpecifierSet(BaseSpecifier):
     specifiers (``>=3.0,!=3.1``), or no specifier at all.
     """
 
-    def __init__(self, specifiers: str = "", prereleases: bool | None = None) -> None:
+    def __init__(
+        self,
+        specifiers: str | Iterable[Specifier] = "",
+        prereleases: bool | None = None,
+    ) -> None:
         """Initialize a SpecifierSet instance.
 
         :param specifiers:
             The string representation of a specifier or a comma-separated list of
             specifiers which will be parsed and normalized before use.
+            May also be an iterable of ``Specifier`` instances, which will be used
+            as is.
         :param prereleases:
             This tells the SpecifierSet if it should accept prerelease versions if
             applicable or not. The default of ``None`` will autodetect it from the
@@ -710,12 +716,17 @@ def __init__(self, specifiers: str = "", prereleases: bool | None = None) -> Non
             raised.
         """
 
-        # Split on `,` to break each individual specifier into it's own item, and
-        # strip each item to remove leading/trailing whitespace.
-        split_specifiers = [s.strip() for s in specifiers.split(",") if s.strip()]
+        if isinstance(specifiers, str):
+            # Split on `,` to break each individual specifier into its own item, and
+            # strip each item to remove leading/trailing whitespace.
+            split_specifiers = [s.strip() for s in specifiers.split(",") if s.strip()]
 
-        # Make each individual specifier a Specifier and save in a frozen set for later.
-        self._specs = frozenset(map(Specifier, split_specifiers))
+            # Make each individual specifier a Specifier and save in a frozen set
+            # for later.
+            self._specs = frozenset(map(Specifier, split_specifiers))
+        else:
+            # Save the supplied specifiers in a frozen set.
+            self._specs = frozenset(specifiers)
 
         # Store our prereleases value so we can use it later to determine if
         # we accept prereleases or not.

tests/test_specifiers.py

@@ -646,6 +646,12 @@ def test_empty_specifier(self, version):
         assert parse(version) in spec
         assert spec.contains(parse(version))
 
+    def test_create_from_specifiers(self):
+        spec_strs = [">=1.0", "!=1.1", "!=1.2", "<2.0"]
+        specs = [Specifier(s) for s in spec_strs]
+        spec = SpecifierSet(iter(specs))
+        assert set(spec) == set(specs)
+
     def test_specifier_prereleases_explicit(self):
         spec = SpecifierSet()
         assert not spec.prereleases