Revert SelectionPreferences as it's performance-sensitive

src/pip/_internal/models/selection_prefs.py

@@ -1,32 +1,53 @@
-from dataclasses import dataclass
 from typing import Optional
 
 from pip._internal.models.format_control import FormatControl
 
 
-@dataclass(frozen=True)
+# TODO: This needs Python 3.10's improved slots support for dataclasses
+# to be converted into a dataclass.
 class SelectionPreferences:
     """
     Encapsulates the candidate selection preferences for downloading
     and installing files.
-
-    :param allow_yanked: Whether files marked as yanked (in the sense
-        of PEP 592) are permitted to be candidates for install.
-    :param format_control: A FormatControl object or None. Used to control
-        the selection of source packages / binary packages when consulting
-        the index and links.
-    :param prefer_binary: Whether to prefer an old, but valid, binary
-        dist over a new source dist.
-    :param ignore_requires_python: Whether to ignore incompatible
-        "Requires-Python" values in links. Defaults to False.
     """
 
+    __slots__ = [
+        "allow_yanked",
+        "allow_all_prereleases",
+        "format_control",
+        "prefer_binary",
+        "ignore_requires_python",
+    ]
+
     # Don't include an allow_yanked default value to make sure each call
     # site considers whether yanked releases are allowed. This also causes
     # that decision to be made explicit in the calling code, which helps
     # people when reading the code.
-    allow_yanked: bool
-    allow_all_prereleases: bool = False
-    format_control: Optional[FormatControl] = None
-    prefer_binary: bool = False
-    ignore_requires_python: Optional[bool] = False
+    def __init__(
+        self,
+        allow_yanked: bool,
+        allow_all_prereleases: bool = False,
+        format_control: Optional[FormatControl] = None,
+        prefer_binary: bool = False,
+        ignore_requires_python: Optional[bool] = None,
+    ) -> None:
+        """Create a SelectionPreferences object.
+
+        :param allow_yanked: Whether files marked as yanked (in the sense
+            of PEP 592) are permitted to be candidates for install.
+        :param format_control: A FormatControl object or None. Used to control
+            the selection of source packages / binary packages when consulting
+            the index and links.
+        :param prefer_binary: Whether to prefer an old, but valid, binary
+            dist over a new source dist.
+        :param ignore_requires_python: Whether to ignore incompatible
+            "Requires-Python" values in links. Defaults to False.
+        """
+        if ignore_requires_python is None:
+            ignore_requires_python = False
+
+        self.allow_yanked = allow_yanked
+        self.allow_all_prereleases = allow_all_prereleases
+        self.format_control = format_control
+        self.prefer_binary = prefer_binary
+        self.ignore_requires_python = ignore_requires_python