Implement __replace__ on 3.13

Fixes #1313

Author: hynek

.github/workflows/ci.yml

@@ -185,7 +185,7 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           # Keep in sync with tox/docs and .readthedocs.yaml.
-          python-version: "3.12"
+          python-version: "3.13"
       - uses: hynek/setup-cached-uv@v2
 
       - run: uvx --with=tox-uv tox run -e docs,changelog

.readthedocs.yaml

@@ -5,7 +5,7 @@ build:
   os: ubuntu-lts-latest
   tools:
     # Keep version in sync with tox.ini/docs and ci.yml/docs.
-    python: "3.12"
+    python: "3.13"
   jobs:
     # Need the tags to calculate the version (sometimes).
     post_checkout:

docs/examples.md

@@ -674,6 +674,19 @@ C(x=1, y=3)
 False
 ```
 
+On Python 3.13 and later, you can also use {func}`copy.replace` from the standard library:
+
+```{doctest}
+>>> import copy
+>>> @frozen
+... class C:
+...     x: int
+...     y: int
+>>> i = C(1, 2)
+>>> copy.replace(i, y=3)
+C(x=1, y=3)
+```
+
 
 ## Other Goodies
 

src/attr/__init__.py

@@ -10,7 +10,7 @@
 from . import converters, exceptions, filters, setters, validators
 from ._cmp import cmp_using
 from ._config import get_run_validators, set_run_validators
-from ._funcs import asdict, assoc, astuple, evolve, has, resolve_types
+from ._funcs import asdict, assoc, astuple, has, resolve_types
 from ._make import (
     NOTHING,
     Attribute,
@@ -19,6 +19,7 @@
     _Nothing,
     attrib,
     attrs,
+    evolve,
     fields,
     fields_dict,
     make_class,

src/attr/_funcs.py

@@ -394,60 +394,6 @@ def assoc(inst, **changes):
     return new
 
 
-def evolve(*args, **changes):
-    """
-    Create a new instance, based on the first positional argument with
-    *changes* applied.
-
-    Args:
-
-        inst:
-            Instance of a class with *attrs* attributes. *inst* must be passed
-            as a positional argument.
-
-        changes:
-            Keyword changes in the new copy.
-
-    Returns:
-        A copy of inst with *changes* incorporated.
-
-    Raises:
-        TypeError:
-            If *attr_name* couldn't be found in the class ``__init__``.
-
-        attrs.exceptions.NotAnAttrsClassError:
-            If *cls* is not an *attrs* class.
-
-    .. versionadded:: 17.1.0
-    .. deprecated:: 23.1.0
-       It is now deprecated to pass the instance using the keyword argument
-       *inst*. It will raise a warning until at least April 2024, after which
-       it will become an error. Always pass the instance as a positional
-       argument.
-    .. versionchanged:: 24.1.0
-       *inst* can't be passed as a keyword argument anymore.
-    """
-    try:
-        (inst,) = args
-    except ValueError:
-        msg = (
-            f"evolve() takes 1 positional argument, but {len(args)} were given"
-        )
-        raise TypeError(msg) from None
-
-    cls = inst.__class__
-    attrs = fields(cls)
-    for a in attrs:
-        if not a.init:
-            continue
-        attr_name = a.name  # To deal with private attributes.
-        init_name = a.alias
-        if init_name not in changes:
-            changes[init_name] = getattr(inst, attr_name)
-
-    return cls(**changes)
-
-
 def resolve_types(
     cls, globalns=None, localns=None, attribs=None, include_extras=True
 ):

src/attr/_make.py

@@ -22,6 +22,7 @@
 from ._compat import (
     PY_3_10_PLUS,
     PY_3_11_PLUS,
+    PY_3_13_PLUS,
     _AnnotationExtractor,
     _get_annotations,
     get_generic_base,
@@ -565,6 +566,60 @@ def _frozen_delattrs(self, name):
     raise FrozenInstanceError
 
 
+def evolve(*args, **changes):
+    """
+    Create a new instance, based on the first positional argument with
+    *changes* applied.
+
+    Args:
+
+        inst:
+            Instance of a class with *attrs* attributes. *inst* must be passed
+            as a positional argument.
+
+        changes:
+            Keyword changes in the new copy.
+
+    Returns:
+        A copy of inst with *changes* incorporated.
+
+    Raises:
+        TypeError:
+            If *attr_name* couldn't be found in the class ``__init__``.
+
+        attrs.exceptions.NotAnAttrsClassError:
+            If *cls* is not an *attrs* class.
+
+    .. versionadded:: 17.1.0
+    .. deprecated:: 23.1.0
+       It is now deprecated to pass the instance using the keyword argument
+       *inst*. It will raise a warning until at least April 2024, after which
+       it will become an error. Always pass the instance as a positional
+       argument.
+    .. versionchanged:: 24.1.0
+       *inst* can't be passed as a keyword argument anymore.
+    """
+    try:
+        (inst,) = args
+    except ValueError:
+        msg = (
+            f"evolve() takes 1 positional argument, but {len(args)} were given"
+        )
+        raise TypeError(msg) from None
+
+    cls = inst.__class__
+    attrs = fields(cls)
+    for a in attrs:
+        if not a.init:
+            continue
+        attr_name = a.name  # To deal with private attributes.
+        init_name = a.alias
+        if init_name not in changes:
+            changes[init_name] = getattr(inst, attr_name)
+
+    return cls(**changes)
+
+
 class _ClassBuilder:
     """
     Iteratively build *one* class.
@@ -979,6 +1034,12 @@ def add_init(self):
 
         return self
 
+    def add_replace(self):
+        self._cls_dict["__replace__"] = self._add_method_dunders(
+            lambda self, **changes: evolve(self, **changes)
+        )
+        return self
+
     def add_match_args(self):
         self._cls_dict["__match_args__"] = tuple(
             field.name
@@ -1381,6 +1442,9 @@ def wrap(cls):
                 msg = "Invalid value for cache_hash.  To use hash caching, init must be True."
                 raise TypeError(msg)
 
+        if PY_3_13_PLUS and not _has_own_attribute(cls, "__replace__"):
+            builder.add_replace()
+
         if (
             PY_3_10_PLUS
             and match_args

src/attr/_next_gen.py

@@ -316,6 +316,9 @@ def define(
        If a class has an *inherited* classmethod called
        ``__attrs_init_subclass__``, it is executed after the class is created.
     .. deprecated:: 24.1.0 *hash* is deprecated in favor of *unsafe_hash*.
+    .. versionadded:: 24.3.0
+       Unless already present, a ``__replace__`` method is automatically
+       created for `copy.replace` (Python 3.13+ only).
 
     .. note::
 

tests/test_functional.py

@@ -4,6 +4,7 @@
 End-to-end tests.
 """
 
+import copy
 import inspect
 import pickle
 
@@ -16,6 +17,7 @@
 
 import attr
 
+from attr._compat import PY_3_13_PLUS
 from attr._make import NOTHING, Attribute
 from attr.exceptions import FrozenInstanceError
 
@@ -766,3 +768,40 @@ class ToRegister(Base):
             pass
 
         assert [ToRegister] == REGISTRY
+
+
+@pytest.mark.skipif(not PY_3_13_PLUS, reason="requires Python 3.13+")
+class TestReplace:
+    def test_replaces(self):
+        """
+        copy.replace() is added by default and works like `attrs.evolve`.
+        """
+        inst = C1(1, 2)
+
+        assert C1(1, 42) == copy.replace(inst, y=42)
+        assert C1(42, 2) == copy.replace(inst, x=42)
+
+    def test_already_has_one(self):
+        """
+        If the object already has a __replace__, it's left alone.
+        """
+        sentinel = object()
+
+        @attr.s
+        class C:
+            x = attr.ib()
+
+            __replace__ = sentinel
+
+        assert sentinel == C.__replace__
+
+    def test_invalid_field_name(self):
+        """
+        Invalid field names raise a TypeError.
+
+        This is consistent with dataclasses.
+        """
+        inst = C1(1, 2)
+
+        with pytest.raises(TypeError):
+            copy.replace(inst, z=42)

tox.ini

@@ -62,7 +62,7 @@ commands = pytest --codspeed -n auto bench/test_benchmarks.py
 
 [testenv:docs]
 # Keep base_python in-sync with ci.yml/docs and .readthedocs.yaml.
-base_python = py312
+base_python = py313
 extras = docs
 commands =
     sphinx-build -n -T -W -b html -d {envtmpdir}/doctrees docs docs/_build/html