Add a codemod for dataclass changes in 3.11

Python 3.11 changed the field default mutability check for dataclasses to only allow defaults which are hashable.

This codemod helps with the migration by changing all default values that aren't obviously hashable to use `default_factory` instead.

Note: it's impossible to accurately determine if a particular expression produces a hashable value in a codemod, so the codemod significantly over-approximates what's unhashable.

libcst/codemod/commands/dataclass_default_factory.py

@@ -0,0 +1,140 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+# pyre-strict
+import builtins
+
+import libcst as cst
+from libcst import matchers as m
+from libcst.codemod import VisitorBasedCodemodCommand
+from libcst.codemod.visitors._add_imports import AddImportsVisitor
+from libcst.metadata.name_provider import QualifiedNameProvider
+from libcst.helpers import ensure_type
+
+
+def node_with_qname(expected: str) -> m.MatchMetadataIfTrue:
+    return m.MatchMetadataIfTrue(
+        QualifiedNameProvider,
+        lambda qnames: any(qname.name == expected for qname in qnames),
+    )
+
+
+annotation = m.SimpleStatementLine(body=[m.AtLeastN(m.AnnAssign(), n=1)])
+dataclass_ref = node_with_qname("dataclasses.dataclass")
+constant_expressions = [
+    name for name in dir(builtins) if name.capitalize()[0] == name[0]
+]
+literal = (
+    m.Integer()
+    | m.Float()
+    | m.Imaginary()
+    | m.SimpleString()
+    | m.ConcatenatedString()
+    | m.FormattedString()
+)
+
+field_call = m.Call(func=node_with_qname("dataclasses.field"))
+
+default_arg = m.Call(
+    args=[
+        m.AtLeastN(
+            m.Arg(
+                keyword=m.Name("default"),
+                value=m.SaveMatchedNode(m.DoNotCare(), "default_value"),
+            ),
+            n=1,
+        )
+    ],
+)
+
+
+def wrap_in_default_factory(expr: cst.BaseExpression) -> cst.Arg:
+    return cst.Arg(
+        keyword=cst.Name("default_factory"),
+        value=cst.Lambda(params=cst.Parameters(), body=expr),
+    )
+
+
+class DataclassDefaultFactoryCodemod(VisitorBasedCodemodCommand):
+    """
+    Converts dataclass fields with mutable default values to use default_factory.
+
+    For example:
+        @dataclass
+        class Foo:
+            x: list = []  # Mutable default, bad practice
+
+    Becomes:
+        @dataclass
+        class Foo:
+            x: list = field(default_factory=lambda: [])  # Better practice
+    """
+
+    METADATA_DEPENDENCIES = (QualifiedNameProvider,)
+
+    def is_immutable(self, expr: cst.BaseExpression) -> bool:
+        return self.matches(
+            expr,
+            m.OneOf(
+                literal,
+                *[m.Name(expr) for expr in constant_expressions],
+            ),
+        )
+
+    @m.leave(
+        m.ClassDef(
+            decorators=[m.AtLeastN(m.Decorator(dataclass_ref), n=1)],
+            body=m.IndentedBlock(body=[m.AtLeastN(annotation, n=1)]),
+        )
+    )
+    def handle_class(
+        self, original_node: cst.ClassDef, updated_node: cst.ClassDef
+    ) -> cst.ClassDef:
+        new_body: list[cst.BaseStatement] = []
+        for line in ensure_type(original_node.body, cst.IndentedBlock).body:
+            if not self.matches(line, annotation):
+                new_body.append(line)
+                continue
+            new_line_body: list[cst.BaseSmallStatement] = []
+            for stmt in ensure_type(line, cst.SimpleStatementLine).body:
+                if not isinstance(stmt, cst.AnnAssign):
+                    new_line_body.append(stmt)
+                    continue
+                new_line_body.append(self.handle_annotation(stmt))
+            new_body.append(line.with_changes(body=new_line_body))
+
+        return updated_node.with_changes(
+            body=updated_node.body.with_changes(body=new_body)
+        )
+
+    def handle_annotation(self, annotation: cst.AnnAssign) -> cst.AnnAssign:
+        if annotation.value is None or self.is_immutable(annotation.value):
+            return annotation
+
+        if not self.matches(annotation.value, field_call):
+            AddImportsVisitor.add_needed_import(self.context, "dataclasses", "field")
+            return annotation.with_changes(
+                value=cst.Call(
+                    func=cst.Name("field"),
+                    args=[wrap_in_default_factory(annotation.value)],
+                )
+            )
+
+        # we found field(...) on the RHS
+        if (match := self.extract(annotation.value, default_arg)) is None:
+            # no default= kwarg, nothing to do
+            return annotation
+        default = ensure_type(match["default_value"], cst.BaseExpression)
+        if self.is_immutable(default):
+            return annotation
+        # rebuild the args for field(), dropping default= and adding default_factory=
+        new_args: list[cst.Arg] = []
+        for arg in ensure_type(annotation.value, cst.Call).args:
+            if arg.keyword is None or arg.keyword.value != "default":
+                new_args.append(arg)
+                continue
+            new_args.append(wrap_in_default_factory(default))
+        return annotation.with_changes(
+            value=annotation.value.with_changes(args=new_args)
+        )

libcst/codemod/commands/tests/test_dataclass_default_factory.py

@@ -0,0 +1,69 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+# pyre-strict
+
+from libcst.codemod import CodemodTest
+from libcst.codemod.commands.dataclass_default_factory import (
+    DataclassDefaultFactoryCodemod,
+)
+
+
+class TestDataclassCommand(CodemodTest):
+    TRANSFORM = DataclassDefaultFactoryCodemod
+
+    def test_simple_immutable(self) -> None:
+        before = """
+        from dataclasses import dataclass
+        @dataclass
+        class Foo:
+            x: int = 1
+            y: bool = False
+            z: str = "foo"
+        """
+        self.assertCodemod(before, before)
+
+    def test_simple_mutable(self) -> None:
+        before = """
+        from dataclasses import dataclass
+        @dataclass
+        class Foo:
+            x: list[int] = []
+            y: foo = bar()
+        """
+        after = """
+        from dataclasses import field, dataclass
+        @dataclass
+        class Foo:
+            x: list[int] = field(default_factory = lambda: [])
+            y: foo = field(default_factory = lambda: bar())
+        """
+        self.assertCodemod(before, after)
+
+    def test_idempotent(self) -> None:
+        before = """
+        from dataclasses import dataclass, field
+        @dataclass
+        class Foo:
+            x: list[int] = field(default_factory=lambda: [])
+            y: list[int] = field(repr=False)
+        """
+        self.assertCodemod(before, before)
+
+    def test_field_with_default(self) -> None:
+        before = """
+        from dataclasses import dataclass, field
+        @dataclass
+        class Foo:
+            x: list[int] = field(default=[])
+            y: bool = field(default=True)
+        """
+        after = """
+        from dataclasses import dataclass, field
+        @dataclass
+        class Foo:
+            x: list[int] = field(default_factory = lambda: [])
+            y: bool = field(default=True)
+        """
+        self.assertCodemod(before, after)