document dataclass codemod

integration_tests/test_fix_dataclass_defaults.py

@@ -0,0 +1,43 @@
+from codemodder.codemods.test import (
+    BaseIntegrationTest,
+    original_and_expected_from_code_path,
+)
+from core_codemods.fix_dataclass_defaults import FixDataclassDefaults
+
+
+class TestFixDataclassDefaults(BaseIntegrationTest):
+    codemod = FixDataclassDefaults
+    code_path = "tests/samples/fix_dataclass_defaults.py"
+    original_code, expected_new_code = original_and_expected_from_code_path(
+        code_path,
+        [
+            (0, """from dataclasses import field, dataclass\n"""),
+            (5, """    phones: list = field(default_factory=list)\n"""),
+            (6, """    friends: dict = field(default_factory=dict)\n"""),
+            (7, """    family: set = field(default_factory=set)\n"""),
+        ],
+    )
+
+    # fmt: off
+    expected_diff =(
+    """--- \n"""
+    """+++ \n"""
+    """@@ -1,8 +1,8 @@\n"""
+    """-from dataclasses import dataclass\n"""
+    """+from dataclasses import field, dataclass\n"""
+    """ \n"""
+    """ @dataclass\n"""
+    """ class Test:\n"""
+    """     name: str = ""\n"""
+    """-    phones: list = []\n"""
+    """-    friends: dict = {}\n"""
+    """-    family: set = set()\n"""
+    """+    phones: list = field(default_factory=list)\n"""
+    """+    friends: dict = field(default_factory=dict)\n"""
+    """+    family: set = field(default_factory=set)\n"""
+    )
+    # fmt: on
+
+    expected_line_change = "6"
+    change_description = FixDataclassDefaults.change_description
+    num_changes = 3

src/codemodder/scripts/generate_docs.py

@@ -247,6 +247,10 @@ class DocMetadata:
         importance="High",
         guidance_explained="This change may impact performance in some cases, but it is recommended when handling untrusted data.",
     ),
+    "fix-dataclass-defaults": DocMetadata(
+        importance="Medium",
+        guidance_explained="This change is safe and will prevent runtime `ValueError`.",
+    ),
 }
 
 METADATA = CORE_METADATA | {

src/core_codemods/docs/pixee_python_fix-dataclass-defaults.md

@@ -0,0 +1,18 @@
+This codemod will fix instances of `dataclasses.dataclass` that define default lists, sets, or dicts which raise a runtime `ValueError`. The [dataclass documentation](https://docs.python.org/3/library/dataclasses.html#mutable-default-values) provides a clear explanation of why this code is disallowed and explains how to use `field(default_factory=...)` instead.
+
+Our changes look something like this:
+
+```diff
+-from dataclasses import dataclass
++from dataclasses import field, dataclass
+
+ @dataclass
+ class Person:
+     name: str = ""
+-    phones: list = []
+-    friends: dict = {}
+-    family: set = set()
++    phones: list = field(default_factory=list)
++    friends: dict = field(default_factory=dict)
++    family: set = field(default_factory=set)
+```

src/core_codemods/fix_dataclass_defaults.py

@@ -8,15 +8,17 @@
 class FixDataclassDefaults(SimpleCodemod, NameAndAncestorResolutionMixin, UtilsMixin):
     metadata = Metadata(
         name="fix-dataclass-defaults",
-        summary="todo",
+        summary="Replace `dataclass` Mutable Default Values with Call to `field`",
         review_guidance=ReviewGuidance.MERGE_WITHOUT_REVIEW,
         references=[
             Reference(
                 url="https://docs.python.org/3/library/dataclasses.html#mutable-default-values"
             )
         ],
     )
-    change_description = "todo"
+    change_description = (
+        "Replace `dataclass` mutable default values with call to `field`"
+    )
 
     def leave_AnnAssign(
         self, original_node: cst.Assign, updated_node: cst.Assign

tests/samples/fix_dataclass_defaults.py

@@ -0,0 +1,8 @@
+from dataclasses import dataclass
+
+@dataclass
+class Test:
+    name: str = ""
+    phones: list = []
+    friends: dict = {}
+    family: set = set()