Fix - add __post_init__ into defining-attr-methods to avoid `attr…

…ibute-defined-outside-init` in dataclasses.

CONTRIBUTORS.txt

@@ -305,3 +305,5 @@ contributors:
 * Agustin Toledo: contributor
 
 * Nicholas Smith: contributor
+
+* Andrzej Klajnert: contributor

ChangeLog

@@ -158,6 +158,10 @@ Release date: TBA
 
   Close #2963
 
+* Added ``__post_init__`` to ``defining-attr-methods`` in order to avoid ``attribute-defined-outside-init`` in dataclasses.
+
+  Close #2581
+
 What's New in Pylint 2.3.0?
 ===========================
 

examples/pylintrc

@@ -460,7 +460,8 @@ redefining-builtins-modules=six.moves,past.builtins,future.builtins,builtins,io
 # List of method names used to declare (i.e. assign) instance attributes.
 defining-attr-methods=__init__,
                       __new__,
-                      setUp
+                      setUp,
+                      __post_init__
 
 # List of member names, which should be excluded from the protected access
 # warning.

pylint/checkers/classes.py

@@ -666,7 +666,7 @@ class ClassChecker(BaseChecker):
         (
             "defining-attr-methods",
             {
-                "default": ("__init__", "__new__", "setUp"),
+                "default": ("__init__", "__new__", "setUp", "__post_init__"),
                 "type": "csv",
                 "metavar": "<method names>",
                 "help": "List of method names used to declare (i.e. assign) \

pylintrc

@@ -334,7 +334,7 @@ max-public-methods=25
 [CLASSES]
 
 # List of method names used to declare (i.e. assign) instance attributes.
-defining-attr-methods=__init__,__new__,setUp
+defining-attr-methods=__init__,__new__,setUp,__post_init__
 
 # List of valid names for the first argument in a class method.
 valid-classmethod-first-arg=cls

tests/functional/attribute_defined_outside_init.py

@@ -78,3 +78,7 @@ def prop(self):
     @prop.setter
     def prop(self, value):
         self.__prop = value
+
+class DataClass:
+    def __post_init__(self):
+        self.a = 42