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dataclass, slots, __post_init__ and super #111500

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Closed as not planned
dataclass, slots, __post_init__ and super#111500
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sobolevn
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stdlibStandard Library Python modules in the Lib/ directorytype-bugAn unexpected behavior, bug, or error

Description

@voidspace
voidspace
opened on Oct 30, 2023

Bug report

Bug description:

Using super() in __post_init__ on a dataclass with slots fails.

This code:

from dataclasses import dataclass

@dataclass(slots=True)
class Base:
    def __post_init__(self):
        pass

@dataclass(slots=True)
class Thing(Base):
    a: int
    b: int
    c: int = 0

    def __post_init__(self):
        self.c = self.a + self.b
        super().__post_init__()

t = Thing(1,3)

Produces this exception:

Traceback (most recent call last):
  File "C:\Users\michael.foord\Code\one-touch-switch-service\example.py", line 18, in <module>
    t = Thing(1,3)
        ^^^^^^^^^^
  File "<string>", line 6, in __init__
  File "C:\Users\michael.foord\Code\one-touch-switch-service\example.py", line 16, in __post_init__
    super().__post_init__()
    ^^^^^^^
TypeError: super(type, obj): obj must be an instance or subtype of type

The fix is to use Python 2 style super(Class, self).__post_init__().

CPython versions tested on:

3.11

Operating systems tested on:

Windows

Linked PRs

Activity

  1. voidspace
    added
    type-bugAn unexpected behavior, bug, or error
    on Oct 30, 2023
  2. Eclips4
    added
    stdlibStandard Library Python modules in the Lib/ directory
    on Oct 30, 2023

  3. sobolevn commented on Oct 30, 2023

    @sobolevn
    sobolevn
    on Oct 30, 2023
    Member

    I think that I found why this happens :)

    First of all, I was not able to reproduce it without @dataclass, so I assumed it is related to the internal machinery.

    This is my debug patch:

    » git patch
diff --git Lib/dataclasses.py Lib/dataclasses.py
index 2fba32b5ffb..a7ce3c03e9b 100644
--- Lib/dataclasses.py
+++ Lib/dataclasses.py
@@ -1232,7 +1232,7 @@ def _add_slots(cls, is_frozen, weakref_slot):
 
     # And finally create the class.
     qualname = getattr(cls, '__qualname__', None)
-    cls = type(cls)(cls.__name__, cls.__bases__, cls_dict)
+    cls = type(cls)(cls.__name__ + '__slots', cls.__bases__, cls_dict)
     if qualname is not None:
         cls.__qualname__ = qualname
 
diff --git Objects/typeobject.c Objects/typeobject.c
index 25085693070..79db15579e5 100644
--- Objects/typeobject.c
+++ Objects/typeobject.c
@@ -10400,9 +10400,11 @@ supercheck(PyTypeObject *type, PyObject *obj)
         Py_XDECREF(class_attr);
     }
 
-    PyErr_SetString(PyExc_TypeError,
+    PyErr_Format(PyExc_TypeError,
                     "super(type, obj): "
-                    "obj must be an instance or subtype of type");
+                    "obj must be an instance or subtype of type %s %s",
+                    Py_TYPE(obj)->tp_name,
+                    type->tp_name);
     return NULL;
 }

    Turns out, I was right. Given the code in the original bug report, it produces:

    Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython2/ex.py", line 32, in <module>
    Thing2(1, 2)
  File "/Users/sobolev/Desktop/cpython2/ex.py", line 19, in __init__
    self.__post_init__()
  File "/Users/sobolev/Desktop/cpython2/ex.py", line 24, in __post_init__
    super().__post_init__()
    ^^^^^^^^^^^^^^^^^^^^^
TypeError: super(type, obj): obj must be an instance or subtype of type Thing__slots Thing

    Looks like we add slots and regenerate class in the very end. I think we can try doing it at the beggining. Maybe it will help.

    I am working on this idea! :)

  4. sobolevn
    self-assigned this
    on Oct 30, 2023

  5. sobolevn commented on Oct 31, 2023

    @sobolevn
    sobolevn
    on Oct 31, 2023
    Member

    Here are some experiments that I had:

    from dataclasses import dataclass
# from attr import dataclass

@dataclass(slots=True)
class Base:
    def m(self):
        return "DONE!"

class Thing(Base):
    def m(self):
        print(super().m())

Thing2 = dataclass(slots=True)(Thing)
Thing2().m()

    It fails with the same error. However, if you pip install attrs and uncomment # from attr import dataclass it will work and print DONE!

    So, it can be fixed (given that our _add_slots functions are similar): https://github.com/python-attrs/attrs/blob/52c1cd97d723cde6d75259a22d218f9305ecf8ee/src/attr/_make.py#L809-L914

  6. sobolevn commented on Oct 31, 2023

    @sobolevn
    sobolevn
    on Oct 31, 2023
    Member

    Here's the fix: https://github.com/python-attrs/attrs/blob/52c1cd97d723cde6d75259a22d218f9305ecf8ee/src/attr/_make.py#L884-L912 :)

  7. sobolevn
    added a commit that references this issue on Oct 31, 2023

    pythongh-111500: Fix `super()` without args calls for `dataclasses` w…

    d310165

  8. voidspace commented on Nov 6, 2023

    @voidspace
    voidspace
    on Nov 6, 2023
    ContributorAuthor

    Nice work @sobolevn

  9. JelleZijlstra commented on Mar 12, 2024

    @JelleZijlstra
    JelleZijlstra
    on Mar 12, 2024
    Member

    Duplicate of #90562

  10. JelleZijlstra
    marked this as a duplicate of RFE: @dataclasses.dataclass(slots=True) doesn't support methods using zero-arg super() #90562 on Mar 12, 2024
  11. JelleZijlstra
    closed this as not plannedon Mar 12, 2024
  12. ambv
    added a commit that references this issue on Oct 20, 2026

    pythongh-111500: Fix `super()` without args calls for `dataclasses` w…

    994fd50
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