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TypeError when using super() and slots=True #102

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p-himik opened this Issue Sep 28, 2016 · 17 comments

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@p-himik
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p-himik commented Sep 28, 2016

This code

import attr


@attr.s(slots=True)
class A:
    def f(self):
        pass


@attr.s(slots=True)
class B(A):
    def f(self):
        super().f()


B().f()

results in this error

Traceback (most recent call last):
  File "test.py", line 16, in <module>
    B().f()
  File "test.py", line 13, in f
    super().f()
TypeError: super(type, obj): obj must be an instance or subtype of type

If I use super(B, self), everything works just fine.

@Tinche

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Tinche commented Sep 28, 2016

Huh, interesting. Slots are a little tricky since we can't "add" slots to a class that's already defined, so we switch the class with another one in attr.s. This is what's probably causing the problem.

I need to figure out how to debug this :)

@p-himik

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p-himik commented Sep 28, 2016

I guess that some other more intricate workflow involving isinstance will be broken in this case too.
May'be it's possible to switch the parent class with the updated one?

@Tinche

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Tinche commented Sep 28, 2016

Reading up on it a little (https://www.python.org/dev/peps/pep-3135/), there will probably be problems with any function inside the class using __class__ or no-arg super(). (Have to admit I wasn't even aware the __class__ name was available inside of methods.)

Unfortunately these function cells that contain the reference to the enclosing class appear to be immutable.

Anything outside the class should be fine.

At the very least, it will be possible to raise an informative error at class definition time.

More research needed.

@moshez

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moshez commented Sep 28, 2016

Just my obsession with minimal example: notice that this reproduces even in this simpler example:

import attr

@attr.s(slots=True)
class B:
    def f(self):
        super()

B().f()
@hynek

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hynek commented Sep 29, 2016

Something tells me that this might be unsolvable unless there’s some secret backdoor we can use. :|

@moshez

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moshez commented Sep 29, 2016

Perhaps the only way is to error out if any of the functions inside the class refer to "super" or "class" with an explanation.

@perkinslr

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perkinslr commented Oct 10, 2016

Could do it by recreating the function objects, the function cells are immutable, but creating a duplicate function object which contains a cell pointing to the new class is possible. Unfortunately, explicitly creating cell instances from python (à la types.FunctionType()) is not possible, so it would require including a stub method in the new type that references the cell and then creating a new function using the old parameters except for swapping the closure.

It's also possible via the python C API to set the cell contents, which may be the simplest solution. I've tested it a bit and it seems to work.

class A(object):
    def printClass(self):
        print(__class__)

class NewA(object):
    pass

import ctypes
libpython = ctypes.cdll.LoadLibrary('/usr/lib64/libpython3.5.so')
libpython.PyCell_Set(ctypes.py_object(A.printClass.__closure__[0]), ctypes.py_object(NewA))

A().printClass()

It would require figuring out if the cell holds a weakref or a normal reference to the cell contents, so as to not screw up the reference count.

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hynek commented Oct 13, 2016

I wonder if we couldn’t fix this using __new__ once we drop the requirement, that attrs definitions are replaced by Attribute instances (since class attributes on __slots__ classes are immutable).

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p-himik commented Jan 3, 2017

Just got bitten by it again - only this time while using a metaclass.

But we can add __slots__ in a metaclass and according to https://stackoverflow.com/questions/11091609/setting-a-class-metaclass-using-a-decorator you can add a metaclass by using a decorator.
So what if @attr.s(...) instead of directly altering class attributes and/or the class itself, would just add a metaclass, taking already provided metaclass into account?

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p-himik commented Jan 3, 2017

Hmm, this method still doesn't make TypeError go away, although the same error doesn't happen when using metaclass kwarg when creating a class.
Probably it's possible to create a metaclass that would be used in @attr.s(...) when a user doesn't need super() and used as a regular metaclass when there's a need to super().

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hynek commented Jul 27, 2017

Yay, I just ran into the problem too. :|

Currently we have problems on both sides of the fence:

  • If we replace the class, super() stops working
  • If we don’t, a bunch of other assumptions don’t work like #202

Looks like a lose-lose scenario. :|

@Tinche

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Tinche commented Jul 27, 2017

There's a way to fix this on CPython.

Content warning: hairy code ahead ;)

Simplest reproducer:

import attr


@attr.s(slots=True)
class C:
    a = attr.ib()

    def t(self):
        return __class__


>>> print(C(1).t() is C)
False

The problem is this:

>>> C.t.__closure__[0].cell_contents
<class '__main__.C'>   # This is not our class, but the old class.

The fix:

import attr


@attr.s(slots=True)
class C:
    a = attr.ib()

    def t(self):
        return __class__

import ctypes
PyCell_Set = ctypes.pythonapi.PyCell_Set
PyCell_Set.argtypes = (ctypes.py_object, ctypes.py_object)
PyCell_Set.restype = ctypes.c_int
PyCell_Set(C.t.__closure__[0], C)

>>> print(C(1).t() is C)
True

We need to ask our PyPy friends how to handle this on PyPy, though.

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hynek commented Jul 27, 2017

That’s…disgusting. :D

I’d know whom to ask however maybe we should discuss what the canonical answer in attrs should be: always new class (like slots=True, requires interpreter internals) or patched class (classic, requires potentially meta classes to be done properly—I intend to look at a __new__-based approach once we’re rid of .Attribute access)?

Yesterday I’d have said “always new class” but the repercussions seem worse than patching around.

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Tinche commented Jul 27, 2017

I don't actually know how a metaclass approach would work. Could you show me a prototype just for me to get the gist?

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hynek commented Jul 27, 2017

Haven’t tried anything yet but you can set __slots__ in __new__. I hope to remember, that the reason why we can’t do it, is that currently the slots are filled and thus immutable.

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Tinche commented Jul 27, 2017

Alright, got a response from the PyPy team (Armin Rigo of course :)

It's actually easier there:

C.t.__closure__[0].__setstate__((C,))

@hynek hynek added this to the 17.3 milestone Jul 29, 2017

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hynek commented Aug 3, 2017

Fixed by #226

@hynek hynek closed this Aug 3, 2017

bors-fusion bot added a commit to fusionapp/fusion-index that referenced this issue Nov 15, 2017

Merge #167
167: Scheduled weekly dependency update for week 46 r=mithrandi




## Updates
Here's a list of all the updates bundled in this pull request. I've added some links to make it easier for you to find all the information you need.
<table align="center">

<tr>
<td><b>attrs</b></td>
<td align="center">17.2.0</td>
<td align="center">&raquo;</td>
<td align="center">17.3.0</td>
<td>
     <a href="https://pypi.python.org/pypi/attrs">PyPI</a> | <a href="https://pyup.io/changelogs/attrs/">Changelog</a> | <a href="http://www.attrs.org/">Homepage</a> 

</td>

<tr>
<td><b>cryptography</b></td>
<td align="center">2.1.2</td>
<td align="center">&raquo;</td>
<td align="center">2.1.3</td>
<td>
     <a href="https://pypi.python.org/pypi/cryptography">PyPI</a> | <a href="https://pyup.io/changelogs/cryptography/">Changelog</a> | <a href="https://github.com/pyca/cryptography">Repo</a> 

</td>

<tr>
<td><b>hypothesis</b></td>
<td align="center">3.33.0</td>
<td align="center">&raquo;</td>
<td align="center">3.37.0</td>
<td>
     <a href="https://pypi.python.org/pypi/hypothesis">PyPI</a> | <a href="https://pyup.io/changelogs/hypothesis/">Changelog</a> | <a href="https://github.com/HypothesisWorks/hypothesis/issues">Repo</a> 

</td>

</tr>
</table>



## Changelogs


### attrs 17.2.0 -> 17.3.0

>### 17.3.0

>-------------------

>Backward-incompatible Changes
>^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

>- Attributes are not defined on the class body anymore.

>  This means that if you define a class ``C`` with an attribute ``x``, the class will *not* have an attribute ``x`` for introspection anymore.
>  Instead of ``C.x``, use ``attr.fields(C).x`` or look at ``C.__attrs_attrs__``.
>  The old behavior has been deprecated since version 16.1.
>  (`253 &lt;https://github.com/python-attrs/attrs/issues/253&gt;`_)


>Changes
>^^^^^^^

>- ``super()`` and ``__class__`` now work on Python 3 when ``slots=True``.
>  (`102 &lt;https://github.com/python-attrs/attrs/issues/102&gt;`_, `226 &lt;https://github.com/python-attrs/attrs/issues/226&gt;`_, `269 &lt;https://github.com/python-attrs/attrs/issues/269&gt;`_, `270 &lt;https://github.com/python-attrs/attrs/issues/270&gt;`_, `272 &lt;https://github.com/python-attrs/attrs/issues/272&gt;`_)
>- Added ``type`` argument to ``attr.ib()`` and corresponding ``type`` attribute to ``attr.Attribute``.

>  This change paves the way for automatic type checking and serialization (though as of this release ``attrs`` does not make use of it).
>  In Python 3.6 or higher, the value of ``attr.Attribute.type`` can alternately be set using variable type annotations
>  (see `PEP 526 &lt;https://www.python.org/dev/peps/pep-0526/&gt;`_). (`151 &lt;https://github.com/python-attrs/attrs/issues/151&gt;`_, `214 &lt;https://github.com/python-attrs/attrs/issues/214&gt;`_, `215 &lt;https://github.com/python-attrs/attrs/issues/215&gt;`_, `239 &lt;https://github.com/python-attrs/attrs/issues/239&gt;`_)
>- The combination of ``str=True`` and ``slots=True`` now works on Python 2.
>  (`198 &lt;https://github.com/python-attrs/attrs/issues/198&gt;`_)
>- ``attr.Factory`` is hashable again. (`204
>  &lt;https://github.com/python-attrs/attrs/issues/204&gt;`_)
>- Subclasses now can overwrite attribute definitions of their superclass.

>  That means that you can -- for example -- change the default value for an attribute by redefining it.
>  (`221 &lt;https://github.com/python-attrs/attrs/issues/221&gt;`_, `229 &lt;https://github.com/python-attrs/attrs/issues/229&gt;`_)
>- Added new option ``auto_attribs`` to ``attr.s`` that allows to collect annotated fields without setting them to ``attr.ib()``.

>  Setting a field to an ``attr.ib()`` is still possible to supply options like validators.
>  Setting it to any other value is treated like it was passed as ``attr.ib(default=value)`` -- passing an instance of ``attr.Factory`` also works as expected.
>  (`262 &lt;https://github.com/python-attrs/attrs/issues/262&gt;`_, `277 &lt;https://github.com/python-attrs/attrs/issues/277&gt;`_)
>- Instances of classes created using ``attr.make_class()`` can now be pickled.
>  (`282 &lt;https://github.com/python-attrs/attrs/issues/282&gt;`_)


>----








### hypothesis 3.33.0 -> 3.37.0

>### 3.37.0

>-------------------

>This is a deprecation release for some health check related features.

>The following are now deprecated:

>* Passing :attr:`~hypothesis.HealthCheck.exception_in_generation` to
>  :attr:`~hypothesis.settings.suppress_health_check`. This no longer does
>  anything even when passed -  All errors that occur during data generation
>  will now be immediately reraised rather than going through the health check
>  mechanism.
>* Passing :attr:`~hypothesis.HealthCheck.random_module` to
>  :attr:`~hypothesis.settings.suppress_health_check`. This hasn&#39;t done anything
>  for a long time, but was never explicitly deprecated. Hypothesis always seeds
>  the random module when running given tests, so this is no longer an error
>  and suppressing it doesn&#39;t do anything.
>* Passing non-:class:`~hypothesis.HealthCheck` values in
>  :attr:`~hypothesis.settings.suppress_health_check`. This was previously
>  allowed but never did anything useful.

>In addition, passing a non-iterable value as :attr:`~hypothesis.settings.suppress_health_check`
>will now raise an error immediately (it would never have worked correctly, but
>it would previously have failed later). Some validation error messages have
>also been updated.

>This work was funded by `Smarkets &lt;https://smarkets.com/&gt;`_.

>-------------------


>### 3.36.1

>-------------------

>This is a yak shaving release, mostly concerned with our own tests.

>While :func:`~python:inspect.getfullargspec` was documented as deprecated
>in Python 3.5, it never actually emitted a warning.  Our code to silence
>this (nonexistent) warning has therefore been removed.

>We now run our tests with ``DeprecationWarning`` as an error, and made some
>minor changes to our own tests as a result.  This required similar upstream
>updates to :pypi:`coverage` and :pypi:`execnet` (a test-time dependency via
>:pypi:`pytest-xdist`).

>There is no user-visible change in Hypothesis itself, but we encourage you
>to consider enabling deprecations as errors in your own tests.

>-------------------


>### 3.36.0

>-------------------

>This release adds a setting to the public API, and does some internal cleanup:

>- The :attr:`~hypothesis.settings.derandomize` setting is now documented (:issue:`890`)
>- Removed - and disallowed - all &#39;bare excepts&#39; in Hypothesis (:issue:`953`)
>- Documented the :attr:`~hypothesis.settings.strict` setting as deprecated, and
>  updated the build so our docs always match deprecations in the code.

>-------------------


>### 3.35.0

>-------------------

>This minor release supports constraining :func:`~hypothesis.strategies.uuids`
>to generate :class:`~python:uuid.UUID`s of a particular version.
>(:issue:`721`)

>Thanks to Dion Misic for this feature.

>-------------------


>### 3.34.1

>-------------------

>This patch updates the documentation to suggest
>:func:`builds(callable) &lt;hypothesis.strategies.builds&gt;` instead of
>:func:`just(callable()) &lt;hypothesis.strategies.just&gt;`.

>-------------------


>### 3.34.0

>-------------------

>Hypothesis now emits deprecation warnings if you apply
>:func:`given &lt;hypothesis.given&gt;` more than once to a target.

>Applying :func:`given &lt;hypothesis.given&gt;` repeatedly wraps the target multiple
>times. Each wrapper will search the space of of possible parameters separately.
>This is equivalent but will be much more inefficient than doing it with a
>single call to :func:`given &lt;hypothesis.given&gt;`.

>For example, instead of
>``given(booleans()) given(integers())``, you could write
>``given(booleans(), integers())``

>-------------------


>### 3.33.1

>-------------------

>This is a bugfix release:

>- :func:`~hypothesis.strategies.builds` would try to infer a strategy for
>  required positional arguments of the target from type hints, even if they
>  had been given to :func:`~hypothesis.strategies.builds` as positional
>  arguments (:issue:`946`).  Now it only infers missing required arguments.
>- An internal introspection function wrongly reported ``self`` as a required
>  argument for bound methods, which might also have affected
>  :func:`~hypothesis.strategies.builds`.  Now it knows better.

>-------------------









That's it for now!

Happy merging! 🤖

bors-fusion bot added a commit to fusionapp/entropy that referenced this issue Nov 15, 2017

Merge #161
161: Scheduled weekly dependency update for week 46 r=mithrandi




## Updates
Here's a list of all the updates bundled in this pull request. I've added some links to make it easier for you to find all the information you need.
<table align="center">

<tr>
<td><b>attrs</b></td>
<td align="center">17.2.0</td>
<td align="center">&raquo;</td>
<td align="center">17.3.0</td>
<td>
     <a href="https://pypi.python.org/pypi/attrs">PyPI</a> | <a href="https://pyup.io/changelogs/attrs/">Changelog</a> | <a href="http://www.attrs.org/">Homepage</a> 

</td>

<tr>
<td><b>cryptography</b></td>
<td align="center">2.1.2</td>
<td align="center">&raquo;</td>
<td align="center">2.1.3</td>
<td>
     <a href="https://pypi.python.org/pypi/cryptography">PyPI</a> | <a href="https://pyup.io/changelogs/cryptography/">Changelog</a> | <a href="https://github.com/pyca/cryptography">Repo</a> 

</td>

<tr>
<td><b>lxml</b></td>
<td align="center">4.1.0</td>
<td align="center">&raquo;</td>
<td align="center">4.1.1</td>
<td>
     <a href="https://pypi.python.org/pypi/lxml">PyPI</a> | <a href="https://pyup.io/changelogs/lxml/">Changelog</a> | <a href="http://lxml.de/">Homepage</a> | <a href="https://bugs.launchpad.net/lxml">Bugtracker</a> 

</td>

<tr>
<td><b>pytz</b></td>
<td align="center">2017.2</td>
<td align="center">&raquo;</td>
<td align="center">2017.3</td>
<td>
     <a href="https://pypi.python.org/pypi/pytz">PyPI</a> | <a href="http://pythonhosted.org/pytz">Homepage</a> | <a href="http://pythonhosted.org/pytz/">Docs</a> 

</td>

</tr>
</table>



## Changelogs


### attrs 17.2.0 -> 17.3.0

>### 17.3.0

>-------------------

>Backward-incompatible Changes
>^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

>- Attributes are not defined on the class body anymore.

>  This means that if you define a class ``C`` with an attribute ``x``, the class will *not* have an attribute ``x`` for introspection anymore.
>  Instead of ``C.x``, use ``attr.fields(C).x`` or look at ``C.__attrs_attrs__``.
>  The old behavior has been deprecated since version 16.1.
>  (`253 &lt;https://github.com/python-attrs/attrs/issues/253&gt;`_)


>Changes
>^^^^^^^

>- ``super()`` and ``__class__`` now work on Python 3 when ``slots=True``.
>  (`102 &lt;https://github.com/python-attrs/attrs/issues/102&gt;`_, `226 &lt;https://github.com/python-attrs/attrs/issues/226&gt;`_, `269 &lt;https://github.com/python-attrs/attrs/issues/269&gt;`_, `270 &lt;https://github.com/python-attrs/attrs/issues/270&gt;`_, `272 &lt;https://github.com/python-attrs/attrs/issues/272&gt;`_)
>- Added ``type`` argument to ``attr.ib()`` and corresponding ``type`` attribute to ``attr.Attribute``.

>  This change paves the way for automatic type checking and serialization (though as of this release ``attrs`` does not make use of it).
>  In Python 3.6 or higher, the value of ``attr.Attribute.type`` can alternately be set using variable type annotations
>  (see `PEP 526 &lt;https://www.python.org/dev/peps/pep-0526/&gt;`_). (`151 &lt;https://github.com/python-attrs/attrs/issues/151&gt;`_, `214 &lt;https://github.com/python-attrs/attrs/issues/214&gt;`_, `215 &lt;https://github.com/python-attrs/attrs/issues/215&gt;`_, `239 &lt;https://github.com/python-attrs/attrs/issues/239&gt;`_)
>- The combination of ``str=True`` and ``slots=True`` now works on Python 2.
>  (`198 &lt;https://github.com/python-attrs/attrs/issues/198&gt;`_)
>- ``attr.Factory`` is hashable again. (`204
>  &lt;https://github.com/python-attrs/attrs/issues/204&gt;`_)
>- Subclasses now can overwrite attribute definitions of their superclass.

>  That means that you can -- for example -- change the default value for an attribute by redefining it.
>  (`221 &lt;https://github.com/python-attrs/attrs/issues/221&gt;`_, `229 &lt;https://github.com/python-attrs/attrs/issues/229&gt;`_)
>- Added new option ``auto_attribs`` to ``attr.s`` that allows to collect annotated fields without setting them to ``attr.ib()``.

>  Setting a field to an ``attr.ib()`` is still possible to supply options like validators.
>  Setting it to any other value is treated like it was passed as ``attr.ib(default=value)`` -- passing an instance of ``attr.Factory`` also works as expected.
>  (`262 &lt;https://github.com/python-attrs/attrs/issues/262&gt;`_, `277 &lt;https://github.com/python-attrs/attrs/issues/277&gt;`_)
>- Instances of classes created using ``attr.make_class()`` can now be pickled.
>  (`282 &lt;https://github.com/python-attrs/attrs/issues/282&gt;`_)


>----








### lxml 4.1.0 -> 4.1.1

>### 4.1.1

>==================

>* Rebuild with Cython 0.27.3 to improve support for Py3.7.











That's it for now!

Happy merging! 🤖

bors-fusion bot added a commit to fusionapp/documint that referenced this issue Nov 17, 2017

Merge #118
118: Update attrs to 17.3.0 r=mithrandi


There's a new version of [attrs](https://pypi.python.org/pypi/attrs) available.
You are currently using **17.2.0**. I have updated it to **17.3.0**



These links might come in handy:  <a href="https://pypi.python.org/pypi/attrs">PyPI</a> | <a href="https://pyup.io/changelogs/attrs/">Changelog</a> | <a href="http://www.attrs.org/">Homepage</a> 



### Changelog
> 
>### 17.3.0

>-------------------

>Backward-incompatible Changes
>^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

>- Attributes are not defined on the class body anymore.

>  This means that if you define a class ``C`` with an attribute ``x``, the class will *not* have an attribute ``x`` for introspection anymore.
>  Instead of ``C.x``, use ``attr.fields(C).x`` or look at ``C.__attrs_attrs__``.
>  The old behavior has been deprecated since version 16.1.
>  (`253 &lt;https://github.com/python-attrs/attrs/issues/253&gt;`_)


>Changes
>^^^^^^^

>- ``super()`` and ``__class__`` now work on Python 3 when ``slots=True``.
>  (`102 &lt;https://github.com/python-attrs/attrs/issues/102&gt;`_, `226 &lt;https://github.com/python-attrs/attrs/issues/226&gt;`_, `269 &lt;https://github.com/python-attrs/attrs/issues/269&gt;`_, `270 &lt;https://github.com/python-attrs/attrs/issues/270&gt;`_, `272 &lt;https://github.com/python-attrs/attrs/issues/272&gt;`_)
>- Added ``type`` argument to ``attr.ib()`` and corresponding ``type`` attribute to ``attr.Attribute``.

>  This change paves the way for automatic type checking and serialization (though as of this release ``attrs`` does not make use of it).
>  In Python 3.6 or higher, the value of ``attr.Attribute.type`` can alternately be set using variable type annotations
>  (see `PEP 526 &lt;https://www.python.org/dev/peps/pep-0526/&gt;`_). (`151 &lt;https://github.com/python-attrs/attrs/issues/151&gt;`_, `214 &lt;https://github.com/python-attrs/attrs/issues/214&gt;`_, `215 &lt;https://github.com/python-attrs/attrs/issues/215&gt;`_, `239 &lt;https://github.com/python-attrs/attrs/issues/239&gt;`_)
>- The combination of ``str=True`` and ``slots=True`` now works on Python 2.
>  (`198 &lt;https://github.com/python-attrs/attrs/issues/198&gt;`_)
>- ``attr.Factory`` is hashable again. (`204
>  &lt;https://github.com/python-attrs/attrs/issues/204&gt;`_)
>- Subclasses now can overwrite attribute definitions of their superclass.

>  That means that you can -- for example -- change the default value for an attribute by redefining it.
>  (`221 &lt;https://github.com/python-attrs/attrs/issues/221&gt;`_, `229 &lt;https://github.com/python-attrs/attrs/issues/229&gt;`_)
>- Added new option ``auto_attribs`` to ``attr.s`` that allows to collect annotated fields without setting them to ``attr.ib()``.

>  Setting a field to an ``attr.ib()`` is still possible to supply options like validators.
>  Setting it to any other value is treated like it was passed as ``attr.ib(default=value)`` -- passing an instance of ``attr.Factory`` also works as expected.
>  (`262 &lt;https://github.com/python-attrs/attrs/issues/262&gt;`_, `277 &lt;https://github.com/python-attrs/attrs/issues/277&gt;`_)
>- Instances of classes created using ``attr.make_class()`` can now be pickled.
>  (`282 &lt;https://github.com/python-attrs/attrs/issues/282&gt;`_)


>----








*Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.*

Happy merging! 🤖
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