Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

On 1/7/2018 12:25 PM, Guido van Rossum wrote:
On Sun, Jan 7, 2018 at 9:09 AM, Eric V. Smith > wrote:


On 1/3/2018 1:17 PM, Eric V. Smith wrote:

I’ll open an issue after I have time to read this thread and
comment on it.


https://bugs.python.org/issue32513 
I need to think though how __eq__ and __ne__ work, as well as the
ordering operators.

My specific concern with __ne__ is that there's one flag to control
their generation, but python will use "not __eq__" if you don't
provide __ne__. I need to think through what happens if the user
only provides __eq__: does dataclasses do nothing, does it add
__ne__, and how does this interact with a base class that does
provide __ne__.


Maybe dataclasses should only ever provide __eq__ and always assume 
Python's default for __ne__ kicks in? If that's not acceptable (maybe 
there are cases where a user did write an explicit __ne__ that needs to 
be overridden) I would recommend the following rule:


- If there's an __eq__, don't do anything (regardless of whether there's 
an __ne__)
- If there no __eq__ but there is an __ne__, generate __eq__ but don't 
generate __ne__

- If neither exists, generate both


I've added my proposal on issue 32513:
https://bugs.python.org/issue32513#msg310392

It's long, so I won't repeat it here. The only really confusing part is 
__hash__ and its interaction with __eq__.


Eric.
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

On Sun, Jan 7, 2018 at 9:09 AM, Eric V. Smith  wrote:

> On 1/3/2018 1:17 PM, Eric V. Smith wrote:
>
>> I’ll open an issue after I have time to read this thread and comment on
>> it.
>>
>
> https://bugs.python.org/issue32513
> I need to think though how __eq__ and __ne__ work, as well as the ordering
> operators.
>
> My specific concern with __ne__ is that there's one flag to control their
> generation, but python will use "not __eq__" if you don't provide __ne__. I
> need to think through what happens if the user only provides __eq__: does
> dataclasses do nothing, does it add __ne__, and how does this interact with
> a base class that does provide __ne__.


Maybe dataclasses should only ever provide __eq__ and always assume
Python's default for __ne__ kicks in? If that's not acceptable (maybe there
are cases where a user did write an explicit __ne__ that needs to be
overridden) I would recommend the following rule:

- If there's an __eq__, don't do anything (regardless of whether there's an
__ne__)
- If there no __eq__ but there is an __ne__, generate __eq__ but don't
generate __ne__
- If neither exists, generate both

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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

On 1/3/2018 1:17 PM, Eric V. Smith wrote:

I’ll open an issue after I have time to read this thread and comment on it.


https://bugs.python.org/issue32513
I need to think though how __eq__ and __ne__ work, as well as the 
ordering operators.


My specific concern with __ne__ is that there's one flag to control 
their generation, but python will use "not __eq__" if you don't provide 
__ne__. I need to think through what happens if the user only provides 
__eq__: does dataclasses do nothing, does it add __ne__, and how does 
this interact with a base class that does provide __ne__.


Eric.
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

On 1/5/2018 2:11 PM, Eric V. Smith wrote:

On 1/5/2018 2:09 PM, Guido van Rossum wrote:
I'm normally no big fan of things that take either a class or an 
instance, but since fields() does this, I think is_dataclass() should 
to. And that's the name I'd choose. OK on the pseudo-fields.


Sounds good. I'll open a bpo issue.


https://bugs.python.org/issue32499

I'm slowly reading through the rest of this thread and will respond this 
weekend.


Eric.

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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

On 1/5/2018 2:09 PM, Guido van Rossum wrote:
I'm normally no big fan of things that take either a class or an 
instance, but since fields() does this, I think is_dataclass() should 
to. And that's the name I'd choose. OK on the pseudo-fields.


Sounds good. I'll open a bpo issue.

Eric.

On Fri, Jan 5, 2018 at 11:06 AM, Eric V. Smith > wrote:


On 1/5/2018 12:58 PM, Guido van Rossum wrote:

Hm. I don't know that people will conclude that checking for a
dataclass is an anti-pattern. They'll probably just invent a
myriad of different hacks like the one you showed. I recommend
making it public.


I'm trying to track down the original discussion. We got bogged down
on whether it worked for classes or instances or both, then we got
tied up in naming it (surprise!), then it looks like we decided to
just not include it since you could make those decisions for yourself.

I think the discussion is buried in this thread:
https://mail.python.org/pipermail/python-dev/2017-November/150966.html


Which references:
https://github.com/ericvsmith/dataclasses/issues/99


So, ignoring the naming issue, I think if we want to revive it, the
question is: should isdataclass() return True on just instances,
just classes, or both? And should it ever raise an exception, or
just return False?

I still worry a bit about ClassVar and InitVar being potentially
useful but I concede I have no use case so I'll drop it.


IIRC, we decided that we could add a parameter to
dataclasses.fields() if we ever wanted to return pseudo-fields. But
no one came up with a use case.

Eric.


On Fri, Jan 5, 2018 at 8:43 AM, Eric V. Smith

>> wrote:

     On 1/5/2018 11:24 AM, Guido van Rossum wrote:

         On Fri, Jan 5, 2018 at 5:08 AM, Eric V. Smith
         
>
         

         >
 

         >>


         I was specifically talking about the name and contents of
         __dataclass_fields__, which are not documented by the
PEP. I
         expect it's inevitable that people will be looking at this
         (since they can see it in the source code). Or do you
recommend
         that people use dataclasses.fields() and catch ValueError?


     The expectation is to use dataclasses.fields(). Both it and
     __dataclass_fields__ contain the fields for this class and the
     parents. The only difference is the pseudo-fields.

     I can add some words describing .fields() returning which
fields are
     present.

         I notice that _isdataclass() exists but is private and
I don't
         recall why.


     I think the argument was that it's an anti-pattern, and if you
     really want to know, just call dataclasses.fields() and
catch the
     TypeError. I have this in a helper file:

     def isdataclass(obj):
          """Returns True for dataclass classes and instances."""
          try:
              dataclasses.fields(obj)
              return True
          except TypeError:
              return False


     (Also now I'm curious what

Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

I'm normally no big fan of things that take either a class or an instance,
but since fields() does this, I think is_dataclass() should to. And that's
the name I'd choose. OK on the pseudo-fields.

On Fri, Jan 5, 2018 at 11:06 AM, Eric V. Smith  wrote:

> On 1/5/2018 12:58 PM, Guido van Rossum wrote:
>
>> Hm. I don't know that people will conclude that checking for a dataclass
>> is an anti-pattern. They'll probably just invent a myriad of different
>> hacks like the one you showed. I recommend making it public.
>>
>
> I'm trying to track down the original discussion. We got bogged down on
> whether it worked for classes or instances or both, then we got tied up in
> naming it (surprise!), then it looks like we decided to just not include it
> since you could make those decisions for yourself.
>
> I think the discussion is buried in this thread:
> https://mail.python.org/pipermail/python-dev/2017-November/150966.html
>
> Which references:
> https://github.com/ericvsmith/dataclasses/issues/99
>
> So, ignoring the naming issue, I think if we want to revive it, the
> question is: should isdataclass() return True on just instances, just
> classes, or both? And should it ever raise an exception, or just return
> False?
>
> I still worry a bit about ClassVar and InitVar being potentially useful
>> but I concede I have no use case so I'll drop it.
>>
>
> IIRC, we decided that we could add a parameter to dataclasses.fields() if
> we ever wanted to return pseudo-fields. But no one came up with a use case.
>
> Eric.
>
>
>> On Fri, Jan 5, 2018 at 8:43 AM, Eric V. Smith > > wrote:
>>
>> On 1/5/2018 11:24 AM, Guido van Rossum wrote:
>>
>> On Fri, Jan 5, 2018 at 5:08 AM, Eric V. Smith
>> 
>> >> wrote:
>>
>>  On 1/2/2018 12:01 AM, Guido van Rossum wrote:
>>
>>  Yes, there's a class variable (__dataclass_fields__) that
>>  identifies the parent fields. The PEP doesn't mention
>> this or
>>  the fact that special methods (like __repr__ and
>> __init__) can
>>  tell whether a base class is a dataclass. It probably
>> should
>>  though. (@Eric)
>>
>>
>>  I think that's covered in this section:
>> https://www.python.org/dev/peps/pep-0557/#inheritance
>> 
>>  > >
>>
>>
>> I was specifically talking about the name and contents of
>> __dataclass_fields__, which are not documented by the PEP. I
>> expect it's inevitable that people will be looking at this
>> (since they can see it in the source code). Or do you recommend
>> that people use dataclasses.fields() and catch ValueError?
>>
>>
>> The expectation is to use dataclasses.fields(). Both it and
>> __dataclass_fields__ contain the fields for this class and the
>> parents. The only difference is the pseudo-fields.
>>
>> I can add some words describing .fields() returning which fields are
>> present.
>>
>> I notice that _isdataclass() exists but is private and I don't
>> recall why.
>>
>>
>> I think the argument was that it's an anti-pattern, and if you
>> really want to know, just call dataclasses.fields() and catch the
>> TypeError. I have this in a helper file:
>>
>> def isdataclass(obj):
>>  """Returns True for dataclass classes and instances."""
>>  try:
>>  dataclasses.fields(obj)
>>  return True
>>  except TypeError:
>>  return False
>>
>>
>> (Also now I'm curious what
>>
>> the "pseudo-fields" are that fields() ignores, but that's OT.)
>>
>>
>> ClassVar and InitVar "fields". dataclasses.fields() doesn't return
>> them.
>>
>> Eric.
>>
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>>
>>
>>
>> --
>> --Guido van Rossum (python.org/~guido )
>>
>>
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>>
>


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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

On 1/5/2018 12:58 PM, Guido van Rossum wrote:
Hm. I don't know that people will conclude that checking for a dataclass 
is an anti-pattern. They'll probably just invent a myriad of different 
hacks like the one you showed. I recommend making it public.


I'm trying to track down the original discussion. We got bogged down on 
whether it worked for classes or instances or both, then we got tied up 
in naming it (surprise!), then it looks like we decided to just not 
include it since you could make those decisions for yourself.


I think the discussion is buried in this thread:
https://mail.python.org/pipermail/python-dev/2017-November/150966.html

Which references:
https://github.com/ericvsmith/dataclasses/issues/99

So, ignoring the naming issue, I think if we want to revive it, the 
question is: should isdataclass() return True on just instances, just 
classes, or both? And should it ever raise an exception, or just return 
False?


I still worry a bit about ClassVar and InitVar being potentially useful 
but I concede I have no use case so I'll drop it.


IIRC, we decided that we could add a parameter to dataclasses.fields() 
if we ever wanted to return pseudo-fields. But no one came up with a use 
case.


Eric.



On Fri, Jan 5, 2018 at 8:43 AM, Eric V. Smith > wrote:


On 1/5/2018 11:24 AM, Guido van Rossum wrote:

On Fri, Jan 5, 2018 at 5:08 AM, Eric V. Smith

>> wrote:

     On 1/2/2018 12:01 AM, Guido van Rossum wrote:

         Yes, there's a class variable (__dataclass_fields__) that
         identifies the parent fields. The PEP doesn't mention
this or
         the fact that special methods (like __repr__ and
__init__) can
         tell whether a base class is a dataclass. It probably
should
         though. (@Eric)


     I think that's covered in this section:
https://www.python.org/dev/peps/pep-0557/#inheritance

     >


I was specifically talking about the name and contents of
__dataclass_fields__, which are not documented by the PEP. I
expect it's inevitable that people will be looking at this
(since they can see it in the source code). Or do you recommend
that people use dataclasses.fields() and catch ValueError?


The expectation is to use dataclasses.fields(). Both it and
__dataclass_fields__ contain the fields for this class and the
parents. The only difference is the pseudo-fields.

I can add some words describing .fields() returning which fields are
present.

I notice that _isdataclass() exists but is private and I don't
recall why.


I think the argument was that it's an anti-pattern, and if you
really want to know, just call dataclasses.fields() and catch the
TypeError. I have this in a helper file:

def isdataclass(obj):
     """Returns True for dataclass classes and instances."""
     try:
         dataclasses.fields(obj)
         return True
     except TypeError:
         return False


(Also now I'm curious what

the "pseudo-fields" are that fields() ignores, but that's OT.)


ClassVar and InitVar "fields". dataclasses.fields() doesn't return them.

Eric.

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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

Hm. I don't know that people will conclude that checking for a dataclass is
an anti-pattern. They'll probably just invent a myriad of different hacks
like the one you showed. I recommend making it public.

I still worry a bit about ClassVar and InitVar being potentially useful but
I concede I have no use case so I'll drop it.

On Fri, Jan 5, 2018 at 8:43 AM, Eric V. Smith  wrote:

> On 1/5/2018 11:24 AM, Guido van Rossum wrote:
>
>> On Fri, Jan 5, 2018 at 5:08 AM, Eric V. Smith > > wrote:
>>
>> On 1/2/2018 12:01 AM, Guido van Rossum wrote:
>>
>> Yes, there's a class variable (__dataclass_fields__) that
>> identifies the parent fields. The PEP doesn't mention this or
>> the fact that special methods (like __repr__ and __init__) can
>> tell whether a base class is a dataclass. It probably should
>> though. (@Eric)
>>
>>
>> I think that's covered in this section:
>> https://www.python.org/dev/peps/pep-0557/#inheritance
>> 
>>
>>
>> I was specifically talking about the name and contents of
>> __dataclass_fields__, which are not documented by the PEP. I expect it's
>> inevitable that people will be looking at this (since they can see it in
>> the source code). Or do you recommend that people use dataclasses.fields()
>> and catch ValueError?
>>
>
> The expectation is to use dataclasses.fields(). Both it and
> __dataclass_fields__ contain the fields for this class and the parents. The
> only difference is the pseudo-fields.
>
> I can add some words describing .fields() returning which fields are
> present.
>
> I notice that _isdataclass() exists but is private and I don't recall why.
>>
>
> I think the argument was that it's an anti-pattern, and if you really want
> to know, just call dataclasses.fields() and catch the TypeError. I have
> this in a helper file:
>
> def isdataclass(obj):
> """Returns True for dataclass classes and instances."""
> try:
> dataclasses.fields(obj)
> return True
> except TypeError:
> return False
>
>
> (Also now I'm curious what
>
>> the "pseudo-fields" are that fields() ignores, but that's OT.)
>>
>
> ClassVar and InitVar "fields". dataclasses.fields() doesn't return them.
>
> Eric.
>
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

On 1/5/2018 11:24 AM, Guido van Rossum wrote:
On Fri, Jan 5, 2018 at 5:08 AM, Eric V. Smith > wrote:


On 1/2/2018 12:01 AM, Guido van Rossum wrote:

Yes, there's a class variable (__dataclass_fields__) that
identifies the parent fields. The PEP doesn't mention this or
the fact that special methods (like __repr__ and __init__) can
tell whether a base class is a dataclass. It probably should
though. (@Eric)


I think that's covered in this section:
https://www.python.org/dev/peps/pep-0557/#inheritance



I was specifically talking about the name and contents of 
__dataclass_fields__, which are not documented by the PEP. I expect it's 
inevitable that people will be looking at this (since they can see it in 
the source code). Or do you recommend that people use 
dataclasses.fields() and catch ValueError?


The expectation is to use dataclasses.fields(). Both it and 
__dataclass_fields__ contain the fields for this class and the parents. 
The only difference is the pseudo-fields.


I can add some words describing .fields() returning which fields are 
present.


I notice that _isdataclass() 
exists but is private and I don't recall why. 


I think the argument was that it's an anti-pattern, and if you really 
want to know, just call dataclasses.fields() and catch the TypeError. I 
have this in a helper file:


def isdataclass(obj):
"""Returns True for dataclass classes and instances."""
try:
dataclasses.fields(obj)
return True
except TypeError:
return False


(Also now I'm curious what

the "pseudo-fields" are that fields() ignores, but that's OT.)


ClassVar and InitVar "fields". dataclasses.fields() doesn't return them.

Eric.

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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

On Fri, Jan 5, 2018 at 5:08 AM, Eric V. Smith  wrote:

> On 1/2/2018 12:01 AM, Guido van Rossum wrote:
>
>> Yes, there's a class variable (__dataclass_fields__) that identifies the
>> parent fields. The PEP doesn't mention this or the fact that special
>> methods (like __repr__ and __init__) can tell whether a base class is a
>> dataclass. It probably should though. (@Eric)
>>
>
> I think that's covered in this section: https://www.python.org/dev/pep
> s/pep-0557/#inheritance
>

I was specifically talking about the name and contents of __dataclass_fields__,
which are not documented by the PEP. I expect it's inevitable that people
will be looking at this (since they can see it in the source code). Or do
you recommend that people use dataclasses.fields() and catch ValueError? I
notice that _isdataclass() exists but is private and I don't recall why.
(Also now I'm curious what the "pseudo-fields" are that fields() ignores,
but that's OT.)

-- 
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

On 1/2/2018 12:01 AM, Guido van Rossum wrote:
On Mon, Jan 1, 2018 at 8:50 PM, Ethan Smith > wrote:




On Mon, Jan 1, 2018 at 5:03 PM, Chris Barker > wrote:

On Sat, Dec 30, 2017 at 7:27 AM, Stephen J. Turnbull
> wrote:

   Just use the simple rule that a new
__repr__ is generated unless provided in the dataclass.


are we only talking about __repr__ here ???

I interpreted Guido's proposal as being about all methods -- we
_may_ want something special for __repr__, but I hope not.

[...]


I interpreted this to be for all methods as well, which makes sense.
Special casing just __repr__ doesn't make sense to me, but I will
wait for Guido to clarify.


Indeed, I just wrote __repr__ for simplicity. This should apply to all 
special methods. (Though there may be some complications for 
__eq__/__ne__ and for the ordering operators.)


On Mon, Jan 1, 2018 at 9:44 PM, Chris Barker > wrote:


On Mon, Jan 1, 2018 at 7:50 PM, Ethan Smith > wrote:


Will you get the "right" __repr__ now if you derive a
dataclass from a dataclass? That would be a nice feature.


The __repr__ will be generated by the child dataclass unless the
user overrides it. So I believe this is the "right" __repr__.


what I was wondering is if the child will know about all the fields
in the parent -- so it could make a full __repr__.


Yes, there's a class variable (__dataclass_fields__) that identifies the 
parent fields. The PEP doesn't mention this or the fact that special 
methods (like __repr__ and __init__) can tell whether a base class is a 
dataclass. It probably should though. (@Eric)


I think that's covered in this section: 
https://www.python.org/dev/peps/pep-0557/#inheritance


Eric.
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

I’ll open an issue after I have time to read this thread and comment on it. 

--
Eric.

> On Jan 3, 2018, at 12:31 PM, Ivan Levkivskyi  wrote:
> 
> I like the Guido's proposal, i.e.
> 
> if '__repr__' not in cls.__dict__:
> ...  # generate the method
> 
> etc. I didn't find an issue to track this. Maybe we should open one?
> 
> --
> Ivan
> 
> 
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ivan Levkivskyi]

I like the Guido's proposal, i.e.

if '__repr__' not in cls.__dict__:
...  # generate the method

etc. I didn't find an issue to track this. Maybe we should open one?

--
Ivan
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Stephen J. Turnbull]

Chris Barker writes:

 > are we only talking about __repr__ here ???

I am, because I haven't thought about the other methods, except to
note I find it hard to imagine a use case *for me* that would require
any of them.  That sorta disqualifies me from comment. ;-)  I assumed
others were talking about all of the dataclass autogenerated methods,
though.

 > And the fact that you can optionally tell it not to in some particular case
 > keeps full flexibility.

AFAICS there is no question about that, just about *how* you indicate
that you do or don't want autobogotification.

 > Will you get the "right" __repr__ now if you derive a datacalss from a
 > dataclass? That would be a nice feature.

I hadn't thought about that: I wouldn't call it "nice", I'd say it's a
sine-qua-back-to-the-drawing-board.

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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

On Mon, Jan 1, 2018 at 8:50 PM, Ethan Smith  wrote:

>
>
> On Mon, Jan 1, 2018 at 5:03 PM, Chris Barker 
> wrote:
>
>> On Sat, Dec 30, 2017 at 7:27 AM, Stephen J. Turnbull <
>> turnbull.stephen...@u.tsukuba.ac.jp> wrote:
>>
>>>   Just use the simple rule that a new
>>> __repr__ is generated unless provided in the dataclass.
>>>
>>
>> are we only talking about __repr__ here ???
>>
>> I interpreted Guido's proposal as being about all methods -- we _may_
>> want something special for __repr__, but I hope not.
>>
> [...]
>>
>
> I interpreted this to be for all methods as well, which makes sense.
> Special casing just __repr__ doesn't make sense to me, but I will wait for
> Guido to clarify.
>

Indeed, I just wrote __repr__ for simplicity. This should apply to all
special methods. (Though there may be some complications for __eq__/__ne__
and for the ordering operators.)

On Mon, Jan 1, 2018 at 9:44 PM, Chris Barker  wrote:

> On Mon, Jan 1, 2018 at 7:50 PM, Ethan Smith  wrote:
>
>>
>> Will you get the "right" __repr__ now if you derive a dataclass from a
>>> dataclass? That would be a nice feature.
>>>
>>
>>
>>
> The __repr__ will be generated by the child dataclass unless the user
>> overrides it. So I believe this is the "right" __repr__.
>>
>
> what I was wondering is if the child will know about all the fields in the
> parent -- so it could make a full __repr__.
>

Yes, there's a class variable (__dataclass_fields__) that identifies the
parent fields. The PEP doesn't mention this or the fact that special
methods (like __repr__ and __init__) can tell whether a base class is a
dataclass. It probably should though. (@Eric)

-- 
--Guido van Rossum (python.org/~guido)
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Chris Barker]

On Mon, Jan 1, 2018 at 7:50 PM, Ethan Smith  wrote:

>
> Will you get the "right" __repr__ now if you derive a datacalss from a
>> dataclass? That would be a nice feature.
>>
>
>
> The __repr__ will be generated by the child dataclass unless the user
> overrides it. So I believe this is the "right" __repr__.
>

what I was wondering is if the child will know about all the fields in the
parent -- so it could make a full __repr__.

-CHB




> ~>Ethan Smith
>
>>
>> -CHB
>>
>> --
>>
>> Christopher Barker, Ph.D.
>> Oceanographer
>>
>> Emergency Response Division
>> NOAA/NOS/OR(206) 526-6959   voice
>> 7600 Sand Point Way NE
>> 
>>   (206) 526-6329   fax
>> Seattle, WA  98115   (206) 526-6317   main reception
>>
>> chris.bar...@noaa.gov
>>
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>


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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Smith]

On Mon, Jan 1, 2018 at 5:03 PM, Chris Barker  wrote:

> On Sat, Dec 30, 2017 at 7:27 AM, Stephen J. Turnbull <
> turnbull.stephen...@u.tsukuba.ac.jp> wrote:
>
>>   Just use the simple rule that a new
>> __repr__ is generated unless provided in the dataclass.
>>
>
> are we only talking about __repr__ here ???
>
> I interpretted Guido's proposal as being about all methods -- we _may_
> want something special for __repr__, but I hope not.
>
> But +1 for Guido's proposal, not only because it's easy to explain, but
> because it more naturally follows the usual inheritance logic:
>
> The decorator's entire point is to auto-generate boilerplate code for you.
> Once it's done that it shouldn't, in the end, behave any differently than
> if you hand wrote that code. If you hand wrote the methods that the
> decorator creates for you, they would override any base class versions. So
> that's what it should do.
>
> And the fact that you can optionally tell it not to in some particular
> case keeps full flexibility.
>
> -CHB
>

I interpreted this to be for all methods as well, which makes sense.
Special casing just __repr__ doesn't make sense to me, but I will wait for
Guido to clarify.

>
> > I grant that there may be many reasons why one would be deriving
>
>> dataclasses from dataclasses
>
>
> Will you get the "right" __repr__ now if you derive a datacalss from a
> dataclass? That would be a nice feature.
>

The __repr__ will be generated by the child dataclass unless the user
overrides it. So I believe this is the "right" __repr__.

~>Ethan Smith

>
> -CHB
>
> --
>
> Christopher Barker, Ph.D.
> Oceanographer
>
> Emergency Response Division
> NOAA/NOS/OR(206) 526-6959   voice
> 7600 Sand Point Way NE   (206) 526-6329   fax
> Seattle, WA  98115   (206) 526-6317   main reception
>
> chris.bar...@noaa.gov
>
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Chris Barker]

On Sat, Dec 30, 2017 at 7:27 AM, Stephen J. Turnbull <
turnbull.stephen...@u.tsukuba.ac.jp> wrote:

>   Just use the simple rule that a new
> __repr__ is generated unless provided in the dataclass.
>

are we only talking about __repr__ here ???

I interpretted Guido's proposal as being about all methods -- we _may_ want
something special for __repr__, but I hope not.

But +1 for Guido's proposal, not only because it's easy to explain, but
because it more naturally follows the usual inheritance logic:

The decorator's entire point is to auto-generate boilerplate code for you.
Once it's done that it shouldn't, in the end, behave any differently than
if you hand wrote that code. If you hand wrote the methods that the
decorator creates for you, they would override any base class versions. So
that's what it should do.

And the fact that you can optionally tell it not to in some particular case
keeps full flexibility.

-CHB

> I grant that there may be many reasons why one would be deriving

> dataclasses from dataclasses


Will you get the "right" __repr__ now if you derive a datacalss from a
dataclass? That would be a nice feature.

-CHB

-- 

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR(206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115   (206) 526-6317   main reception

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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Stephen J. Turnbull]

Ethan Furman writes:

 > Good point.  So auto-generate a new __repr__ if:
 > 
 > - one is not provided, and
 > - existing __repr__ is either:
 >- object.__repr__, or
 >- a previous dataclass __repr__

-0.5 I'm with Guido here.  Just use the simple rule that a new
__repr__ is generated unless provided in the dataclass.

The logic I use (Guido seems to be just arguing for "simple" for now)
is that a dataclass is "usually" going to add fields, which you
"normally" want exposed in the repr, and that means that an
*inherited* __repr__ is going to be broken in some sense.  The code
author will disagree in "a few" cases, and in those cases they will
use repr=False to override.

I grant that there may be many reasons why one would be deriving
dataclasses from dataclasses without adding fields that should be in
the repr, so the quote marks above may be taken to be indications of
my lack of imagination. ;-)

Here's to 2018.  It *has* to be better than 2017 -- there will be a
Python feature release!

Steve

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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Eric V. Smith]

I’m traveling until next week, and haven’t had time to read any of these 
emails. I’ll look at them when I return. 

--
Eric.

> On Dec 30, 2017, at 5:20 AM, Raymond Hettinger  
> wrote:
> 
> 
>> On Dec 29, 2017, at 4:52 PM, Guido van Rossum  wrote:
>> 
>> I still think it should overrides anything that's just inherited but nothing 
>> that's defined in the class being decorated.
> 
> This has the virtue of being easy to explain, and it will help with debugging 
> by honoring the code proximate to the decorator :-)
> 
> For what it is worth, the functools.total_ordering class decorator does 
> something similar -- though not exactly the same.  A root comparison method 
> is considered user-specified if it is different than the default method 
> provided by object: 
> 
>def total_ordering(cls):
>"""Class decorator that fills in missing ordering methods"""
># Find user-defined comparisons (not those inherited from object).
>roots = {op for op in _convert if getattr(cls, op, None) is not 
> getattr(object, op, None)}
>...
> 
> The @dataclass decorator has a much broader mandate and we have almost no 
> experience with it, so it is hard to know what legitimate use cases will 
> arise.
> 
> 
> Raymond
> 
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Raymond Hettinger]

> On Dec 29, 2017, at 4:52 PM, Guido van Rossum  wrote:
> 
> I still think it should overrides anything that's just inherited but nothing 
> that's defined in the class being decorated.

This has the virtue of being easy to explain, and it will help with debugging 
by honoring the code proximate to the decorator :-)

For what it is worth, the functools.total_ordering class decorator does 
something similar -- though not exactly the same.  A root comparison method is 
considered user-specified if it is different than the default method provided 
by object: 

def total_ordering(cls):
"""Class decorator that fills in missing ordering methods"""
# Find user-defined comparisons (not those inherited from object).
roots = {op for op in _convert if getattr(cls, op, None) is not 
getattr(object, op, None)}
...

The @dataclass decorator has a much broader mandate and we have almost no 
experience with it, so it is hard to know what legitimate use cases will arise.


Raymond

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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Smith]

Okay, I think Guido's proposal is a good compromise.

I already have a branch of dataclasses that should implement that behavior,
so perhaps it was meant to be. :)

~>Ethan Smith

On Fri, Dec 29, 2017 at 5:13 PM, Nick Coghlan  wrote:

>
>
> On 30 Dec. 2017 11:01 am, "Ethan Smith"  wrote:
>
>
>
> On Fri, Dec 29, 2017 at 4:52 PM, Guido van Rossum 
> wrote:
>
>> I still think it should overrides anything that's just inherited but
>> nothing that's defined in the class being decorated.
>>
>>
> Could you explain why you are of this opinion? Is it a concern about
> complexity of implementation?
>
>
> Adding a new method to a base class shouldn't risk breaking existing
> subclasses.
>
> If folks want to retain the base class implementation, they can request
> that explicitly (and doing so isn't redundant at the point of subclass
> definition the way it is for methods defined in the class body).
>
> Cheers,
> Nick.
>
>
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Nick Coghlan]

On 30 Dec. 2017 11:01 am, "Ethan Smith"  wrote:



On Fri, Dec 29, 2017 at 4:52 PM, Guido van Rossum  wrote:

> I still think it should overrides anything that's just inherited but
> nothing that's defined in the class being decorated.
>
>
Could you explain why you are of this opinion? Is it a concern about
complexity of implementation?


Adding a new method to a base class shouldn't risk breaking existing
subclasses.

If folks want to retain the base class implementation, they can request
that explicitly (and doing so isn't redundant at the point of subclass
definition the way it is for methods defined in the class body).

Cheers,
Nick.
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

No, I am concerned about the rule being too complex to explain, and about
surprising effects when the base changes (action at a distance).

I also don't necessarily think "we all agree" that what attrs does is
wrong, but the rule I propose seems reasonable.

On Dec 29, 2017 5:58 PM, "Ethan Smith"  wrote:

>
>
> On Fri, Dec 29, 2017 at 4:52 PM, Guido van Rossum 
> wrote:
>
>> I still think it should overrides anything that's just inherited but
>> nothing that's defined in the class being decorated.
>>
>>
> Could you explain why you are of this opinion? Is it a concern about
> complexity of implementation?
>
>
>> On Dec 29, 2017 5:43 PM, "Nathaniel Smith"  wrote:
>>
>>> On Fri, Dec 29, 2017 at 12:30 PM, Ethan Furman 
>>> wrote:
>>> > Good point.  So auto-generate a new __repr__ if:
>>> >
>>> > - one is not provided, and
>>> > - existing __repr__ is either:
>>> >   - object.__repr__, or
>>> >   - a previous dataclass __repr__
>>> >
>>> > And if the auto default doesn't work for one's use-case, use the
>>> keyword
>>> > parameter to specify what you want.
>>>
>>> What does attrs do here?
>>>
>>> -n
>>>
>>> --
>>> Nathaniel J. Smith -- https://vorpus.org
>>> ___
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Furman]

On Fri, Dec 29, 2017 at 12:30 PM, Ethan Furman wrote:
> Good point.  So auto-generate a new __repr__ if:
>
> - one is not provided, and
> - existing __repr__ is either:
>   - object.__repr__, or
>   - a previous dataclass __repr__
>
> And if the auto default doesn't work for one's use-case, use the keyword
> parameter to specify what you want.

On Dec 29, 2017 5:43 PM, "Nathaniel Smith" wrote:
> What does attrs do here?

On 12/29/2017 04:52 PM, Ethan Smith wrote:
> attrs just silently overwrites any user provided __repr__ unless you provide
> repr=False to attr.s.

On 12/29/2017 04:52 PM, Guido van Rossum wrote:
> I still think it should overrides anything that's just inherited but nothing
> that's defined in the class being decorated.

I can certainly live with that.

--
~Ethan~
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Smith]

On Fri, Dec 29, 2017 at 4:52 PM, Guido van Rossum  wrote:

> I still think it should overrides anything that's just inherited but
> nothing that's defined in the class being decorated.
>
>
Could you explain why you are of this opinion? Is it a concern about
complexity of implementation?


> On Dec 29, 2017 5:43 PM, "Nathaniel Smith"  wrote:
>
>> On Fri, Dec 29, 2017 at 12:30 PM, Ethan Furman 
>> wrote:
>> > Good point.  So auto-generate a new __repr__ if:
>> >
>> > - one is not provided, and
>> > - existing __repr__ is either:
>> >   - object.__repr__, or
>> >   - a previous dataclass __repr__
>> >
>> > And if the auto default doesn't work for one's use-case, use the keyword
>> > parameter to specify what you want.
>>
>> What does attrs do here?
>>
>> -n
>>
>> --
>> Nathaniel J. Smith -- https://vorpus.org
>> ___
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Smith]

attrs just silently overwrites any user provided __repr__ unless you
provide repr=False to attr.s.

I think we can all agree that if nothing else, silently overwriting
unconditionally is not what we want for dataclasses.

On Fri, Dec 29, 2017 at 4:38 PM, Nathaniel Smith  wrote:

> On Fri, Dec 29, 2017 at 12:30 PM, Ethan Furman  wrote:
> > Good point.  So auto-generate a new __repr__ if:
> >
> > - one is not provided, and
> > - existing __repr__ is either:
> >   - object.__repr__, or
> >   - a previous dataclass __repr__
> >
> > And if the auto default doesn't work for one's use-case, use the keyword
> > parameter to specify what you want.
>
> What does attrs do here?
>
> -n
>
> --
> Nathaniel J. Smith -- https://vorpus.org
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

I still think it should overrides anything that's just inherited but
nothing that's defined in the class being decorated.

On Dec 29, 2017 5:43 PM, "Nathaniel Smith"  wrote:

> On Fri, Dec 29, 2017 at 12:30 PM, Ethan Furman  wrote:
> > Good point.  So auto-generate a new __repr__ if:
> >
> > - one is not provided, and
> > - existing __repr__ is either:
> >   - object.__repr__, or
> >   - a previous dataclass __repr__
> >
> > And if the auto default doesn't work for one's use-case, use the keyword
> > parameter to specify what you want.
>
> What does attrs do here?
>
> -n
>
> --
> Nathaniel J. Smith -- https://vorpus.org
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Nathaniel Smith]

On Fri, Dec 29, 2017 at 12:30 PM, Ethan Furman  wrote:
> Good point.  So auto-generate a new __repr__ if:
>
> - one is not provided, and
> - existing __repr__ is either:
>   - object.__repr__, or
>   - a previous dataclass __repr__
>
> And if the auto default doesn't work for one's use-case, use the keyword
> parameter to specify what you want.

What does attrs do here?

-n

-- 
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Smith]

On Fri, Dec 29, 2017 at 12:30 PM, Ethan Furman  wrote:

> On 12/29/2017 11:55 AM, Ethan Smith wrote:
>
>> On Fri, Dec 29, 2017 at 11:37 AM, Ethan Furman wrote:
>>
>
> It is possible to determine whether an existing __repr__ is from 'object'
>>>
>> >> or not, and only provide one if that is the case.  I think that should
> be
> >> the default, with 'repr = True' for those cases where a new,
> auto-generated,
>
>> __repr__ is desired.
>>>
>>
>> The only other thing you'd want to handle is to cover inheriting from
>> another dataclass. e.g., if I have dataclass with
>> attribute a, and subclass it to add attribute b, I'd want both in the
>> repr.
>>
>
> Good point.  So auto-generate a new __repr__ if:
>
> - one is not provided, and
> - existing __repr__ is either:
>   - object.__repr__, or
>   - a previous dataclass __repr__
>
> And if the auto default doesn't work for one's use-case, use the keyword
> parameter to specify what you want.
>
>
> --
> ~Ethan~
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Furman]

On 12/29/2017 11:55 AM, Ethan Smith wrote:

On Fri, Dec 29, 2017 at 11:37 AM, Ethan Furman wrote:



It is possible to determine whether an existing __repr__ is from 'object'

>> or not, and only provide one if that is the case.  I think that should be
>> the default, with 'repr = True' for those cases where a new, auto-generated,

__repr__ is desired.


The only other thing you'd want to handle is to cover inheriting from another 
dataclass. e.g., if I have dataclass with
attribute a, and subclass it to add attribute b, I'd want both in the repr.


Good point.  So auto-generate a new __repr__ if:

- one is not provided, and
- existing __repr__ is either:
  - object.__repr__, or
  - a previous dataclass __repr__

And if the auto default doesn't work for one's use-case, use the keyword 
parameter to specify what you want.

--
~Ethan~
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Smith]

On Fri, Dec 29, 2017 at 11:37 AM, Ethan Furman  wrote:

> On 12/29/2017 02:23 AM, Ethan Smith wrote:
>
> The first is that needing both a keyword and method is duplicative and
>> unnecessary. Eric agreed it was a hassle, but
>> felt it was justified considering someone may accidentally override a
>> dataclass method. I disagree with this point of
>> view as dataclasses are billed as providing automatic methods. Overriding
>> via method definition is very natural and
>> idiomatic. I don't really see how someone could accidentally override a
>> dataclass method if methods were not generated
>> by the dataclass decorator that are already defined in the class at
>> definition time.
>>
>
> Accidental or not, the decorator should not be replacing methods defined
> by the class.
>
> The second concern, which I came across more recently, is if I have a base
>> class, and dataclasses inherit from this base
>> class, inherited __repr__ & co are silently overridden by dataclass. This
>> is both unexpected, and also means I need to
>> pass a repr=False to each subclass' decorator to get correct behavior,
>> which somewhat defeats the utility of
>> subclassing. Im not as sure a whole lot can be done about this though.
>>
>
> It is possible to determine whether an existing __repr__ is from 'object'
> or not, and only provide one if that is the case.  I think that should be
> the default, with 'repr = True' for those cases where a new,
> auto-generated, __repr__ is desired.
>

The only other thing you'd want to handle is to cover inheriting from
another dataclass. e.g., if I have dataclass with attribute a, and subclass
it to add attribute b, I'd want both in the repr.

~>Ethan Smith

>
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Furman]

On 12/29/2017 02:23 AM, Ethan Smith wrote:


The first is that needing both a keyword and method is duplicative and 
unnecessary. Eric agreed it was a hassle, but
felt it was justified considering someone may accidentally override a dataclass 
method. I disagree with this point of
view as dataclasses are billed as providing automatic methods. Overriding via 
method definition is very natural and
idiomatic. I don't really see how someone could accidentally override a 
dataclass method if methods were not generated
by the dataclass decorator that are already defined in the class at definition 
time.


Accidental or not, the decorator should not be replacing methods defined by the 
class.


The second concern, which I came across more recently, is if I have a base 
class, and dataclasses inherit from this base
class, inherited __repr__ & co are silently overridden by dataclass. This is 
both unexpected, and also means I need to
pass a repr=False to each subclass' decorator to get correct behavior, which 
somewhat defeats the utility of
subclassing. Im not as sure a whole lot can be done about this though.


It is possible to determine whether an existing __repr__ is from 'object' or not, and only provide one if that is the 
case.  I think that should be the default, with 'repr = True' for those cases where a new, auto-generated, __repr__ is 
desired.


--
~Ethan~
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Antoine Pitrou]

On Fri, 29 Dec 2017 11:12:11 -0800
Ethan Smith  wrote:
> 
> > Agreed as well.  If I make the effort of having a dataclass inherit
> > from a base class, I probably don't want the base class' methods to be
> > silently overriden by machine-generated methods.  Of course, that can
> > be worked around by using multiple inheritance, you just need to be
> > careful and add a small amount of class definition boilerplate.
> 
> I am not sure exactly what you mean by "worked around by using multiple
> inheritance".

I mean you can write:

class _BaseClass:
def __repr__(self):
# ...

@dataclass
class _DataclassMixin:
# your attribute definitions here

class FinalClass(_BaseClass, _BaseDataclass):
pass


Yes, it's tedious and verbose :-)

Regards

Antoine.
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

But you always inherit __repr__, from object. The base class might also
itself be a dataclass. I think it should only skip when the decorated class
itself defines __repr__.

On Dec 29, 2017 3:47 AM, "Antoine Pitrou"  wrote:

> On Fri, 29 Dec 2017 02:23:56 -0800
> Ethan Smith  wrote:
> >
> > In a few cases I want to override the repr of the AST nodes. I wrote a
> > __repr__ and ran the code but lo and behold I got a type error. I
> couldn't
> > override it. I quickly learned that one needs to pass a keyword to the
> > dataclass decorator to tell it *not* to auto generate methods you
> override.
> >
> > I have two usability concerns with the current implementation. I emailed
> > Eric about the first, and he said I should ask for thoughts here. The
> > second I found after a couple of days sitting on this message.
> >
> > The first is that needing both a keyword and method is duplicative and
> > unnecessary. Eric agreed it was a hassle, but felt it was justified
> > considering someone may accidentally override a dataclass method. I
> > disagree with this point of view as dataclasses are billed as providing
> > automatic methods. Overriding via method definition is very natural and
> > idiomatic.
>
> Agreed.  We shouldn't take magic too far just for the sake of
> protecting users against their own (alleged) mistakes.  And I'm not
> sure how you "accidentally" override a dataclass method (if I'm
> implementing a __repr__ I'm doing so deliberately :-)).
>
> > The second concern, which I came across more recently, is if I have a
> base
> > class, and dataclasses inherit from this base class, inherited __repr__ &
> > co are silently overridden by dataclass. This is both unexpected, and
> also
> > means I need to pass a repr=False to each subclass' decorator to get
> > correct behavior, which somewhat defeats the utility of subclassing. Im
> not
> > as sure a whole lot can be done about this though.
>
> Agreed as well.  If I make the effort of having a dataclass inherit
> from a base class, I probably don't want the base class' methods to be
> silently overriden by machine-generated methods.  Of course, that can
> be worked around by using multiple inheritance, you just need to be
> careful and add a small amount of class definition boilerplate.
>
> I would expect dataclass parameters such as `repr` to be tri-state:
>
> * repr=None (the default): only provide a machine-generated
>   implementation if none is already defined (either on a base class or
>   in the dataclass namespace... ignoring runtime-provided defaults such
>   as object.__repr__)
> * repr=False: never provide a machine-generated implementation
> * repr=True: always provide a machine-generated implementation, even
>   overriding a previous user-defined implementation
>
> Regards
>
> Antoine.
>
>
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Smith]

On Fri, Dec 29, 2017 at 2:45 AM, Antoine Pitrou  wrote:

> On Fri, 29 Dec 2017 02:23:56 -0800
> Ethan Smith  wrote:
> >
> > In a few cases I want to override the repr of the AST nodes. I wrote a
> > __repr__ and ran the code but lo and behold I got a type error. I
> couldn't
> > override it. I quickly learned that one needs to pass a keyword to the
> > dataclass decorator to tell it *not* to auto generate methods you
> override.
> >
> > I have two usability concerns with the current implementation. I emailed
> > Eric about the first, and he said I should ask for thoughts here. The
> > second I found after a couple of days sitting on this message.
> >
> > The first is that needing both a keyword and method is duplicative and
> > unnecessary. Eric agreed it was a hassle, but felt it was justified
> > considering someone may accidentally override a dataclass method. I
> > disagree with this point of view as dataclasses are billed as providing
> > automatic methods. Overriding via method definition is very natural and
> > idiomatic.
>
> Agreed.  We shouldn't take magic too far just for the sake of
> protecting users against their own (alleged) mistakes.  And I'm not
> sure how you "accidentally" override a dataclass method (if I'm
> implementing a __repr__ I'm doing so deliberately :-)).
>

My thinking exactly.

>
> > The second concern, which I came across more recently, is if I have a
> base
> > class, and dataclasses inherit from this base class, inherited __repr__ &
> > co are silently overridden by dataclass. This is both unexpected, and
> also
> > means I need to pass a repr=False to each subclass' decorator to get
> > correct behavior, which somewhat defeats the utility of subclassing. Im
> not
> > as sure a whole lot can be done about this though.
>
> Agreed as well.  If I make the effort of having a dataclass inherit
> from a base class, I probably don't want the base class' methods to be
> silently overriden by machine-generated methods.  Of course, that can
> be worked around by using multiple inheritance, you just need to be
> careful and add a small amount of class definition boilerplate.
>

I am not sure exactly what you mean by "worked around by using multiple
inheritance". Do you mean you think
the dataclass decorator should be made into a dataclass base class for a
dataclass class, or that it should look
at the MRO?


>
> I would expect dataclass parameters such as `repr` to be tri-state:
>
> * repr=None (the default): only provide a machine-generated
>   implementation if none is already defined (either on a base class or
>   in the dataclass namespace... ignoring runtime-provided defaults such
>   as object.__repr__)
> * repr=False: never provide a machine-generated implementation
> * repr=True: always provide a machine-generated implementation, even
>   overriding a previous user-defined implementation
>

This is sensible to me.


Thanks for your thoughts!

~>Ethan Smith


> Regards
>
> Antoine.
>
>
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Re: [Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Antoine Pitrou]

On Fri, 29 Dec 2017 02:23:56 -0800
Ethan Smith  wrote:
> 
> In a few cases I want to override the repr of the AST nodes. I wrote a
> __repr__ and ran the code but lo and behold I got a type error. I couldn't
> override it. I quickly learned that one needs to pass a keyword to the
> dataclass decorator to tell it *not* to auto generate methods you override.
> 
> I have two usability concerns with the current implementation. I emailed
> Eric about the first, and he said I should ask for thoughts here. The
> second I found after a couple of days sitting on this message.
> 
> The first is that needing both a keyword and method is duplicative and
> unnecessary. Eric agreed it was a hassle, but felt it was justified
> considering someone may accidentally override a dataclass method. I
> disagree with this point of view as dataclasses are billed as providing
> automatic methods. Overriding via method definition is very natural and
> idiomatic.

Agreed.  We shouldn't take magic too far just for the sake of
protecting users against their own (alleged) mistakes.  And I'm not
sure how you "accidentally" override a dataclass method (if I'm
implementing a __repr__ I'm doing so deliberately :-)).

> The second concern, which I came across more recently, is if I have a base
> class, and dataclasses inherit from this base class, inherited __repr__ &
> co are silently overridden by dataclass. This is both unexpected, and also
> means I need to pass a repr=False to each subclass' decorator to get
> correct behavior, which somewhat defeats the utility of subclassing. Im not
> as sure a whole lot can be done about this though.

Agreed as well.  If I make the effort of having a dataclass inherit
from a base class, I probably don't want the base class' methods to be
silently overriden by machine-generated methods.  Of course, that can
be worked around by using multiple inheritance, you just need to be
careful and add a small amount of class definition boilerplate.

I would expect dataclass parameters such as `repr` to be tri-state:

* repr=None (the default): only provide a machine-generated
  implementation if none is already defined (either on a base class or
  in the dataclass namespace... ignoring runtime-provided defaults such
  as object.__repr__)
* repr=False: never provide a machine-generated implementation
* repr=True: always provide a machine-generated implementation, even
  overriding a previous user-defined implementation

Regards

Antoine.


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[Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Ethan Smith]

Hello all,

I've recently been experimenting with dataclasses. They totally rock! A lot
of the boilerplate for the AST I've designed in Python is automatically
taken care of, it's really great! However, I have a few concerns about the
implementation.

In a few cases I want to override the repr of the AST nodes. I wrote a
__repr__ and ran the code but lo and behold I got a type error. I couldn't
override it. I quickly learned that one needs to pass a keyword to the
dataclass decorator to tell it *not* to auto generate methods you override.

I have two usability concerns with the current implementation. I emailed
Eric about the first, and he said I should ask for thoughts here. The
second I found after a couple of days sitting on this message.

The first is that needing both a keyword and method is duplicative and
unnecessary. Eric agreed it was a hassle, but felt it was justified
considering someone may accidentally override a dataclass method. I
disagree with this point of view as dataclasses are billed as providing
automatic methods. Overriding via method definition is very natural and
idiomatic. I don't really see how someone could accidentally override a
dataclass method if methods were not generated by the dataclass decorator
that are already defined in the class at definition time.

The second concern, which I came across more recently, is if I have a base
class, and dataclasses inherit from this base class, inherited __repr__ &
co are silently overridden by dataclass. This is both unexpected, and also
means I need to pass a repr=False to each subclass' decorator to get
correct behavior, which somewhat defeats the utility of subclassing. Im not
as sure a whole lot can be done about this though.


I appreciate any thoughts folks have related to this.


Cheers,
~>Ethan Smith
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