So I am stuck on a problem. I have a class which I want to use to create another class without having to go through the boiler plate of subclassing. Specifically because the subclass needs to have certain class attributes
and I would like to control how those are passed to provide defaults and
such. What I have working right now is

class Foo():
@classmethod
def method_a(cls): print(cls.name)

Bar = type('Bar', (Foo,), {'name': 'test1'})

Bar.method_a()

This is sort of fine but the user needs to know how to call type and
include all the base classes etc.

I could just wrap the type call in a function like below

def BarMaker(name):
return type('Bar', (Foo,), {'name': name})

But then if the user needs to actually subclass Foo, to add additional
methods that is more difficult. Of course those methods could be added as
part of the third argument to types but that just doesn't feel very Python.

This is all complicated by the fact that I am trying to avoid actually instancating any of these classes because their purpose is to be passed off
to another framework to be executed. In real life most of the classes would fail to instantiate because they are relying on pieces of the framework
that won't be up and running when this setup is happening.

The solution I am hoping for is something like
Foo(name, **kwds) -> Returns new class type of name
For example
test = Foo('test')

So that then optionally a user could do

class UserClass(Foo):
def new_method():
pass
bar = UserClass('Bar')

Or something similar. Since I have a working example with type I feel like there should be a way to do this with the normal class structure but I am
at a loss for how I have messed with the __new__ and __prepare__ methods
and I can't seem to make anything work and the examples of meta classes
don't clearly show how to convert my type example to a class structure.

Is what I want to do possible?

Chris

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Pada Minggu, 23 Juli 2023 pukul 06.13.23 UTC+7, Chris Nyland menulis:

So I am stuck on a problem. I have a class which I want to use to create another class without having to go through the boiler plate of subclassing. Specifically because the subclass needs to have certain class attributes
and I would like to control how those are passed to provide defaults and such. What I have working right now is

class Foo():
@classmethod
def method_a(cls): print(cls.name)

Bar = type('Bar', (Foo,), {'name': 'test1'})

Bar.method_a()

This is sort of fine but the user needs to know how to call type and
include all the base classes etc.

I could just wrap the type call in a function like below

def BarMaker(name):
return type('Bar', (Foo,), {'name': name})

But then if the user needs to actually subclass Foo, to add additional methods that is more difficult. Of course those methods could be added as part of the third argument to types but that just doesn't feel very Python.

This is all complicated by the fact that I am trying to avoid actually instancating any of these classes because their purpose is to be passed off to another framework to be executed. In real life most of the classes would fail to instantiate because they are relying on pieces of the framework
that won't be up and running when this setup is happening.

The solution I am hoping for is something like
Foo(name, **kwds) -> Returns new class type of name
For example
test = Foo('test')

So that then optionally a user could do

class UserClass(Foo):
def new_method():
pass
bar = UserClass('Bar')

Or something similar. Since I have a working example with type I feel like there should be a way to do this with the normal class structure but I am
at a loss for how I have messed with the __new__ and __prepare__ methods
and I can't seem to make anything work and the examples of meta classes
don't clearly show how to convert my type example to a class structure.

Is what I want to do possible?

Chris

https://sirekan.usk.ac.id/uploads/slot4d/

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Chris Nyland wrote at 2023-7-22 19:12 -0400:

So I am stuck on a problem. I have a class which I want to use to create >another class without having to go through the boiler plate of subclassing.

Do you know about `__init_subclass__`?
It is called whenever a class is subclassed and can be used to
check/update the newly created class.

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On 23 Jul 2023, at 02:12, Chris Nyland via Python-list <python-list@python.org> wrote:

So I am stuck on a problem. I have a class which I want to use to create another class without having to go through the boiler plate of subclassing. Specifically because the subclass needs to have certain class attributes
and I would like to control how those are passed to provide defaults and such. What I have working right now is

class Foo():
@classmethod
def method_a(cls): print(cls.name)

Bar = type('Bar', (Foo,), {'name': 'test1’})

Could you write a more expressive example of what you are trying to do and what the problem actually is?

The way you wrote the above doesn’t contain any clues why usual subclassing is not appropriate.

By attributes you mean value attributes or methods? Why going the usual route of subclassing & controlling their behaviour via constructor arguments isn’t working?

This is sort of fine but the user needs to know how to call type and
include all the base classes etc.

I don’t get what you mean by this either. Very hard to follow everything that went after, given I was lost at this point.

Maybe it’s only me... But I if you clarified these, I might be able to help a bit.

DG

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Dom Grigonis <dom.grigonis@gmail.com> writes:

I don’t get what you mean by this either. Very hard to follow
everything that went after, given I was lost at this point.

I had the same experience. The OP should pose his question in
the form of an executable test, e.g.,

|I want a class T, so that this expressions evaluates to True: |T.__init__.__class__ == T.__class__

.

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