On Wed, Apr 15, 2020, 11:01 PM HarshadB <bari.hars...@gmail.com> wrote:
> @alok
> Thank you for your reply.
>
> I already have working class constructors(classmethods) that do what I
> want. That part is done.
> Sorry, I should have asked the question clearly in the first line.
>
> What I am actually doing is to further automate the instance creation of
> the class based on the argument provided, or with no argument.
> For e.g with no argument, it should call cls.from_selection(cls)
> This part is also done but in a hackish sort of way. I have isinstance
> checks in my class __new__ method. *I want to write this code in a
> better, efficient way.*
>
>
> *Problem is when the argument is checked in __new__:*- A class is
> returned from the respective classmethod and passed to init.
> - The original argument is still passed to init after already passing it
> to the classmethod above.
> - So the init gets called twice.
>
Since you are combining classmethod and __new__, you would have to
implement your classmethod to also use super().__new__ so that it doesn't
call the constructor. But that is a bit unintuitive if it is a public
classmethod. So maybe you should define private class methods that can be
called by your __new__.
- I have worked around this for now by having a *isinstance(arg,
> om.mObject)* check in the __init__.
>
> - H
>
> On Wednesday, April 15, 2020 at 3:35:30 PM UTC+9, Alok Gandhi wrote:
>>
>> Python @classmethod were added for this exact reason - Alternative
>> Contructors.
>>
>> class Foo(object):
>> def __init__(self, bar=None, baz=None):
>> self._bar = bar
>> self._baz = baz
>> print 'bar: {}, baz: {}'.format(self._bar, self._baz)
>>
>> @classmethod
>> def from_bar(cls, bar):
>> return cls(bar=bar)
>>
>> @classmethod
>> def from_baz(cls, baz):
>> return cls(baz=baz)
>>
>> # Default
>> foo = Foo('default bar', 'default baz')
>> # bar: default bar, baz: default baz
>>
>> # From bar
>> foo_bar = Foo.from_bar('from bar')
>> # bar: from bar, baz: None
>>
>> # From baz
>> foo_baz = Foo.from_baz('from baz')
>> # bar: None, baz: from baz
>>
>>
>> - Alok
>>
>> On Wed, Apr 15, 2020, 10:44 HarshadB <bari....@gmail.com> wrote:
>>
>>> Hoping for some help with this, I am trying to understand Class
>>> construction(instantiation before init).
>>>
>>> I have a component class which encapsulates a hierarchy of nodes in
>>> Maya. Instantiated using a mObject as input.
>>> BaseComponent(mObject)
>>>
>>> I want to automate the instance construction of this class based on
>>> selection, mObjectHandle, string and have created classmethods (as
>>> alternative constructors) for the same. I wanted to allow initiation just
>>> by BaseComponent() that will take the selection and avoid
>>> BaseComponent.some_method as default.
>>>
>>> Goal:
>>> BaseComponent() # Takes selection
>>> BaseComponent.fromString() # Takes string name of an object in scene
>>> BaseComponent.fromMObjectHandle() # Takes an mObjectHandle for an
>>> object in scene
>>> BaseComponent(mObject) # Takes mObject as default
>>>
>>> Currently trying to implement this with __new__. Previously, all the
>>> "isinstance" checks were done in init which I wanted to simplify.
>>> I couldn't get it to work exactly how I wanted to, but I did get it to
>>> work with all four arg input types (selection, str, mObjectHandle, mObject).
>>>
>>> Problems:
>>> - The problem is there's no clear error/warning if the end-user makes a
>>> mistake.
>>> - All alternative constructors also pass to the init where I had to put
>>> a mObject isinstance check for it to work. If possible, I want to remove
>>> mObject check in init as mObject should be the only one passed to init.
>>>
>>> I have attached my extracted current working code in Pastebin. It has
>>> print statements and can be run in a new Maya session:
>>> https://pastebin.com/tN5bs6g0
>>>
>>> Any suggestions/recommendations are really appreciated. Please let me
>>> know if I wasn't clear enough.
>>> (I have a basic idea about metaclasses and class decorators, so any
>>> advice that includes those is welcome.)
>>>
>>> Thank you!
>>>
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>>>
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