@Alok Thank you for the elaborate answer and clean code example. And thank you for your time.
> May we know what was the challenge in implementing the behavior in > __init__ ? It was more of a personal challenge to try __new__ as I was learning about metaclasses and other intermediate topics. This code is from a personal project where everything was accumulated in init. So, while writing the alternate constructors, I wanted to try that. I would encourage you to read the docs > <https://docs.python.org/2/reference/datamodel.html#object.__new__>for > __new__, generally speaking you can achieve all you want from the __init__, > __new__ is usually required for subclassing immutables like str, tuple, > etc. +1 for the doc reference Actually the object is passed to __init__, not the class. As I mentioned > above, if you are returning the object from __new__, this would cause > __init__ to be called on the object that is returned, which in your case > would be done from one of the alternative constructors. This makes sense. I will continue using init for now but am more familiar with __new__ now than before. Thanks - H On Thursday, April 16, 2020 at 11:28:06 PM UTC+9, Alok Gandhi wrote: > > Harshad, I missed your original question, you already had all the > information in there, I should have given more attention, apologies > > 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). > > May we know what was the challenge in implementing the behavior in > __init__ ? > > I would encourage you to read the docs > <https://docs.python.org/2/reference/datamodel.html#object.__new__>for > __new__, generally speaking you can achieve all you want from the __init__, > __new__ is usually required for subclassing immutables like str, tuple, > etc. I have personally used __new__ in the past for some meta programming > involving ORM/ Db stuff, implementing singleton pattern (though it can be > done in quite a few different ways) etc. > > Further, as the doc says, to avoid calling the __init__ you can simply > choose to not return the instance from __new__, but I guess that is not > what you are looking for, or maybe? > > >> *Problem is when the argument is checked in __new__:*- A class is >> returned from the respective classmethod and passed to init. > > Actually the object is passed to __init__, not the class. As I mentioned > above, if you are returning the object from __new__, this would cause > __init__ to be called on the object that is returned, which in your case > would be done from one of the alternative constructors. > > Finally, I feel that we can still get away in this case without using > __new__ still retaining the required behavior. Please find my code here > <https://pastebin.com/VWYVnys0>. This covers almost everything you need. > Let me know if this does not work. > > Cheers! > > On Thu, Apr 16, 2020 at 1:41 AM Justin Israel <justin...@gmail.com > <javascript:>> wrote: > >> >> >> On Wed, Apr 15, 2020, 11:01 PM HarshadB <bari....@gmail.com <javascript:>> >> 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! >>>>> >>>>> -- >>>>> You received this message because you are subscribed to the Google >>>>> Groups "Python Programming for Autodesk Maya" group. >>>>> To unsubscribe from this group and stop receiving emails from it, send >>>>> an email to python_inside_maya+unsubscr...@googlegroups.com. >>>>> To view this discussion on the web visit >>>>> https://groups.google.com/d/msgid/python_inside_maya/e97d389f-19c1-49b4-a868-79afafe965fc%40googlegroups.com >>>>> >>>>> <https://groups.google.com/d/msgid/python_inside_maya/e97d389f-19c1-49b4-a868-79afafe965fc%40googlegroups.com?utm_medium=email&utm_source=footer> >>>>> . >>>>> >>>> -- >>> You received this message because you are subscribed to the Google >>> Groups "Python Programming for Autodesk Maya" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to python_inside_maya+unsubscr...@googlegroups.com >>> <javascript:>. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/python_inside_maya/4451bc9e-ddec-4296-87b9-9b1b12ce41e9%40googlegroups.com >>> >>> <https://groups.google.com/d/msgid/python_inside_maya/4451bc9e-ddec-4296-87b9-9b1b12ce41e9%40googlegroups.com?utm_medium=email&utm_source=footer> >>> . >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "Python Programming for Autodesk Maya" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to python_inside_maya+unsubscr...@googlegroups.com <javascript:>. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/python_inside_maya/CAPGFgA1a8ebAxcZa-zJZFinxR0cH4BUSC21hx25VeeVymgSHbA%40mail.gmail.com >> >> <https://groups.google.com/d/msgid/python_inside_maya/CAPGFgA1a8ebAxcZa-zJZFinxR0cH4BUSC21hx25VeeVymgSHbA%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "Python Programming for Autodesk Maya" group. 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