Re: [Maya-Python] Controlled Class instantiation with multiple constructors

[HarshadB]

@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  
> 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 
> 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 
> . 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  > wrote:
>
>>
>>
>> On Wed, Apr 15, 2020, 11:01 PM HarshadB > 
>> 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)

 

Re: [Maya-Python] Controlled Class instantiation with multiple constructors

[Alok Gandhi]

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
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
. 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 
wrote:

>
>
> On Wed, Apr 15, 2020, 11:01 PM HarshadB  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  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 

Re: [Maya-Python] Controlled Class instantiation with multiple constructors

[Justin Israel]

On Wed, Apr 15, 2020, 11:01 PM HarshadB  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  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
>>> 
>>> .
>>>
>> --
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> To view this 

Re: [Maya-Python] Controlled Class instantiation with multiple constructors

[HarshadB]

@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.
- 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 > 
> 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
>>  
>> 
>> .
>>
>

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Re: [Maya-Python] Controlled Class instantiation with multiple constructors

[Alok Gandhi]

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  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
> 
> .
>

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