Controlled Class instantiation with multiple constructors

[HarshadB]

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!

[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

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[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 <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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[HarshadB]

Here's an excerpt from the Pastebin code linked in the original post, this is the __new__ method from the class:

[Justin Israel]

On Wed, Apr 15, 2020, 11:01 PM HarshadB <bari.h...@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__. 

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[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!

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[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

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