3
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I had an idea for a __first__ special method for a class. It would be run after __new__ and before __init__ the first time a specific object is instantiated but is not then run if you instantiate the object again. For example, if you run:

class Foo:
    def __first__():
        print('Inside __first__')
    def __init__(self):
        print('Inside __init__')
a = Foo()
b = Foo()

It would return:

Inside __first__
Inside __init__
Inside __init__

In order to try and create this particular syntax I decided to create a meta class so that you could simply add this syntax to a class of your choice. This was my final code for the meta class and it was the first time I have tried meta class syntax so I was interested in what you thought about my code:

class First(type):
    _first = True
    def __call__(cls, *args, **kwargs):
        if cls._first:
            cls._first = False
            cls._oldinit = cls.__init__
            def init(*args, **kwargs):
                cls.__first__()
                cls._oldinit(*args, **kwargs)
            cls.__init__ = init
            cls.__init__.__doc__ = cls._oldinit.__doc__
        else:
            cls.__init__ = cls._oldinit
        return super(First, cls).__call__(*args, **kwargs)
\$\endgroup\$
  • 1
    \$\begingroup\$ Would be nice to see this in context rather than an abstract idea - why do you want to do this? Practice with metaclasses, a specific application, etc. ? Should first be a class method? Perhaps that's not appropriate but context would be useful here. \$\endgroup\$ – Greedo Feb 15 at 15:17
  • \$\begingroup\$ @Greedo It is basically just practice of metaclasses but I also thought that this idea could potentially be a useful special method. From the tests I have done I have not specified that __first__ is a class method but I suppose it could be \$\endgroup\$ – 13ros27 Feb 15 at 15:30
  • \$\begingroup\$ Have you looked at subclasses? If class D extends B, will B._first interfere with D? \$\endgroup\$ – Austin Hastings Feb 15 at 21:07
  • \$\begingroup\$ @AustinHastings Thanks for the suggestion for a way to break it. From what I have seen in the testing I did just now everything works fine with subclasses in that if B has a __first__ it will be run on the first time of D being run if either B or D has First set as its metaclass and then everything else will be normal \$\endgroup\$ – 13ros27 Feb 15 at 21:12

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