a new object definition

Discussion in 'Python' started by Sylvain Ferriol, Sep 1, 2006.

  1. hello everybody,

    i want to talk with you about a question i have in mind and i do not
    find a answer. it 's simple:
    why do we not have a beatiful syntax for object definition as we have
    for class definition ?

    we can define a class in python in 2 ways:
    1. by using the metaclass constructor
    my_class = MyMetaClass(....)
    2. by using the classic definition syntax:
    class my_class(object):
    __metaclass__ = MyMetaClass

    if i want to instanciate an object, i only have one way to define it:
    my_obj = my_class(....)

    why not a better syntax like class syntax ?


    with this syntax, we are coherent between objects and classes.
    why do we have a specific syntax for the class object definition and not
    for objects of different type?

    so if i want to define a new class object (my_class for example) with a
    new object syntax:

    Sylvain Ferriol
    Ingénieur de recherche
    Laboratoire TIMC/IMAG
    Sylvain Ferriol, Sep 1, 2006
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  2. See http://www.python.org/dev/peps/pep-0359 (already rejected by

    Michele Simionato
    Michele Simionato, Sep 1, 2006
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  3. Python's classes are objects too - instances of their metaclass.
    I fail to see how it's "better", nor what would be the use case. But
    anyway I think it could be possible by using the metaclass as class and
    the class as instance. Just make sure the metaclass wraps all methods
    into classmethods and you should be done.
    Bruno Desthuilliers, Sep 1, 2006
  4. Michele Simionato a écrit :
    i do not understand the withdrawal note, what do "different level" mean ?
    do you have an example or is it python core implemantation problem ?
    Sylvain Ferriol, Sep 1, 2006
  5. I asked Guido in person at EuroPython. He said the syntax didn't look
    "right" to him. It is as simple as that.

    Michele Simionato
    Michele Simionato, Sep 1, 2006
  6. Michele Simionato a écrit :
    note that we can not use the "make syntax" for a function definition
    because the block is not executed until the function call, while the
    block for a class is executed before the class instanciation
    Sylvain Ferriol, Sep 1, 2006
  7. Michele Simionato already pointed you to `PEP 359`_. One of the reasons
    that I withdrew it was that people seemed to feel that you could get
    most of what you want now by defining appropriate metaclasses. In your
    case, for example, the appropriate metaclass and its usage might look like::
    ... cls = dict.pop('__class__')
    ... dict.pop('__metaclass__')
    ... dict.pop('__module__') # silently added by class statement
    ... return cls(**dict)
    ... ... def __init__(self, a, b):
    ... self.a = a
    ... self.b = b
    ... ... __metaclass__ = instance
    ... __class__ = C
    ... a = 'foo'
    ... b = 'bar'
    ... ('foo', 'bar')

    Sure, it's misleading to use a class statement when you're not actually
    creating a class, but I guess people felt that wanting to do this was
    uncommon enough that they weren't worried about it.

    ... _PEP 359: http://www.python.org/dev/peps/pep-0359/

    Steven Bethard, Sep 1, 2006
  8. Michele Simionato already pointed you to `PEP 359`_. One of the reasons
    i know that there is always a solution. But this problem show that
    python and others are not coherent in the syntax (contrary to lisp for

    with the 'make' syntax, it will be really easy to translate a program or
    a data structure defined in XML format into python syntax.

    i do not know how many use-cases we need for changing a PEP status :)

    another advantage is that we have the same syntax in all definition
    levels: metaclass, class, instance.
    and if we just want to use objects and do a sort of 'prototype
    programming', we can with this syntax.
    instance my_obj(prototype_obj):
    ... object specialisation

    Sylvain Ferriol, Sep 4, 2006
  9. Only if there are no ordering constraints and no need for multiple
    elements with the same name. The make statement was built to mirror the
    the class statement, so the body is just executed in a regular Python
    dict. Hence, you can only have one value for each name, and the order
    in which the names appear is not maintained. There's some discussion of
    workarounds_ for this in the PEP, but they're pretty hackish.

    ... _workarounds:

    Steven Bethard, Sep 6, 2006
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