generators as decorators simple issue

Discussion in 'Python' started by j.m.dagenhart@gmail.com, Sep 12, 2012.

  1. Guest

    I'm trying to call SetName on an object to prevent me from ever having to call it explictly again on that object. Best explained by example.


    def setname(cls):
    '''this is the proposed generator to call SetName on the object'''
    try:
    cls.SetName(cls.__name__)
    finally:
    yield cls


    class Trial:
    '''class to demonstrate with'''
    def SetName(self, name):
    print 1, 1

    @setname
    class Test(Trial):
    '''i want SetName to be called by using setname as a decorator'''
    def __init__(self):

    print 'Yay! or Invalid.'

    if __name__ == '__main__':
    test = Test()


    How can i fix this?
    This is my exact error: python decors2.py
    Traceback (most recent call last):
    File "decors2.py", line 23, in <module>
    test = Test()
    TypeError: 'generator' object is not callable
     
    , Sep 12, 2012
    #1
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  2. On Wednesday, 12 September 2012 07:58:10 UTC+5:30, pyjoshsys wrote:
    > I'm trying to call SetName on an object to prevent me from ever having to call it explictly again on that object. Best explained by example.
    >

    [snip]
    In your decorator, you are using `yield cls` - it should be `return cls` 99.99% of the time.
     
    Ramchandra Apte, Sep 12, 2012
    #2
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  3. alex23 Guest

    On Sep 12, 12:28 pm, wrote:
    > def setname(cls):
    >     '''this is the proposed generator to call SetName on the object'''
    >     try:
    >         cls.SetName(cls.__name__)
    >     finally:
    >         yield cls


    A generator is (basically) a callable that acts like an iterator.
    You'd use a generator if you wanted to loop with for or a list
    comprehension across the output of the generator: for foo in
    setname(Test)

    A decorator is a callable that takes another callable as an argument,
    either modifying it or returning a wrapped version of it: Test =
    setname(Test)

    You don't want to iterate over anything, so you should change `yield`
    to `return`.
     
    alex23, Sep 12, 2012
    #3
  4. Am 12.09.2012 04:28 schrieb :
    > I'm trying to call SetName on an object to prevent me from ever having to call it explictly again on that object. Best explained by example.
    >
    >
    > def setname(cls):
    > '''this is the proposed generator to call SetName on the object'''
    > try:
    > cls.SetName(cls.__name__)
    > finally:
    > yield cls
    >
    >
    > class Trial:
    > '''class to demonstrate with'''
    > def SetName(self, name):
    > print 1, 1
    >
    > @setname
    > class Test(Trial):
    > '''i want SetName to be called by using setname as a decorator'''
    > def __init__(self):
    >
    > print 'Yay! or Invalid.'
    >
    > if __name__ == '__main__':
    > test = Test()
    >
    >
    > How can i fix this?


    I am not sure what exactly you want to achieve, but I see 2 problems here:

    1. Your setname operates on a class, but your SetName() is an instance
    function.

    2. I don't really understand the try...finally yield stuff. As others
    already said, you probably just want to return. I don't see what a
    generator would be useful for here...

    def setname(cls):
    '''this is the proposed generator to call SetName on the object'''
    try:
    cls.SetName(cls.__name__)
    finally:
    return cls

    and

    class Trial(object):
    '''class to demonstrate with'''
    @classmethod
    def SetName(cls, name):
    print 1, 1

    should solve your problems.
     
    Thomas Rachel, Sep 12, 2012
    #4
  5. pyjoshsys Guest

    The output is still not what I want. Now runtime error free, however the output is not what I desire.



    def setname(cls):
    '''this is the proposed generator to call SetName on the object'''

    try:
    cls.SetName(cls.__name__)
    except Exception as e:
    print e
    finally:
    return cls

    class Trial(object):
    '''class to demonstrate with'''
    def __init__(self):
    object.__init__(self)
    self.name = None

    @classmethod
    def SetName(cls, name):
    cls.name = name

    @setname
    class Test(Trial):
    '''i want SetName to be called by using setname as a decorator'''
    def __init__(self):
    Trial.__init__(self)



    if __name__ == '__main__':
    test = Test()
    print 'instance'
    print '', test.name #should be Test
    print 'class'
    print '', Test.name


    The output is: python decors2.py
    instance
    None
    class
    Test

    I want:
    instance
    Test
    class
    Test

    Is this possible in this manner?
     
    pyjoshsys, Sep 12, 2012
    #5
  6. On Wed, 12 Sep 2012 03:22:31 -0700 (PDT), pyjoshsys
    <> wrote:
    > The output is still not what I want. Now runtime error free,

    however the output is not what I desire.

    > def setname(cls):
    > '''this is the proposed generator to call SetName on the

    object'''

    > try:
    > cls.SetName(cls.__name__)
    > except Exception as e:
    > print e
    > finally:
    > return cls


    I would write the function above in one line:

    cls.name = name


    > class Trial(object):
    > '''class to demonstrate with'''
    > def __init__(self):
    > object.__init__(self)
    > self.name = None


    Remove the line above. The instance attribute self.name is hiding the
    class attribute cls.name.

    Oscar
     
    Oscar Benjamin, Sep 12, 2012
    #6
  7. pyjoshsys Guest

    so decorators only pass the object and not any instance of the object as the implied argument? Is this right?

    The idea was to use @setname instead of instance.SetName(instance.__name__).

    I thought decorators would do this, but it seems not.
     
    pyjoshsys, Sep 12, 2012
    #7
  8. Ian Kelly Guest

    On Wed, Sep 12, 2012 at 4:22 AM, pyjoshsys <> wrote:
    > The output is still not what I want. Now runtime error free, however the output is not what I desire.


    [SNIP]

    > class Trial(object):
    > '''class to demonstrate with'''
    > def __init__(self):
    > object.__init__(self)
    > self.name = None
    >
    > @classmethod
    > def SetName(cls, name):
    > cls.name = name


    [SNIP]

    > if __name__ == '__main__':
    > test = Test()
    > print 'instance'
    > print '', test.name #should be Test
    > print 'class'
    > print '', Test.name
    >
    >
    > The output is: python decors2.py
    > instance
    > None
    > class
    > Test
    >
    > I want:
    > instance
    > Test
    > class
    > Test
    >
    > Is this possible in this manner?



    The SetName class method sets the name on the *class* dictionary. The
    class's __init__ method also sets a name (None) on the *instance*
    dictionary. From an instance's perspective, the instance dictionary
    will shadow the class dictionary. If you remove the attribute from
    the instance dictionary entirely (delete the "self.name = None" line),
    and leave the class dictionary as is, then you will get the output you
    want (although from your later post I am not certain that this is the
    behaviour you want).


    On Wed, Sep 12, 2012 at 5:15 AM, pyjoshsys <> wrote:
    > so decorators only pass the object and not any instance of the object as the implied argument? Is this right?


    Right.

    > The idea was to use @setname instead of instance.SetName(instance.__name__).


    The appropriate place to do this so that it applies to all instances
    of the class rather than to the class would be inside the __init__
    method.

    Also, instances don't have a __name__ attribute, so it's still unclear
    to me what you're looking for. Did you mean the effect to be that of
    "instance.SetName(cls.__name__)"? If so, then the decorator approach
    (with the line "self.name = None" removed) should be fine for your
    purposes -- you'll just have the name stored in the class dict instead
    of in each instance dict, but it will still be visible as long as you
    haven't shadowed it.
     
    Ian Kelly, Sep 12, 2012
    #8
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