getattr for static/class variables in python

Question

I have a class like:

class MyClass:
     Foo = 1
     Bar = 2

Whenever MyClass.Foo or MyClass.Bar is invoked, I need a custom method to be invoked before the value is returned. Is it possible in Python? I know it is possible if I create an instance of the class and I can define my own __getattr__ method. But my scnenario involves using this class as such without creating any instance of it.

Also I need a custom __str__ method to be invoked when str(MyClass.Foo) is invoked. Does Python provide such an option?

score 87 · Accepted Answer · edited Aug 27 '18 at 10:35

__getattr__() and __str__() for an object are found on its class, so if you want to customize those things for a class, you need the class-of-a-class. A metaclass.

class FooType(type):
    def _foo_func(cls):
        return 'foo!'

    def _bar_func(cls):
        return 'bar!'

    def __getattr__(cls, key):
        if key == 'Foo':
            return cls._foo_func()
        elif key == 'Bar':
            return cls._bar_func()
        raise AttributeError(key)

    def __str__(cls):
        return 'custom str for %s' % (cls.__name__,)

class MyClass:
    __metaclass__ = FooType

# in python 3:
# class MyClass(metaclass=FooType):
#    pass


print MyClass.Foo
print MyClass.Bar
print str(MyClass)

printing:

foo!
bar!
custom str for MyClass

And no, an object can't intercept a request for a stringifying one of its attributes. The object returned for the attribute must define its own __str__() behavior.

Thanks Ignacio and Matt. I think now I understand how class (not instance) creation works and also I thought the required __str__ behaviour was difficult to achieve. I think metaclasses should be good enough for my requirement. :) — Tuxdude, Jul 01 '10 at 06:49
If you always get `AttributeError` check out the other answer, which has the syntax for python3 (this example seems to be python2) — Chris, Jun 21 '17 at 09:03

shx2 · Answer 2 · 2016-02-19T06:26:07.323

20

(I know this is an old question, but since all the other answers use a metaclass...)

You can use the following simple classproperty descriptor:

class classproperty(object):
    """ @classmethod+@property """
    def __init__(self, f):
        self.f = classmethod(f)
    def __get__(self, *a):
        return self.f.__get__(*a)()

Use it like:

class MyClass(object):

     @classproperty
     def Foo(cls):
        do_something()
        return 1

     @classproperty
     def Bar(cls):
        do_something_else()
        return 2

edited Feb 19 '16 at 06:26

answered Mar 29 '14 at 09:43

shx2

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This will only help if you know the names of your properties beforehand. If you read them from file and need to react dynamically, this will not work. – Chris Jun 21 '17 at 09:02

score 12 · Answer 3 · answered Jul 01 '10 at 06:26

12

For the first, you'll need to create a metaclass, and define __getattr__() on that.

class MyMetaclass(type):
  def __getattr__(self, name):
    return '%s result' % name

class MyClass(object):
  __metaclass__ = MyMetaclass

print MyClass.Foo

For the second, no. Calling str(MyClass.Foo) invokes MyClass.Foo.__str__(), so you'll need to return an appropriate type for MyClass.Foo.

answered Jul 01 '10 at 06:26

Ignacio Vazquez-Abrams

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Tested this example and it only works for me in Python 2.x. After changing the print to a function, Python 3.2 gives me a "AttributeError: type object 'MyClass' has no attribute 'Foo'" error. Any suggestions? – CyberFonic Nov 17 '11 at 07:44
1

@CyberED: http://docs.python.org/py3k/reference/datamodel.html#metaclasses : "When the class definition is read, if a callable `metaclass` keyword argument is passed after the bases in the class definition, the callable given will be called instead of `type()`." – Ignacio Vazquez-Abrams Nov 17 '11 at 07:51
4

Couldn't wait .... after a bit of Googling and testing : The example is for Python 2.x. In Python 3.x you need : class MyClass(metaclass=MyMetaclass): pass – CyberFonic Nov 17 '11 at 08:00

score 9 · Answer 4 · edited Jul 24 '19 at 04:15

9

Surprised no one pointed this one out:

class FooType(type):
    @property
    def Foo(cls):
        return "foo!"

    @property
    def Bar(cls):
        return "bar!"

class MyClass(metaclass=FooType):
    pass

Works:

>>> MyClass.Foo
'foo!'
>>> MyClass.Bar
'bar!'

(for Python 2.x, change definition of MyClass to:

class MyClass(object):
    __metaclass__ = FooType

)

What the other answers say about str holds true for this solution: It must be implemented on the type actually returned.

edited Jul 24 '19 at 04:15

xaxxon

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answered Jan 28 '14 at 20:26

Jonas Schäfer

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While I usually try to refrain from posting "Thanks", I seriously need to post "Thanks" right now: Thanks! – Chris Jun 21 '17 at 09:11

score 1 · Answer 5 · answered Sep 21 '18 at 14:37

Depending on the case I use this pattern

class _TheRealClass:
    def __getattr__(self, attr):
       pass

LooksLikeAClass = _TheRealClass()

Then you import and use it.

from foo import LooksLikeAClass
LooksLikeAClass.some_attribute

This avoid use of metaclass, and handle some use cases.

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