Get attributes for class and instance in python

Question

In python work next code:

class MyClass(object):
    field = 1

>>> MyClass.field
1

>>> MyClass().field
1

When I want return value for custom fields I use next code:

class MyClass(object):
    def __getattr__(self, name):
       if name.startswith('fake'):
           return name
       raise AttributeError("%r object has no attribute %r" %
                            (type(self).__name__, name))

>>> MyClass().fake
fake

But:

>>> MyClass.fake
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: class MyClass has no attribute 'fake'

Ok, for classes I can use next code:

class MyClassMeta(type):
    def __getattr__(cls, name):
       if name.startswith('fake'):
           return name
       raise AttributeError("%r object has no attribute %r" %
                            (type(self).__name__, name))

class MyClass(object):
    __metaclass__ = MyClassMeta

>>> MyClass.fake
fake

But:

>>> MyClass().fake
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'MyClass' object has no attribute 'fake'

To resolve this problem I use next code:

class FakeAttrMixin():
   def __getattr__(self, name):
       if name.startswith('fake'):
           return name
       raise AttributeError("%r object has no attribute %r" %
                            (type(self).__name__, name))

class MyClassMeta(type, FakeAttrMixin):
    pass

class MyClass(object, FakeAttrMixin):
    __metaclass__ = MyClassMeta

>>> MyClass.fake
fake

>>> MyClass().fake
fake

MyClass.fake will call __getattr__ with MyClass and fake arguments.

MyClass().fake will call __getattr__ with MyClass instance and fake arguments.

And it's ok if I implement __getattr__ logic only on my mixin and don't use self argument.

Can I write custom value resolving by class and instance more beautiful and why field value resolving for MyClass.field and MyClass().field with MyClass(object): field = 1 definition works different if compare with __getattr__ method? Because when I want get field it at first searching in instance, then in class, but I can't understand why __getattr__ works another way.

Similar questions: __getattr__ on a class and not (or as well as) an instance and Difference between accessing an instance attribute and a class attribute.

Community · Accepted Answer · 2017-05-23 11:51:27Z

3

No, if you have to support both arbitrary attribute lookup on the class as well as the instance, then your only option is to implement a __getattr__ hook method on both the metaclass and the class, one each to support lookups on the class and the instance.

This is because special hook methods are always looked up on the type, so type(obj).__getattr__. Hence, for MyClass.fake the metaclass __getattr__ is used. See Special method lookup for new-style classes; I explained why this is in a previous answer.

The short reason is that in your case, MyClass.fake would translate into MyClass.__getattr__('fake') and __getattr__ is then an unbound method expecting two arguments (self and name), which would fail.

edited May 23, 2017 at 11:51

CommunityBot

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answered Mar 13, 2013 at 14:13

Martijn Pieters

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