Dynamic class attribute access (get/set) via string names

Today I'd like to share some of my experience concerning dynamic class attribute access in python. In most cases you can easily predict the attributes you will want to access, but sometimes - especially when the attributes/fields depend on some input data this may be a problem. 

There are 3 magic python build in functions that may help a lot: setattr, getattr, hasattr responsible for setting, getting named class attributes and checking if they exist, respectively.

A simple example to get familiar how it works:

>>> class A():
...     pass
... 
>>> my_cls = A()
>>> hasattr(my_cls, 'test_attr')
False
>>> my_cls.test_attr
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: A instance has no attribute 'test_attr'
>>> setattr(my_cls, 'test_attr', 'some value')
>>> hasattr(my_cls, 'test_attr')
True
>>> my_cls.test_attr
'some value'
>>> getattr(my_cls, 'test_attr', 'default_value')
'some value'

This is quite intuitive, you may use these methods to update class attributes according to a dict:

>>> my_dict = {'attr1': 1, 'attr2' : 5, 'attr3' : 10}
>>> for k,v in my_dict.items():
...     setattr(my_cls, k, v)
... 
>>> my_cls.attr1
1
>>> getattr(my_cls, "attr3", -1)
10
>>> getattr(my_cls, "attr4", -1)
-1

Since getattr supports default values, you can manage without using hasattr.

These functions are great, when you can't predict what attributes you will need to access or when you are not sure whether some attributes are set. A good usage example are global configuration files. For example in django:

from django.conf import settings



some_param = getattr(settings, 'MY_CONF_PARAM',  100)

Just keep in mind that using by using setattr you may override ANY attribute, so make sure you implement some validation mechanisms. And if you ever need it - deleting attributes is also possible, try delattr.

~KR