Python property for array operations

Question

When using Python properties (setters and getters), usually following is used:

class MyClass(object):
 ... 
 @property
 def my_attr(self):
 ...
 @my_attr.setter
 def my_attr(self, value):
 ... 

However, is there any similar approach for appending / removing arrays? For example, in a bi-directional relationship between two objects, when removing object A, it would be nice to dereference the relationship to A in object B. I know that SQLAlchemy has implemeneted a similar function.

I also know that I can implement methods like

def add_element_to_some_array(element):
 some_array.append(element)
 element.some_parent(self)

but I would prefer to do it like "properties" in Python.. do you know some way?

Answer 1

To make your class act array-like (or dict-like), you can override __getitem__ and __setitem__.

class HappyArray(object):
 #
 def __getitem__(self, key):
 # We skip the real logic and only demo the effect
 return 'We have an excellent %r for you!' % key
 #
 def __setitem__(self, key, value):
 print('From now on, %r maps to %r' % (key, value))
>>> h = HappyArray()
>>> h[3]
'We have an excellent 3 for you!'
>>> h[3] = 'foo'
From now on, 3 maps to 'foo'

If you want several attributes of your object to exhibit such behavior, you need several array-like objects, one for each attribute, constructed and linked at your master object's creation time.

• kungcc: Thanks, I was hopeing for some more property like solution, but I guess that is not possible.. I will upvote your answer =)

Answer 2

The getter property will return reference to the array. You may do array operations with that. Like this

class MyClass(object):
 ... 
 @property
 def my_attr(self):
 ...
 @my_attr.setter
 def my_attr(self, value):
 ... 
m = MyClass()
m.my_attr.append(0) # <- array operations like this