Python Classes: adding dynamic attributes to methods

Question

Say we have a class:

class Foo (object):
...     def __init__(self,d):
...         self.d=d
...     def return_d(self):
...         return self.d

... and a dict:

d={'k1':1,'k2':2}

... and an instance:

inst=Foo(d)

Is there a way to dynamically add attributes to return_d so:

inst.return_d.k1 would return 1?

Why won't inst.return_d()['k1'] do? The . operator does an attribute lookup, so whatever return_d returns, should support attribute lookups, perhaps via the __getattr__ hook. Note that you expect return_d to be an attribute, but you defined it as a method, so the example you state would not work on that account alone. — Martijn Pieters
– Martijn Pieters, Commented Sep 13, 2012 at 10:05
Partial dupe of Accessing dict keys like an attribute in Python? — Martijn Pieters
– Martijn Pieters, Commented Sep 13, 2012 at 10:06
@Pieter: it would do. It is a silly example, just trying to lear more about classes and what I can and cannot do. — root
– root, Commented Sep 13, 2012 at 10:08

Martijn Pieters · Accepted Answer · 2012-09-13 10:09:23Z

9

You'd need to do two things: declare return_d as an attribute or property, and return a dict-like object that allows attribute access for dictionary keys. The following would work:

class AttributeDict(dict): 
    __getattr__ = dict.__getitem__

class Foo (object):
    def __init__(self,d):
        self.d=d

    @property
    def return_d(self):
        return AttributeDict(self.d)

Short demo:

>>> foo = Foo({'k1':1,'k2':2})
>>> foo.return_d.k1
1

The property decorator turns methods into attributes, and the __getattr__ hook allows the AttributeDict class to look up dict keys via attribute access (the . operator).

answered Sep 13, 2012 at 10:09

Martijn Pieters

1.1m326 gold badges4.2k silver badges3.4k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user1726343 Over a year ago

Does the dict class have any important attributes that this might obscure?

Martijn Pieters Over a year ago

__getattr__ is only consulted for attributes not found otherwise; methods on the dict object trump keys with the same name.

Collectives™ on Stack Overflow

Python Classes: adding dynamic attributes to methods

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related