Are there any 'gotchas' with this Python pattern?

Question

Here's the pattern I'm thinking of using:

class Dicty(dict): 
    def __init__(self): 
         self.__dict__ = self 

d = Dicty()
d.foo = 'bar' 
print d['foo']
>>> bar 
d['foo'] = 'baz'
print d.foo
>>> 'baz'

Generally, I prefer the semantics of object attribute access over dict get/set access, but there are some circumstances where dict-like access is required (for example, d['foo-bar'] = 'baz') and I'd prefer not to have special getter setter methods for these cases, so thus, the dual behavior of dict & object at the same time with shared attributes.

Are there any gotchas with the above pattern?

isinstance(dict, object). You don't need to inherit from object. And the implementation is more than hacky. Also: possible duplicate of Accessing dict keys like an attribute in Python? — user395760
– user395760, Commented Feb 16, 2011 at 19:21
Objects created in this manner can be pickled without problem but objects created in the style suggested by the accepted answer cannot. — Gearoid Murphy
– Gearoid Murphy, Commented Oct 20, 2012 at 15:22

shang · Accepted Answer · 2011-02-16 19:23:51Z

10

Here's a less "hacky" way to achieve the same effect:

class Dicty(dict):
    def __getattr__(self, key):
        return self[key]

    def __setattr__(self, key, value):
        self[key] = value

I think that your way may work fine as well, but setting the __dict__ attribute like that seems a bit iffy style-wise, and is bound to raise some questions if anyone else ends up reading your code.

answered Feb 16, 2011 at 19:23

shang

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7 Comments

Cat Plus Plus Over a year ago

You should catch and translate KeyError — __getattr__ is supposed to throw AttributeError.

kindall Over a year ago

If you're going to do that, it's better to just do __getattr__ = __getitem__ and __setattr__ = __setitem__. Simpler, and one less function call.

shang Over a year ago

@kindall @PiotrLegnica: You both make excellent (although mutually exclusive) points! Thanks! :)

slacy Over a year ago

@kindall, that's an awesome idea!

Amelio Vazquez-Reina Over a year ago

@GearoidMurphy. I am interested in pickling data containers that support the container.attribute syntax . Is there a reason why these objects cannot be pickled? Do you have any suggestions for an alternative solution that supports pickling?

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nmichaels · Accepted Answer · 2011-02-16 19:20:49Z

3

Don't set self.__dict__. Call __init__(self, *args, **kwargs) on the superclass. Also, dict inherits from object so you don't need to specify it.

answered Feb 16, 2011 at 19:20

nmichaels

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Comments

Keith · Accepted Answer · 2011-02-16 19:43:45Z

2

A couple of things. One is if you try and use a dictionary method, such as keys, you won't be able to get it now. There were some other issues I ran into, such as being pickle-able and copy-able.

But I have implemented something that does this without those problems. You can see it here, it's the AttrDict class in the dictlib.py module. That module also contains a class that can wrap another mapping style object, in cases where it can't be subclassed.

answered Feb 16, 2011 at 19:43

Keith

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