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?

See: http://stackoverflow.com/questions/3031219/python-recursively-access-dict-via-attributes-as-well-as-index-access — Josh Lee, Feb 16 '11 at 19:19
`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?](http://stackoverflow.com/questions/4984647/accessing-dict-keys-like-an-attribute-in-python) — , Feb 16 '11 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, Oct 20 '12 at 15:22

score 10 · Accepted Answer · answered Feb 16 '11 at 19:23

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 '11 at 19:23

shang

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4

You should catch and translate `KeyError` — `__getattr__` is supposed to throw `AttributeError`. – Cat Plus Plus Feb 16 '11 at 19:27
2

If you're going to do that, it's better to just do `__getattr__ = __getitem__` and `__setattr__ = __setitem__`. Simpler, and one less function call. – kindall Feb 16 '11 at 19:39
@kindall @PiotrLegnica: You both make excellent (although mutually exclusive) points! Thanks! :) – shang Feb 16 '11 at 19:44
@kindall, that's an awesome idea! – slacy Feb 16 '11 at 19:56
@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? – Amelio Vazquez-Reina Feb 05 '13 at 20:44
Use the approach that 'slacy' describes in his question, it works fine – Gearoid Murphy Feb 08 '13 at 11:14
doesnt seem to work for nested object. And what if the key in the dict has spaces.. – viveksinghggits Feb 07 '19 at 10:19

score 3 · Answer 2 · answered Feb 16 '11 at 19:20

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 '11 at 19:20

nmichaels

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score 2 · Answer 3 · answered Feb 16 '11 at 19:43

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.

Are there any 'gotchas' with this Python pattern?

3 Answers3

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