ActiveState Code

Recipe 299777: Simplified Mutable Instances

Often you want to just create an instance with nothing in it, then modify arbitrary values. According to the standard you should do: class Something: pass I propose the following better solution:

Python 
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class MutableInstance(dict):
 def __init__(self):
  self.__dict__ = self

# This makes common tasks easier, not by much but conceptually it unifies things

Foo = MutableInstance()
Foo.x = 5
assert Foo['x'] == 5
Foo.y = 7
assert Foo.keys() == ['x', 'y']
assert Foo.values() == [5, 7]

# And now you can pass it to anything that wants a dictionary too.

Discussion

It's a suprisingly small but powerful change over just inheriting object (or nothing at all).

Comments

  1. 1. At 2:25 p.m. on 15 aug 2004, Hamish Lawson said:

    Care needed with allowing arbitrary attributes to be set. Consider Foo['keys'] == 5. Now you've clobbered your keys method. That's the reason why attribute access and dictionary lookup are separate mechanisms and have separate namespaces. Generally dictionaries should be used for arbitrary fields.

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