1st Sketch at at ReadOnly dict

[Charles Hixson]

On 01/20/2014 12:52 PM, Peter Otten wrote:
> Charles Hixson wrote:
>
>> This is just a first sketch, and I haven't yet attempted to test it, so
>> what I'm hoping for is criticisms on the general approach.
>>
>> class RODict:
>>       def __init__ (self, ddict = {}):
> Default values are evaluted just once when the method is created. Mutable
> default values mean trouble:
>
>>>> class D:
> ...     def __init__(self, dict={}):
> ...             self.dict = dict
> ...     def __setitem__(self, key, value):
> ...             self.dict[key] = value
> ...     def __repr__(self): return repr(self.dict)
> ...
>>>> d1 = D()
>>>> d2 = D()
>>>> d1[1] = 42
>>>> d2[2] = 42
>>>> d1
> {1: 42, 2: 42}
>>>> d2
> {1: 42, 2: 42}
>
>>           if not isinstance(ddict, dict):
>>               raise    TypeError("ddict must be a dict.  It is " +
>> repr(ddict))
>>           self._ddict    =    ddict
> I think instead of the type check I would build a new dict from the
> argument. The usual initializers
>
> dict({1:2, 3:4})
> dict([(1,2), (3,4)])
> dict(a=1, b=2)
>
> should work with your read-only dict.
>
>>       ##    Test for containment.
>>       #    @param    key    The item to be found.
>>       #    @return    True    If key is in the instance, otherwise False.
> Docstrings are usually prefered over comments like the above.
>
>>       def __contains__(self, key):
>>           return    key in self._ddict
> Did you know
>
> http://docs.python.org/dev/library/collections.abc.html#collections.abc.Mapping
>
> Subclass from it to ensure that all the usuall methods are defined.
>
That link, 
http://docs.python.org/dev/library/collections.abc.html#collections.abc.Mapping 
, is a very good one.  I hadn't realized that it separated Mapping from 
MutableMapping.
It would be nice if there were some examples of it's usage around, 
though.  I can't really tell how to use it.  (E.g., it mixes in 
__contains__, but it doesn't show how to specify what you are testing 
for cotainment in.  So it looks as if I need to specify everything 
anyway because I can't tell what might be implemented in some 
inappropriate way.)  OTOH, it's a good list of what needs to be 
implemented.  (I see that I left out implementation of eq and ne, which 
is pretty straightfoward, but apparently needed (unless it's 
automatically being handled by the mixin...which I can't tell).

-- 
Charles Hixson