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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: 1st Sketch at at ReadOnly dict
Date: Mon, 20 Jan 2014 21:52:57 +0100
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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.