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
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