On Wed, Oct 07, 2015 at 04:10:20PM +0000, Albert-Jan Roskam wrote:
> Hi,
> I wanted to create a read-only dict to hold some constants. I looked around
> on the internet and created two implementations:-FrozenDict (derives from
> collections.mapping)-ChillyDict (derives from dict, which seems more obvious
> to me)
> The code can be found here: http://pastebin.com/QJ3V2mSK
> Some questions:1. one doctest from FrozenDict fails: fd.keys() returns
> an empty list. Why?
No it doesn't.
py> fd = FrozenDict(a=1, b=2)
py> fd.keys()
['a', 'b']
That's under Python 2.7.
In 3.3, you will have a problem that FrozenDict is not a proper
iterator. You can't set self.__next__ = self.next, that won't work.
Dunder methods have to be on the class, not on the instance, so instead
of making the assignment in the __init__ method, put this in the body of
your class:
def next(self):
# Python 2 method
...
__next__ = next # Python 3 method.
Unfortunately that's not enough to get it working in Python 3. I need
more time to think about that.
> 2. Is FrozenDict the way to use collections.mapping (aside from the
> error!). I just discovered this and i seems quite cool (pun intended)
I think the error is quite significant...
> 3. Which implementation is better, and why? I like ChillyDict better
> because it is more straightforward and shorter.
I dislike the ChillyDict implementation because it looks like it
supports item assignment:
hasattr(ChillyDict, '__setitem__')
will return True, but it actually doesn't. That could make it risky in
code that assumes that the existence of __setitem__ means you can set
items.
--
Steve
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