[Python-ideas] Add dict.getkey() and set.get()

[Oscar Benjamin]

On 15 September 2013 00:07, Steven D'Aprano <steve at pearwood.info> wrote:
> On Sat, Sep 14, 2013 at 09:52:30AM -0700, David Mertz wrote:
>
>> def getexact(m, v):
>>     for x in m:
>>         if x==v: return x
>>     else:
>>         raise KeyError(v)
>
> This has the flaw that it is O(N) rather than O(1). It's really quite
> unfortunate to have dicts and sets able to access keys in (almost)
> constant time, but not be able to communicate that key back to the
> caller except by walking the entire dict/set.

I don't know whether this is relying on undefined behaviour but the
following is O(1) and seems to work:

>>> def canonical_key(d, k):
...     k, = {k} & d.keys()
...     return k
...
>>> canonical_key({1:'q', 2.0:'w'}, 1.0)
1
>>> canonical_key({1:'q', 2.0:'w'}, 2)
2.0


Oscar