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

[Serhiy Storchaka]

I propose to add two methods:

dict.getkey(key) returns original key stored in the dict which is equal 
to specified key. E.g.

 >>> d = {2: 'a', 5.0: 'b'}
 >>> d.getkey(2.0)
2
 >>> d.getkey(5)
5.0
 >>> d.getkey(17)
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
KeyError: 17

set.get(value) returns original value stored in the set which is equal 
to specified value. E.g.

 >>> s = {2, 5.0}
 >>> s.get(2.0)
2
 >>> s.get(5)
5.0
 >>> s.get(17)
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
KeyError: 17