[dictionary] how to get key by item

[Delaney, Timothy C (Timothy)]

Roy Smith wrote:

>>     >>> forward = {10 : 50, 2 : 12, 4 : 43}
>>     >>> reverse = dict([(v,k) for (k,v) in forward.iteritems()])    
>>     >>> print forward {10: 50, 4: 43, 2: 12}
>>     >>> print reverse
>>     {50: 10, 43: 4, 12: 2}
> 
> BTW, does Python really build the intermediate list and throw it away
> after using it to initialize the dictionary, or is it smart enough to
> know that it doesn't really need to build the whole list in memory?

Python will do what you tell it. In the above case, it will build a
list.

Using Python 2.4, the above can be rewritten as a generator expression:

>>> forward = {10 : 50, 2 : 12, 4 : 43}
>>> reverse = dict((v,k) for (k,v) in forward.iteritems())    
>>> print forward {10: 50, 4: 43, 2: 12}
>>> print reverse
{50: 10, 43: 4, 12: 2}

which does not build an intermediate list. Note that all I've done is
remove the [ and ] - that's because a genexp must be surrounded by
parentheses, but *any* parentheses will do - for example, the
parentheses surrounding the parameters in a function call.

Tim Delaney