[Tutor] How to get a key from dictionary?

[Ike Hall]

Oh I just realized....in the function I gave earlier, I meant to use 
dict.keys() instead of dict.has_key().....Ive got physics on the brain today 
instead of python....tomorrow Ill be better  :)

Ike

On Monday 25 March 2002 07:26 pm, you wrote:
> At 05:23 PM 3/25/2002 -0600, Isaac Hall wrote:
> >If that is the case, then the best way I have thought of
> >so far is to place a list as the value for each new key.
>
> Yes, I had to do something like this recently.  I wanted
> a dictionary of spheres keyed by floating point distance
> from the origin, but with several values possible at the
> same distance (same key).  So I decided to just accumulate
> all those sharing a key in a list.  Sometimes this is a
> good solution.
>
> >def addtodict(dict):           # Import a normal dictionary
>
> Not really importing -- just passing a dictionary as a
> parameter.
>
> >     for key in dict.has_key(): # loop over all keys
> >         if dict[dict[key]]:     #see if we already have an
> >                                 #entry for each value
> >             dict[dict[key]].append(key) #if so, append to it
> >         else:
> >             dict[dict[key]] = [key] #if not create it
> >
> >I haven't tested this, but something of this nature should work.
>
> Very untested :-D.  dict.has_key returns 1 or 0 (true or
> false) so isn't something to iterate over with for.
>
> An easy way to accumulate:
>   >>> def addtodict(thedict, thekey, thevalue):
>
>          thedict[thekey] = thedict.get(thekey,[]) + [thevalue]
>
>   >>> thedict = {}
>   >>> addtodict(thedict,'aa',1)
>   >>> addtodict(thedict,'bb',3)
>   >>> addtodict(thedict,'aa',2)
>   >>> addtodict(thedict,'cc',4)
>   >>> addtodict(thedict,'bb','r')
>   >>> thedict
>
>   {'aa': [1, 2], 'cc': [4], 'bb': [3, 'r']}
>
> What this does is use thedict.get(thekey,[]) + [thevalue]
> to return return the existing value appended to the new
> value, or, if there's no existing value, to initialize
> with [thevalue] (because if there's no existing value,
> thedict.get(thekey,[]) returns [] and [] + [thevalue]
> is just [thevalue].
>
> The above is sort of non-functional programming in that
> it uses a function to change a dictionary object passed
> as a parameter, but returns nothing.  You can return
>
> thedict if you prefer syntax like:
>    >>> thedict = add2dict(thedict, 'rr', 78)
>
> You can also define the function with thedict assumed
> global.  Or have a function factory to bind a specific
>
> dictionary (extending a thread I was working on earlier):
>   >>> def addto(thedict):
>
>           def main(thekey,thevalue):
>              thedict[thekey] = thedict.get(thekey,[]) + [thevalue]
>           return main
>
>   >>> it = {}             # some dictionary
>   >>> add2it = addto(it)  # a function for this dict in particular
>   >>> add2it
>
>   <function main at 0x01256F20>
>
>   >>> add2it('aa',1)       # no need to mention which dictionary
>   >>> add2it('aa',2)
>   >>> add2it('bb',3)
>   >>> it
>
>   {'aa': [1, 2], 'bb': [3]}
>
> Kirby