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join dictionaries using keys from one & values

I'm still learning python so this might be a crazy question but I
thought I would ask anyway. Can anyone tell me if it is possible to
join two dictionaries together to create a new dictionary using the
keys from the old dictionaries?

The keys in the new dictionary would be the keys from the old
dictionary one (dict1) and the values in the new dictionary would be
the keys from the old dictionary two (dict2). The keys would be joined
by matching the values from dict1 and dict2. The keys in each
dictionary are unique.

dict1 = {1:'bbb', 2:'aaa', 3:'ccc'}

dict2 = {5.01:'bbb', 6.01:'ccc', 7.01:'aaa'}

dict3 = {1 : 5.01, 3 : 6.01, 2 : 7.01}

I looked at "update" but I don't think it's what I'm looking for.

Thanks,

Greg

Dec 6 '05 #1
7 7713
ProvoWallis wrote:
I'm still learning python so this might be a crazy question but I
thought I would ask anyway. Can anyone tell me if it is possible to
join two dictionaries together to create a new dictionary using the
keys from the old dictionaries?


There is no builtin method. The usual way is to just wrap a class
around two dictionaries, one for mapping keys to values and the other
for mapping values back to keys.

--
Erik Max Francis && ma*@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
Yes I'm / Learning from falling / Hard lessons
-- Lamya
Dec 6 '05 #2

ProvoWallis wrote:
I'm still learning python so this might be a crazy question but I
thought I would ask anyway. Can anyone tell me if it is possible to
join two dictionaries together to create a new dictionary using the
keys from the old dictionaries?

The keys in the new dictionary would be the keys from the old
dictionary one (dict1) and the values in the new dictionary would be
the keys from the old dictionary two (dict2). The keys would be joined
by matching the values from dict1 and dict2. The keys in each
dictionary are unique.

dict1 = {1:'bbb', 2:'aaa', 3:'ccc'}

dict2 = {5.01:'bbb', 6.01:'ccc', 7.01:'aaa'}

dict3 = {1 : 5.01, 3 : 6.01, 2 : 7.01}

I looked at "update" but I don't think it's what I'm looking for.

Thanks,

If you can be sure that the value is hashable, I think you can just
invert one of the dict(key/value flipped) and a for loop to create the
new dict

dict2x = dict( ((dict2[k], k) for k in dict2.iterkeys()))
dict3 = dict(((k, dict2x[v]) for k,v in dict1.iteritems()))

This doesn't handle the case where v is in dict1 but not in dict2, it
can be filtered out though.

Dec 6 '05 #3
Thanks so much. I never would have been able to figure this out on my
own.

def dictionary_join(one, two):

dict2x = dict( ((dict2[k], k) for k in dict2.iterkeys()))
dict3 = dict(((k, dict2x[v]) for k,v in dict1.iteritems()))
print dict3

dict1 = {1:'bbb', 2:'aaa', 3:'ccc'}

dict2 = {'5.01':'bbb', '6.01':'ccc', '7.01':'aaa'}

dictionary_join(dict1, dict2)

Dec 6 '05 #4

ProvoWallis wrote:
Thanks so much. I never would have been able to figure this out on my
own.

def dictionary_join(one, two):

dict2x = dict( ((dict2[k], k) for k in dict2.iterkeys()))
dict3 = dict(((k, dict2x[v]) for k,v in dict1.iteritems()))
print dict3

dict1 = {1:'bbb', 2:'aaa', 3:'ccc'}

dict2 = {'5.01':'bbb', '6.01':'ccc', '7.01':'aaa'}

dictionary_join(dict1, dict2)


You might want to make a working function.

def join_dicts(d1,d2):
temp = dict(((d2[k], k) for k in d2.iterkeys()))
joined = dict(((k, temp[v]) for k,v in d1.iteritems()))
return joined

Dec 6 '05 #5
ProvoWallis <gs**********@earthlink.net> wrote:
...
The keys in the new dictionary would be the keys from the old
dictionary one (dict1) and the values in the new dictionary would be
the keys from the old dictionary two (dict2). The keys would be joined
by matching the values from dict1 and dict2. The keys in each
dictionary are unique.
....but are the VALUES unique...? That's the crucial issue and you don't
mention anything about it.
dict1 = {1:'bbb', 2:'aaa', 3:'ccc'}

dict2 = {5.01:'bbb', 6.01:'ccc', 7.01:'aaa'}

dict3 = {1 : 5.01, 3 : 6.01, 2 : 7.01}


But what if in dict1 both keys 2 and 3 had a corresponding value of
'ccc' -- what would you want as a result then? What if key 1 had a
corresponding value of 'ddd' -- not a value in dict2; what would you
want THEN? Without a more complete specification, it's impossible to
tell, and one key Python principle is "in the face of ambiguity, refuse
the temptation to guess".

If values are assured to be unique, and the sets of values of the two
dictionaries are assured to be identical, then the suggestion (already
given in another post) to invert dict2 is a good idea, i.e., as a
function:

def PWmerge(d1, d2):
invd = dict((v2, k2) for k2, v2 in d2.iteritems())
return dict((k1,invd[v1]) for k1,v1 in d1.iteritems())

but without all of the above assurances, different tweaks may be needed
depending on what exactly you want to happen in the several "anomalous"
cases.
Alex
Dec 6 '05 #6
Super simple:

dict3 = {}
for k1 in dict1.keys():
for k2 in dict2.keys():
if dict1.get(k1) == dict2[k2]:
dict3[k1] = k2

works in all cases and can be simplified to an iterated dictionary in
python 2.4

Dec 6 '05 #7
Thanks again. This is very helpful.

Dec 7 '05 #8

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