Re: join dictionaries using keys from one values

[danplawson]

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

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Re: join dictionaries using keys from one values

[ProvoWallis]

Thanks again. This is very helpful.

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Re: join dictionaries using keys from one values

[Erik Max Francis]

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.

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Re: join dictionaries using keys from one values

[bonono]

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.

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Re: join dictionaries using keys from one values

[ProvoWallis]

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)

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Re: join dictionaries using keys from one values

[Devan L]

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

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Re: join dictionaries using keys from one values

[Alex Martelli]

ProvoWallis [EMAIL PROTECTED] 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
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