Divide the values of two dictionaries in python

Question

I have two dictionaries with the same keys and I would like to do division on the values to update or create a new dictionary, keeping the keys intact, with the quotient as the new value for each of the keys.

d1 = { 'a':12 , 'b':10 , 'c':2 }
d2 = { 'a':0 , 'c':2 , 'b':5}
d3 = d2 / d1

d3 = { 'a':0 , 'b':0.5 , 'c':1 }

Aside from iterating through the key, value pairs and creating ordered lists of the values, then dividing, I'm not sure how to do this. I was hoping for a more elegant solution.

Answer 1

Using viewkeys (python2.7):

{k: float(d2[k])/d1[k] for k in d1.viewkeys() & d2}

Same in python 3 (where we can drop the float() call altogether):

{k: d2[k]/d1[k] for k in d1.keys() & d2}

Yes, I am using a key intersection here; if you are absolutely sure your keys are the same in both, just use d2:

{k: float(d2[k])/d1[k] for k in d2}

And to be complete, In Python 2.6 and before you'll have to use a dict() constructor with a generator expression to achieve the same:

dict((k, float(d2[k])/d1[k]) for k in d2)

which generates a sequence of key-value tuples.

Answer 2

This works for all pythons, I would however recommend the solution by @MartijnPieters if have Py 2.7+

>>> d1 = { 'a':12 , 'b':10 , 'c':2 }
>>> d2 = { 'a':0 , 'c':2 , 'b':5}
>>> d3 = dict((k, float(d2[k]) / d1[k]) for k in d2)
>>> d3
{'a': 0.0, 'c': 1.0, 'b': 0.5}