how to append to a dictionary values from another dictionary if keys match

Question

let's say I have two dictionaries

dict_1 ={'A': 'a', 'B':'b', 'C': 'c', 'D':'d', 'E':'e','F':f}
dict_2 ={'A': None, 'G': None, 'H': None, 'I': None,'L': None}

I'd like a function that returns the second dictionary as

dict_2 ={'A': 'a', 'G': None, 'H': None, 'I': None,'L': None}

which is matching the keys of dict_1 against those in dict_2. If one matches replace the value in dict_2 with the value in dict_1 for that key. Otherwise nothing.

Answer 1

A simple way to do this by iterating over dict_2 's items and using dict_1.get() providing default value as dict_2 corresponding value -

>>> dict_1 ={'A': 'a', 'B':'b', 'C': 'c', 'D':'d', 'E':'e','F':f}
>>> dict_2 ={'A': None, 'G': None, 'H': None, 'I': None,'L': None}
>>> for k,v in dict_2.items():
...     dict_2[k] = dict_1.get(k,v)
...
>>> dict_2
{'G': None, 'H': None, 'I': None, 'L': None, 'A': 'a'}

---

Same using dict comprehension -

>>> dict_1 ={'A': 'a', 'B':'b', 'C': 'c', 'D':'d', 'E':'e','F':f}
>>> dict_2 ={'A': None, 'G': None, 'H': None, 'I': None,'L': None}
>>> dict_2 = {k:dict_1.get(k,v) for k,v in dict_2.items()}
>>> dict_2
{'G': None, 'H': None, 'I': None, 'L': None, 'A': 'a'}

Answer 2

Another way is to find the common keys, and iterate over them like this:

dict_1 ={'A': 'a', 'B':'b', 'C': 'c', 'D':'d', 'E':'e','F':f}
dict_2 ={'A': None, 'G': None, 'H': None, 'I': None,'L': None}

for key in set(dict_1.iterkeys()) & set(dict_2.iterkeys()):
    dict_2[key] = dict_1[key]

This should be much less computational expensive if it's relatively few common entries compared to the total number of entries in the dictionaries.

Answer 3

You could use dict comprehension and if else to do it :

dict_1 ={'A': 'a', 'B':'b', 'C': 'c', 'D':'d', 'E':'e','F':f}
dict_2 ={'A': None, 'G': None, 'H': None, 'I': None,'L': None}
dict_2={a : dict_1[a] if a in dict_1 else dict_2[a] for a in dict_2.keys() }
print dict_2

output:

{'A': 'a', 'H': None, 'I': None, 'L': None, 'G': None}

But this creates a new dict object

Answer 4

You can use dict.viewkeys to find the common keys:

dict_1 ={'A': 'a', 'B':'b', 'C': 'c', 'D':'d', 'E':'e','F':'f'}
dict_2 ={'A': None, 'G': None, 'H': None, 'I': None,'L': None}

for k in dict_1.viewkeys() & dict_2.viewkeys():
    dict_2[k] = dict_1[k]

print(dict_2)
{'A': 'a', 'H': None, 'I': None, 'L': None, 'G': None}

For python3 just use .keys as it returns a dictionary-view-object not a list:

for k in dict_1.keys() & dict_2.keys():
    dict_2[k] = dict_1[k]

print(dict_2)