Python: Dictionary merge by updating but not overwriting if value exists

Question

If I have 2 dicts as follows:

d1 = {(\'unit1\',\'test1\'):2,(\'unit1\',\'test2\'):4}
d2 = {(\'unit1\',\'test1\'):2,(\'unit1\',\'test2\'):\'\'}

Answer 1

To add to d2 keys/values from d1 which do not exist in d2 without overwriting any existing keys/values in d2:

temp = d2.copy()
d2.update(d1)
d2.update(temp)