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

Just switch the order:

z = dict(d2.items() + d1.items())

By the way, you may also be interested in the potentially faster update method.

In Python 3, you have to cast the view objects to lists first:

z = dict(list(d2.items()) + list(d1.items())) 

If you want to special-case empty strings, you can do the following:

def mergeDictsOverwriteEmpty(d1, d2):
    res = d2.copy()
    for k,v in d2.items():
        if k not in d1 or d1[k] == '':
            res[k] = v
    return res