How do I merge dictionaries together in Python?

Question

d3 = dict(d1, **d2)

I understand that this merges the dictionary. But, is it unique? What if d1 has the same key as d2 but different value? I would like d1 and d2 to be merged, but d1 has priority if there is duplicate key.

Answer 1

You can use the .update() method if you don't need the original d2 any more:

Update the dictionary with the key/value pairs from other, overwriting existing keys. Return None.

E.g.:

>>> d1 = {'a': 1, 'b': 2} 
>>> d2 = {'b': 1, 'c': 3}
>>> d2.update(d1)
>>> d2
{'a': 1, 'c': 3, 'b': 2}

Update:

Of course you can copy the dictionary first in order to create a new merged one. This might or might not be necessary. In case you have objects in your dictionary, copy.deepcopy should also be considered.

Answer 2

.

d1={'a':1,'b':2}
d2={'a':10,'c':3}

d1 overrides d2:

dict(d2,**d1)
# {'a': 1, 'c': 3, 'b': 2}

d2 overrides d1:

dict(d1,**d2)
# {'a': 10, 'c': 3, 'b': 2}

This behavior is not just a fluke of implementation; it is guaranteed in the documentation:

If a key is specified both in the positional argument and as a keyword argument, the value associated with the keyword is retained in the dictionary.

Answer 3

If you want d1 to have priority in the conflicts, do:

d3 = d2.copy()
d3.update(d1)

Otherwise, reverse d2 and d1.