How do I merge dictionaries together in Python?

In Python2,

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.

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

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

Otherwise, reverse d2 and d1.

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 compound objects (objects that contain other objects, like lists or class instances) in your dictionary, copy.deepcopy should also be considered.