Union of dict objects in Python [duplicate]

Question

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• How to merge two dictionaries in a single expression? 44 answers

How do you calculate the union of two dict objects in Python, where a (key, value) pair is present in the result iff key is in either dict (unless there are duplicates)?

For example, the union of {'a' : 0, 'b' : 1} and {'c' : 2} is {'a' : 0, 'b' : 1, 'c' : 2}.

Preferably you can do this without modifying either input dict. Example of where this is useful: Get a dict of all variables currently in scope and their values

Answer 1

Two dictionaries

def union2(dict1, dict2):
    return dict(list(dict1.items()) + list(dict2.items()))

n dictionaries

def union(*dicts):
    return dict(itertools.chain.from_iterable(dct.items() for dct in dicts))

Answer 2

If you need both dicts to remain independent, and updatable, you can create a single object that queries both dictionaries in its __getitem__ method (and implement get, __contains__ and other mapping method as you need them).

A minimalist example could be like this:

class UDict(object):
   def __init__(self, d1, d2):
       self.d1, self.d2 = d1, d2
   def __getitem__(self, item):
       if item in self.d1:
           return self.d1[item]
       return self.d2[item]

And it works:

>>> a = UDict({1:1}, {2:2})
>>> a[2]
2
>>> a[1]
1
>>> a[3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in __getitem__
KeyError: 3
>>> 

Answer 3

This question provides an idiom. You use one of the dicts as keyword arguments to the dict() constructor:

dict(y, **x)

Duplicates are resolved in favor of the value in x; for example

dict({'a' : 'y[a]'}, **{'a', 'x[a]'}) == {'a' : 'x[a]'}

Answer 4

You can also use update method of dict like

a = {'a' : 0, 'b' : 1}
b = {'c' : 2}

a.update(b)
print a