How to merge two dictionaries in Python 2 and 3

Suppose we have two dictionaries A and B to be merged, where the values in B will replace those values in A if they share the same key.

A = {'x': 10, 'y': 20}
B = {'y': 30, 'z': 40}

Python dictionary object has an instrinsic method update() method to update dictionary A with B,

A.update(B)

But the data of A will be replaced in-place instead of returning a new dictionary containing merge A and B.

We will introduce the methods of how to merge two dictionaries and return a new dictionary.

Python 2.7 Dictionary Merge

Dictionary comprehension method - 1

C = {key: value for d in (A, B) for key, value in d.items()}

d.itmes() returns a list of (key, value) pairs as 2-tuples of dictionary d.

This method uses the nested dictionary comprehension to merge two dictionary. The right order of for should be paid extra attention. It should be,

flattern_patterns = [planet
                    for sublist in planets
                    for planet in sublist]

Dictionary comprehension method - 2

We could also use dict() method to initialize the new dictionary.

C = dict((key, value) for d in (A, B) for key, value in d.items())

Technically speaking, it is almost the same as the above method but differs in the performance that will be mentioned afterward.

itertools.chain Method

itertools module standardizes a core set of iterator building blocks. It has features like fast and memory efficient.

itertools.chain returns a chain object whose .next() method returns elements from the first iterable until it is exhausted, then the next iterable(s), until all are exhausted.

dict(itertools.chain(A.iteritems(), B.iteritems()))

iteritems() returns an iterator over the (key, value) items of the dictionary.

Therefore, the scripts above will return a dictionary containing items of A and B.

copy and update Method

As mentioned at the beginning, update() could merge A and B, but will replace the dictionary in-place. We could use copy() method to make a copy of the dictionary A.

m = A.copy()
C = m.update(B)

Merge Methods Performance Analysis and Comparison

import timeit

A = {'x': 10, 'y': 20}
B = {'y': 30, 'z': 40}

SETUP_CODE = '''
A = {'x': 10, 'y': 20}
B = {'y': 30, 'z': 40}
'''

TEST_CODE = '''
{key: value for d in (A, B) for key, value in d.items()}
'''
print min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000))

TEST_CODE = '''
dict((key, value) for d in (A, B) for key, value in d.items())
'''
print min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000))

TEST_CODE = '''
dict(itertools.chain(A.iteritems(), B.iteritems()))
'''
print min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000))

SETUP_CODE = '''
def merge_dicts(a, b):
    m = a.copy()
    m.update(b)
    return m

A = {'x': 10, 'y': 20}
B = {'y': 30, 'z': 40}
'''

TEST_CODE = '''
merge_dicts(A, B)
'''
print min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000))

The result is

0.0162378
0.029774
0.019975
0.0110059

Python 3.5 (and above) Dictionary Merge Method

From Python 3.5, besides the same methods as in Python 2.7, it has also ** unpacking operator of dictionary, as introduced in PEP-448. It allows unpacking an arbitrary number of items.

>>> C = {**A, **B}
>>> C
{'x': 10, 'y': 30, 'z': 40}

import timeit

A = {'x': 10, 'y': 20}
B = {'y': 30, 'z': 40}

SETUP_CODE = '''
A = {'x': 10, 'y': 20}
B = {'y': 30, 'z': 40}
'''

TEST_CODE = '''
{**A, **B}
'''
print(min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000)))

TEST_CODE = '''
{key: value for d in (A, B) for key, value in d.items()}
'''
print(min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000)))

TEST_CODE = '''
dict((key, value) for d in (A, B) for key, value in d.items())
'''
print(min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000)))

TEST_CODE = '''
dict(itertools.chain(A.items(), B.items()))
'''
print(min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000)))

SETUP_CODE = '''
def merge_dicts(a, b):
    m = a.copy()
    m.update(b)
    return m

A = {'x': 10, 'y': 20}
B = {'y': 30, 'z': 40}
'''

TEST_CODE = '''
merge_dicts(A, B)
'''
print(min(timeit.repeat(setup = SETUP_CODE, stmt = TEST_CODE, repeat = 3, number=10000)))

0.0017047999999999508
0.009127499999999955
0.0168952
0.01078009999999996
0.005767999999999995

Merge Methods Conclusion

In Python 2.7, copy and update is the best method.

m = A.copy()
C = m.update(B)

In Python 3.5+, dictionary unpacking method is the best.

{**A, **B}