Dictionary is the only mapping type in the Python language, which we often encounter in our daily work. The following article mainly introduces you to the relevant information on how to elegantly merge two dictionaries (dict) in Python. In the article The introduction through the sample code is very detailed. Friends in need can refer to it. Let’s take a look together.
Preface
Dictionary is one of the most powerful data types in Python. This article will give you a detailed introduction to Python merging two dictionaries. (dict) related content is shared for everyone’s reference and study. Not much to say, let’s take a look at the detailed introduction.
One line of code merges two dicts
Suppose there are two dicts x and y, merge them into a new dict, no Change the values of x and y, for example
x = {'a': 1, 'b': 2} y = {'b': 3, 'c': 4}
expect to get a new result Z, if the key is the same, y covers x. The expected result is
>>> z {'a': 1, 'b': 3, 'c': 4}
In PEP448, there is a new syntax that can be implemented, and this syntax is supported in python3.5. The merged code is as follows
z = {**x, **y}
A proper line of code. Since many people are still using python2, for people with python2 and python3.0-python3.4, there is a more elegant method, but it requires two lines of code.
z = x.copy() z.update(y)
In the above method, y will overwrite the content in x, so the final result is b=3.
## How to do it in one line without using python3.5
def merge_two_dicts(x, y): """Given two dicts, merge them into a new dict as a shallow copy.""" z = x.copy() z.update(y) return z
z = merge_two_dicts(x, y)
def merge_dicts(*dict_args): """ Given any number of dicts, shallow copy and merge into a new dict, precedence goes to key value pairs in latter dicts. """ result = {} for dictionary in dict_args: result.update(dictionary) return result
z = merge_dicts(a, b, c, d, e, f, g)
Some less elegant demonstrations
items
z = dict(x.items() + y.items())
>>> c = dict(a.items() + b.items()) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unsupported operand type(s) for +: 'dict_items' and 'dict_items'
z = dict(list(x.items()) list(y.items ())) , this is such a waste of performance. In addition, the union method based on the list returned by
items() will also fail for python3. Moreover, the union method leads to uncertainty in the value of repeated keys. Therefore, if you have priority requirements for merging two dicts, this method is completely inappropriate.
>>> x = {'a': []} >>> y = {'b': []} >>> dict(x.items() | y.items()) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'list'
>>> x = {'a': 2} >>> y = {'a': 1} >>> dict(x.items() | y.items()) {'a': 2}
Constructor
z = dict(x, **y)
>>> c = dict(a, **b) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: keyword arguments must be strings
dict({}, {1:3}) is illegal because it is an abuse of the mechanism after all. Although this method is more hacky, it is too opportunistic.
Some poor performance but more elegant methods
{k: v for d in dicts for k, v in d.items()}
dict((k, v) for d in dicts for k, v in d.items())
import itertools z = dict(itertools.chain(x.iteritems(), y.iteritems()))
##Performance Test
The following was done on Ubuntu 14.04, in Python 2.7 (System Python):
>>> min(timeit.repeat(lambda: merge_two_dicts(x, y))) 0.5726828575134277 >>> min(timeit.repeat(lambda: {k: v for d in (x, y) for k, v in d.items()} )) 1.163769006729126 >>> min(timeit.repeat(lambda: dict(itertools.chain(x.iteritems(),y.iteritems())))) 1.1614501476287842 >>> min(timeit.repeat(lambda: dict((k, v) for d in (x, y) for k, v in d.items()))) 2.2345519065856934
In python3.5
>>> min(timeit.repeat(lambda: {**x, **y})) 0.4094954460160807 >>> min(timeit.repeat(lambda: merge_two_dicts(x, y))) 0.7881555100320838 >>> min(timeit.repeat(lambda: {k: v for d in (x, y) for k, v in d.items()} )) 1.4525277839857154 >>> min(timeit.repeat(lambda: dict(itertools.chain(x.items(), y.items())))) 2.3143140770262107 >>> min(timeit.repeat(lambda: dict((k, v) for d in (x, y) for k, v in d.items()))) 3.2069112799945287
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