How Can I Efficiently Merge Two Dictionaries in Python?

How Do I Merge Two Dictionaries in a Single Expression in Python?

Problem:

Merge two dictionaries into a new dictionary, with values from the second dictionary overwriting those from the first.

Possible Solutions:

In Python 3.9.0 or Greater:

z = x | y

In Python 3.5 or Greater:

z = {**x, **y}

In Python 2, 3.4 or Lower:

def merge_two_dicts(x, y):
    z = x.copy()  # Start with keys and values of x
    z.update(y)  # Modifies z with keys and values of y
    return z

z = merge_two_dicts(x, y)

Explanation:

1. Python 3.9.0 or Greater:

The pipe operator (|) is introduced in PEP-584 and combines both dictionaries using dictionary unpacking syntax.

2. Python 3.5 or Greater:

The double asterisk operator (**) is used for dictionary unpacking. Each dictionary is unpacked as key-value pairs, and then merged into a new dictionary.

3. Python 2, 3.4 or Lower:

In Python versions prior to 3.5, you must manually merge the dictionaries using the copy() and update() methods:

• copy() creates a shallow copy of the original dictionary.
• update() updates the copied dictionary with the key-value pairs from the second dictionary.

Note:

Using the dictionary unpacking syntax ({x, y}) is more performant than using the dicta comprehension ({k: v for d in (x, y) for k, v in d.items()}) or the chain() function from itertools.

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