How do I merge two dictionaries in a single expression?

You can use the update() method of one dictionary to merge the key-value pairs from another dictionary into it. For example:

merge dictionaries by update method in Python

d1 = {'a': 1, 'b': 2}
d2 = {'c': 3, 'd': 4}
d1.update(d2)
print(d1)  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}

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Alternatively, you can use the {**d1, **d2} syntax to create a new dictionary that contains the key-value pairs from both d1 and d2. This syntax is available in Python 3.5 and newer.

merge dictionaries by ** in Python

d1 = {'a': 1, 'b': 2}
d2 = {'c': 3, 'd': 4}
d3 = {**d1, **d2}
print(d3)  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}

This syntax is known as the double-starred dictionary (or "unpacking") syntax. It allows you to expand the key-value pairs in one dictionary into another dictionary.