Merge Python dictionaries - The difference of double asterisks operation and update method

The unpacking technique (from Python 3.5) enables you to merge some dictionaries.

a = {'apple': 3, 'lemon': 2}
b = {'apple': 15, 'peach': 7}

c = {**a, **b}

print(c)  # {'apple': 15, 'lemon': 2, 'peach': 7}

Unpacking, which uses double asterisks, generates multiple pairs so c has all the pairs in a and b. A Python dictionary can't have duplicate keys so the value of apple is 15 in the merged dictionary.

Example

a = {'apple': 3, 'lemon': 2}
b = {'apple': 15, 'peach': 7}
c = {'lemon': 99}

d = {**a, **b, **c}

print(d)  # {'apple': 15, 'lemon': 99, 'peach': 7}

Double asterisks are needed

a = {'apple': 3, 'lemon': 2}
b = {'apple': 15, 'peach': 7}

c = {a, b}
# TypeError: unhashable type: 'dict'

You can't omit asterisks to merge. a is an object and **a is the result of unpacking a. a itself doesn't mean its pairs.

Merge vs Update

Python dictionary has update method that differs from merging.

a = {'apple': 3, 'lemon': 2}
b = {'apple': 15, 'peach': 7}

c = a.update(b)

print(c)  # None
print(a)  # {'apple': 15, 'lemon': 2, 'peach': 7}
print(b)  # {'apple': 15, 'peach': 7}

update returns None and updates the original dictionary while merging with unpacking creates a new dictionary.