Python dictionary fromkeys() usage and examples

Python dictionary methods

The fromkeys() method creates a new dictionary based on the given element sequence, with the values provided by the user.

The syntax of the fromkeys() method is:

dictionary.fromkeys(sequence[, value])

fromkeys() parameters

The fromkeys() method takes two parameters:

• sequence -The element sequence used as the new dictionary key

• value (optional) -The value set for each element of the dictionary

fromkeys() return value

The fromkeys() method returns a new dictionary with the given element sequence as the dictionary keys.

If the value parameter is set, each element of the newly created dictionary will be set to the provided value.

Example1: Create a dictionary based on the sequence of keys

# Vowel keys
keys = {'a', 'e', 'i', 'o', 'u'}
vowels = dict.fromkeys(keys)
print(vowels)

When running the program, the output is:

{'a': None, 'u': None, 'o': None, 'e': None, 'i': None}

Example2: Create a dictionary based on the sequence of keys with values

# Vowel keys
keys = {'a', 'e', 'i', 'o', 'u'}
value = 'vowel'
vowels = dict.fromkeys(keys, value)
print(vowels)

When running the program, the output is:

{'a': 'vowel', 'u': 'vowel', 'o': 'vowel', 'e': 'vowel', 'i': 'vowel'}

Example3: Create a dictionary from a list of mutable objects

# Vowel keys
keys = {'a', 'e', 'i', 'o', 'u'}
value = [1]
vowels = dict.fromkeys(keys, value)
print(vowels)
# Updated value
value.append(2)
print(vowels)

When running the program, the output is:

{'a': [1], 'u': [1], 'o': [1], 'e': [1], 'i': [1}]
{'a': [1,　2], 'u': [1,　2], 'o': [1,　2], 'e': [1,　2], 'i': [1,　2}]

If the provided value is a mutable object (whose value can be changed), such aslist,dictionaryWhen, if a mutable object is modified, each element in the sequence will also be updated.

This is because, for each element, a reference to the same object (pointing to the same object in memory) is assigned.

To avoid this problem, we use dictionary comprehension.

# Vowel keys
keys = {'a', 'e', 'i', 'o', 'u'}
value = [1]
vowels = { key: list(value) for key in keys }
# You can also use { key: value[:] for key in keys }
print(vowels)
# Updated value
value.append(2)
print(vowels)

When running the program, the output is:

{'a': [1], 'u': [1], 'o': [1], 'e': [1], 'i': [1}]
{'a': [1], 'u': [1], 'o': [1], 'e': [1], 'i': [1}]

Here, for each key in keys, a new list is created from value and assigned to it.

Essentially, value is not assigned to the element, but a new list is created from it, and then it is assigned to each element in the dictionary.

Python dictionary methods