Python Dictionary fromkeys() Method

Python dictionary fromkeys() function returns the dictionary with key mapped and specific value. It creates a new dictionary from the given sequence with the specific value.

Python Dictionary fromkeys() Method Syntax:

Syntax : fromkeys(seq, val)

Parameters :

• seq : The sequence to be transformed into a dictionary.
• val : Initial values that need to be assigned to the generated keys. Defaults to None.

Returns : A dictionary with keys mapped to None if no value is provided, else to the value provided in the field. 

Python Dictionary fromkeys() Method Example:

seq = ('a', 'b', 'c')
print(dict.fromkeys(seq, None))

Output:

{'a': None, 'b': None, 'c': None}

Example 1: Demonstrating the working of fromkeys() 

seq = {'a', 'b', 'c', 'd', 'e'}

# creating dict with default values as None
res_dict = dict.fromkeys(seq)

print("The newly created dict with None values : " + str(res_dict))

# creating dict with default values as 1
res_dict2 = dict.fromkeys(seq, 1)

print("The newly created dict with 1 as value : " + str(res_dict2))

Output : 

The newly created dict with None values : {'d': None, 'a': None, 'b': None, 'c': None, 'e': None} 
The newly created dict with 1 as value : {'d': 1, 'a': 1, 'b': 1, 'c': 1, 'e': 1}

Behavior of Python Dictionary fromkeys() Method with Mutable objects as values, fromdict() can also be supplied with the mutable object as the default value. But in this case, a shallow copy is made of the dictionary, i.e. if we append a value in the original list, the append takes place in all the values of keys.

Prevention: Certain dictionary comprehension techniques can be used to create a new list of key values, that do not point to the original list of values of keys.

Example 2: Demonstrating the behavior with mutable objects

seq = {'a', 'b', 'c', 'd', 'e'}
lis1 = [2, 3]

res_dict = dict.fromkeys(seq, lis1)

# Printing created dict
print("The newly created dict with list values : "
      + str(res_dict))

# appending to lis1
lis1.append(4)

print("The dict with list values after appending : ", str(res_dict))

lis1 = [2, 3]
print('\n')

# using fromkeys() to convert sequence to dict
# using dict. comprehension
res_dict2 = {key: list(lis1) for key in seq}

# Printing created dict
print("The newly created dict with list values : "
      + str(res_dict2))

# appending to lis1
lis1.append(4)

# Printing dict after appending
# Notice that append doesnt take place now.
print("The dict with list values after appending (no change) : ", str(res_dict2))

Output:

The newly created dict with list values : {'d': [2, 3], 'e': [2, 3], 'c': [2, 3], 'a': [2, 3], 'b': [2, 3]} 
The dict with list values after appending : {'d': [2, 3, 4], 'e': [2, 3, 4], 'c': [2, 3, 4], 'a': [2, 3, 4], 'b': [2, 3, 4]}
The newly created dict with list values : {'d': [2, 3], 'e': [2, 3], 'c': [2, 3], 'a': [2, 3], 'b': [2, 3]} 
The dict with list values after appending (no change) : {'d': [2, 3], 'e': [2, 3], 'c': [2, 3], 'a': [2, 3], 'b': [2, 3]}

Example 3: Python Dictionary fromkeys() with an empty list

# Python3 code to demonstrate 
# to initialize dictionary with list 
# using fromkeys()
  
# using fromkeys() to construct
new_dict = dict.fromkeys(range(4), [])
      
# printing result
print ("New dictionary with empty lists as keys : " + str(new_dict))

Output:

New dictionary with empty lists as keys : {0: [], 1: [], 2: [], 3: []}