Python – Extracting keys not in values

Sometimes, while working with Python dictionaries, we can have a problem in which we require to get all the keys that do not occur in values lists. This can have applications in the domains such as day-day programming. Lets discuss certain ways in which this task can be performed.

Method #1 : Using set() + values() + keys() + loop
This is brute way to approach this task. In this, we test for elements in value lists, and keep adding them in separate list. Then we subtract this from extracted keys using keys().

# Python3 code to demonstrate working of
# Extracting keys not in values
# Using set() + keys() + values() + loop

# initializing Dictionary
test_dict = {3 : [1, 3, 4], 5 : [1, 2], 6 : [4, 3], 4 : [1, 3]}

# printing original dictionary
print("The original dictionary is : " + str(test_dict))

# Extracting keys not in values
# Using set() + keys() + values() + loop
temp1 = set(test_dict.keys())
temp2 = set()
for ele in test_dict.values():
    temp2.update(ele)
res = list(temp1 - temp2)

# printing result
print("The extracted keys are : " + str(res))

Output :

The original dictionary is : {3: [1, 3, 4], 4: [1, 3], 5: [1, 2], 6: [4, 3]}
The extracted keys are : [5, 6]

Method #2 : Using generator expression + set()
This method is similar to above method. The difference is that it is performed as one liner way in compact format using generator expression.

# Python3 code to demonstrate working of
# Extracting keys not in values
# Using generator expression + set()

# initializing Dictionary
test_dict = {3 : [1, 3, 4], 5 : [1, 2], 6 : [4, 3], 4 : [1, 3]}

# printing original dictionary
print("The original dictionary is : " + str(test_dict))

# Extracting keys not in values
# Using generator expression + set()
res = list(set(test_dict) - set(ele for sub in test_dict.values() for ele in sub))

# printing result
print("The extracted keys are : " + str(res))

Output :

The original dictionary is : {3: [1, 3, 4], 4: [1, 3], 5: [1, 2], 6: [4, 3]}
The extracted keys are : [5, 6]