How to Common keys in list and dictionary using Python

In this article, we will learn how to find common keys between a list and dictionary in Python. This is useful when you need to filter dictionary keys based on a list of allowed values.

Methods Used

The following are the various methods to accomplish this task ?

• Using the 'in' operator and List Comprehension

• Using set() and intersection() functions

• Using keys() function & in operator

• Using the Counter() function

Example Setup

Assume we have taken an input dictionary and list. We will find the common elements between the input list and keys of a dictionary using the above methods ?

Input

inputDict = {"hello": 2, "all": 4, "welcome": 6, "to": 8, "tutorialspoint": 10}
inputList = ["hello", "tutorialspoint", "python"]

Expected Output

Resultant list: ['hello', 'tutorialspoint']

In the above example, 'hello' and 'tutorialspoint' are the common elements between the input list and keys of the dictionary.

Using List Comprehension with 'in' Operator

List comprehension provides a concise way to create lists based on existing sequences. The 'in' operator checks membership in a sequence ?

# input dictionary
inputDict = {"hello": 2, "all": 4, "welcome": 6, "to": 8, "tutorialspoint": 10}

# input list
inputList = ["hello", "tutorialspoint", "python"]

print("Input dictionary:", inputDict)
print("Input list:", inputList)

# checking whether any input list element matches the keys of a dictionary
outputList = [key for key in inputDict if key in inputList]

# printing the resultant list
print("Resultant list:", outputList)

Input dictionary: {'hello': 2, 'all': 4, 'welcome': 6, 'to': 8, 'tutorialspoint': 10}
Input list: ['hello', 'tutorialspoint', 'python']
Resultant list: ['hello', 'tutorialspoint']

Using set() and intersection() Functions

The set() function creates a set object that removes duplicates. The intersection() method returns elements that are present in both sets ?

# input dictionary
inputDict = {"hello": 2, "all": 4, "welcome": 6, "to": 8, "tutorialspoint": 10}

# input list
inputList = ["hello", "tutorialspoint", "python"]

print("Input dictionary:", inputDict)
print("Input list:", inputList)

# getting common elements using set intersection
outputList = list(set(inputList).intersection(set(inputDict.keys())))

# printing the resultant list
print("Resultant list:", outputList)

Input dictionary: {'hello': 2, 'all': 4, 'welcome': 6, 'to': 8, 'tutorialspoint': 10}
Input list: ['hello', 'tutorialspoint', 'python']
Resultant list: ['hello', 'tutorialspoint']

Using keys() Function with 'in' Operator

The keys() method provides a view of all dictionary keys. We can iterate through these keys and check membership in the input list ?

# input dictionary
inputDict = {"hello": 2, "all": 4, "welcome": 6, "to": 8, "tutorialspoint": 10}

# input list
inputList = ["hello", "tutorialspoint", "python"]

print("Input dictionary:", inputDict)
print("Input list:", inputList)

# empty list for storing common elements
outputList = []

# getting the list of keys of a dictionary
keysList = list(inputDict.keys())

# traversing through the keys list
for key in keysList:
    # checking whether the current key is present in the input list
    if key in inputList:
        # appending that key to the output list
        outputList.append(key)

# printing the resultant list
print("Resultant list:", outputList)

Input dictionary: {'hello': 2, 'all': 4, 'welcome': 6, 'to': 8, 'tutorialspoint': 10}
Input list: ['hello', 'tutorialspoint', 'python']
Resultant list: ['hello', 'tutorialspoint']

Using Counter() Function

The Counter() function from the collections module counts hashable objects. It creates a dictionary with element frequencies ?

# importing Counter function from the collections module
from collections import Counter

# input dictionary
inputDict = {"hello": 2, "all": 4, "welcome": 6, "to": 8, "tutorialspoint": 10}

# input list
inputList = ["hello", "tutorialspoint", "python"]

print("Input dictionary:", inputDict)
print("Input list:", inputList)

# getting the frequency of input list elements as a dictionary
frequency = Counter(inputList)

# empty list for storing common elements
outputList = []

# traversing through dictionary keys
for key in inputDict.keys():
    # checking whether the current key is present in the input list
    if key in frequency:
        # appending that key to the output list
        outputList.append(key)

# printing the resultant list
print("Resultant list:", outputList)

Input dictionary: {'hello': 2, 'all': 4, 'welcome': 6, 'to': 8, 'tutorialspoint': 10}
Input list: ['hello', 'tutorialspoint', 'python']
Resultant list: ['hello', 'tutorialspoint']

Comparison

Conclusion

We explored four methods for finding common keys between a list and dictionary. The set intersection method is most efficient for large datasets, while list comprehension offers the most readable syntax.