How to Compare Two Lists in Python

Python developers commonly use lists to store and manipulate data. Often they need to compare two or more lists to see if they are identical, or they have common elements. There are several easy ways to do this in Python. In this article, we will learn how to compare two lists in Python.

How to Compare Two Lists in Python

Here are the different ways to compare two lists in Python. When we compare two lists, there are many different things that people like to check, depending on their requirement. For example, you may want to see if there are any duplicates, find elements that exist in one list but not the other.

1. Using Set contructor

The set() constructor converts a list into a set of unique items. Thereafter, you can apply various functions available to the sets to compare lists. These functions are similar to mathematical operations applied to sets. Here is an example to check if two lists have any common elements.

a=[1, 2, 3, 4]
b=[4, 5, 6, 7]

print(set(a)&set(b)) # output is {4}
print(set(a).intersection(b)) # output is {4}

If you want the result to be a list, then you can wrap the result of above code in list() constructor.

print(list(set(a) & set(b))) # output is [4]

Here is the command to find the union of two lists. This will return the set of union of all elements.

print(set(a).union(b)) # output is {1, 2, 3, 4, 5, 6, 7}

Similarly, if you want to find the elements that are present in one list but not the other then you can use ‘-‘ operator to get the difference.

print(set(a)-set(b))  # output is {1, 2, 3}
print(set(b)-set(a))  # output is {5, 6, 7}

2. Using Zip function

Zip function accepts two or more iterables such as lists, and zips them together, that is, pairs their 1st items together, then 2nd item together, and so on.

print(all(i == j for i, j in zip(a,b)) ) # output is False

In the above code, we compare the individual items of each pair of items generated using zip() function, for equality. This will return a generator object of boolean values. The All() function will return True, if the result of comparison of each pair of items is True. Else it will return False. Since our lists are not identical, it will return False as the result.

3. Using List Comprehension

List comprehensions are useful to quickly loop through one or more lists and evaluate them for a condition. Here is an example to compare two lists using list comprehensions.

print([x for x in a if x in b]) # output is [4]

In the above code, we loop through list a and in each iteration, we check if the item is also present in list. If so, it is included in the result. Otherwise, it is omitted.

4. Using itertools

Itertools is a useful library that contains plenty of useful functions to work with iterable data types. You can use its product() function to obtain a cartesian product of two lists, and stores the result in sorted order. We then loop through it to check if any two consecutive items are identical. If so, it is stored in the new list.

import itertools
c=[]
for i in list(itertools.product(a,b)):
    if i[0] == i[1]:
        c.append(i[0])    
print(c) # output is [4]

5. Using nested for loop

If you are finding it difficult to understand all these different functions and want a more intuitive solution, then you can go for the good old fashioned nested for loops. In this method, we loop through one list, and in each iteration we check if each item is present in the other loop, by looping through the other list.

new_list = []
for element in a:
    if element in b:
        new_list.append(element)
        
print(new_list)   # output is [4]

One of the advantages of this solution is that you can easily customize it as per your requirement to include/exclude specific items.

6. Using sort and == operator

Python lists support sort() function that sorts them. Once they are sorted, you can use equality operator ‘==’ to check if they are equal. It will compare the lists item by item, so identical items need to be at the same location. Here is an example.

a=[1, 2, 3, 4]
b=[5, 4, 6, 7]
a.sort()
b.sort()
print(a) # output is [1, 2, 3, 4]
print(b) # output is [4, 5, 6, 7]
if a == b:
    print('equal')
else: print('not equal')

Output is ‘not equal’. This method works whether the lists have same number of elements or not.

7. Using collections.counter()

In this solution, we check equality of lists by comparing the frequency of occurrences of its individual items. We will compare the frequency of each item of first list with that of the second. It does not depend on the order of items of the list.

import collections
a=[1, 2, 3, 4]
b=[5, 4, 6, 7]
if(collections.Counter(a)==collections.Counter(b)):
    print( "Equal")
else:
    print( "Non equal")

In the above code, we use collections.Counter() function to get frequency of list items. It returns a Counter object such as Counter({1: 1, 2: 1, 3: 1, 4: 1}). We use equality operator ‘==’ to compare the result of Counter objects for both lists.

Conclusion

In this article, we have learnt several different ways to compare two lists. If your lists are small-medium size, then you can use set() constructor to convert them into set, since it offers many useful functions to compare sets. This may have an overhead for large lists, so you can use list comprehensions in such cases since they are fast. If you need to customize the comparison in a specific way, you can go for the basic nested loop solution mentioned above.

Also read:

How to Create Dictionary from Lists in Python
How to Remove Duplicates in Python List
How to Insert New Column to Pandas DataFrame