Perform List Subtraction in Python

Convert List to
set
to Perform List Subtraction in Python  Use List Comprehension to Get List Difference in Python
This tutorial demonstrates how to perform the list subtraction, or in other words, list minus list in Python.
As defined by the set theory in mathematics, the difference of two sets refers to the elements from one set that do not exist in the other set.
For example, if we declare these two lists:
list1 = [1, 2, 4]
list2 = [2, 3]
The difference of list1  list2
would be [1, 4]
, while list2  list1
would be [3]
.
Convert List to set
to Perform List Subtraction in Python
Set theory operations are supported in Python. However, only the set
data type support these operations. Therefore, to use the set
operation, lists have to be converted into sets. This is possible by wrapping a list around the function set()
.
Converting a list to a set will remove any type of order and remove duplicate values from the list.
listA = [1, 2, 4, 7, 9, 11, 11, 14, 14]
listB = [2, 3, 7, 8, 11, 13, 13, 16]
setA = set(listA)
setB = set(listB)
print('A  B = ', setA  setB)
Output:
A  B = {1, 4, 9, 14}
The result outputs the difference between the two sets and removes the duplicate values.
We can use the function list()
to convert the result from a set
to a list.
listA = [1, 2, 4, 7, 9, 11, 11, 14, 14]
listB = [2, 3, 7, 8, 11, 13, 13, 16]
setA = set(listA)
setB = set(listB)
list_diff = list(setA  setB)
print('A  B: ', list_diff)
Output:
A  B: [1, 4, 9, 14]
Use List Comprehension to Get List Difference in Python
List comprehension can be used to check if an element exists only in the first list but does not exist in the second list. This solution makes it possible to perform the difference operation without converting the list to a set.
listA = [1, 2, 4, 7, 9, 11, 11, 14, 14]
listB = [2, 3, 7, 8, 11, 13, 13, 16]
listSub = [elem for elem in listA if elem not in listB]
print('A  B =', listSub)
Output:
A  B = [1, 4, 9, 14, 14]
This solution does not mess with the order of the list and removes duplicates.
However, the value 11
is repeated twice in listA
, and both iterations of 11
are removed from the result of A  B
since 11
exists in both sets. This behavior is as expected.