Python Set difference_update(): Examples and Questions

The Python set difference update() is a built-in function that allows you to update a set by removing items from the specified set (passed as arguments in the function).

Python set difference_update

To use the difference_update() method, we follow the syntax:

Copy Code

                    
P.difference_update(Q)

Here, P and Q are two sets. If we perform (P-Q) using the difference_update() method, only set P gets modified to the difference value (P-Q). Set Q does not change.

If we use Q.difference_update(P), set Q is updated to (Q-P), and set P remains unchanged.

Return Value from difference_update()

Unlike the set difference() method, Python set difference_update() does not return any value (None). It indicates that the set has been mutated/changed.

In the code:

Copy Code

                    
result = X.difference_update(Y)

The result will be: None, X will be equal to (X-Y), and Y will remain unchanged.

Example of how difference_update() works

Let us now look at an example to understand how the Python set difference_update() method works in a program.

Copy Code

                    
# creating two sets
num1 = { 45, 23, 55, 11, 46, 33, 94, 29, 40}
num2 = { 11, 45, 38, 82, 33, 40, 55, 73, 98}

# using difference_update()
result = num1.difference_update(num2)

print ('First row =', num1)
print ('Second row =', num2)
print ('Result =', result)

Output:

Copy Code

                    
First row = {46, 23, 29, 94}
Second row = {33, 98, 38, 40, 73, 11, 45, 82, 55}
Result = None

Questions and Answers

Q1. What does the Python set difference_update() method do?

The set difference_update() is a built-in function of Python that allows you to remove elements of a set that are members of the specified set arguments. The set that calls the method is modified to the difference, and the set argument remains unchanged.

Q2. How does set difference_update() work in Python?

To use the difference_update() with Python sets, we follow the syntax:

Copy Code

                    
P.difference_update(Q)

Where P and Q are two sets.

Example:

Copy Code

                    
# set
hob1 = {'swimming', 'skating', 'singing', 'dancing'}
hob2 = {'swimming', 'singing'}

hob1.difference_update(hob2)

print ('Previous Hobbies:', hob1)
print ('Current Hobbies:', hob2)

Output:

Copy Code

                    
Previous Hobbies: {'skating', 'dancing'}
Current Hobbies: {'swimming', 'singing'}

Q3. Is the Python set difference_update() different from difference()?

Yes. The set difference() method finds the difference between two sets and returns a new set with the value. The Python set difference_update() method modifies/updates the existing caller set.

Example:

Copy Code

                    
# sets
num1 = { 45, 23, 55, 11, 46, 33, 94, 29, 40}
num2 = { 11, 45, 38, 82, 33, 40, 55, 73, 98}

# difference() method
num = num1.difference(num2)

# difference_update()
result = num1.difference_update(num2)

print ('With difference():', num)
print ('With difference_update(): num1=', num1)
print ('With difference_update(): num2=', num2)

Output:

Copy Code

                    
With difference(): {94, 29, 46, 23}
With difference_update(): num1= {46, 23, 29, 94}
With difference_update(): num2= {33, 98, 38, 40, 73, 11, 45, 82, 55}

Q4. Can I use difference_update() with multiple sets?

Yes. You can use the difference_update method() with multiple sets by separating them with commas.

Copy Code

                    
# create three sets
A = {'rose', 'lily', 'marigold', 'cactus', ' orchid', 'lotus'}
B = {'lily', 'rafflesia', 'tulip', 'rose', 'carnation', 'hyacinth'}
C = {'freesia', 'chrysanthemum', 'daffodil', 'rose', 'tulip'}

# using difference_update()
result = A.difference_update(B,C)

print ('A =', A)
print ('B =', B)
print ('C =', C)
print ('Result =', result)

Output:

Copy Code

                    
A = {'cactus', 'marigold', ' orchid', 'lotus'}
B = {'lily', 'tulip', 'hyacinth', 'rafflesia', 'carnation', 'rose'}
C = {'tulip', 'freesia', 'chrysanthemum', 'daffodil', 'rose'}
Result = None

Share with friends