RuntimeError: Set changed size during iteration in Python

RuntimeError: Set changed size during iteration in Python #

The Python "RuntimeError: Set changed size during iteration in Python" occurs when we change the size of a set object when iterating over it. To solve the error, use the copy() method to create a shallow copy of the set that you can iterate over, e.g. my_set.copy().

Here is an example of how the error occurs.

Copied!
my_set = {'Alice', 'Bob', 'Carl'}

for i in my_set:
    print(i)

    if i == 'Bob':
        # ⛔️ RuntimeError: Set changed size during iteration
        my_set.remove(i)

It is not allowed to change the size of a set while iterating over it.

One way to solve the error is to use the set.copy() method to create a shallow copy of the set object and iterate over the copy.

Copied!
my_set = {'Alice', 'Bob', 'Carl'}

for i in my_set.copy():
    print(i)

    if i == 'Bob':
        my_set.remove(i)

print(my_set)  # 👉️ {'Alice', 'Carl'}

We could have also used a list comprehension to filter out elements from the set.

Copied!
my_set = {'Alice', 'Bob', 'Carl'}

my_new_set = set([i for i in my_set if i != 'Bob'])

print(my_new_set)  # 👉️ {'Carl', 'Alice'}

Make sure to pass the list to the set() class to convert the result to a set object.

Here is another example of using a list comprehension to filter out elements from a set.

Copied!
my_set = {1, 2, 3, 4, 5, 6}

my_new_set = set([i for i in my_set if i % 2 == 0])

print(my_new_set)  # 👉️ {2, 4, 6}

The final set contains only elements that are divisible by 2.

Set objects are an unordered collection of unique elements.