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Python Introducing Lists Using Lists Mutability

Posted by wc93
wc93
25,725 Points

Unexpected result when looping through mutable object such as lists?

Just wondering what exactly is going on? Does the for in loop get the index length once and then refer back to the list after the remove method has removed an item so the second iteration through the list it removes the third item?

I wrote a little python to try and figure out what was going on...

#######      Unexpected results when looping through mutable list

nums = [1, 2, 3, 4, 5, 6]
nums2 = nums.copy()

# Unexpected result when looping through list and removing items

print("\nUnexpected looping results on mutable object\n")
for item in nums:
    print(f"Item to be Removed: {item}")
    print(f"List Before Remove: {nums}")
    nums.remove(item)
    print(f"List After Remove: {nums}")

# Using copy to remove unexpected results as each time through the list it is making a new copy of list
print(f"\n\nUsing copy to mitigate unexpected results while looping through mutable object\n")
for item in nums2.copy():
    print(f"Item to be Removed: {item}")
    print(f"List Before Remove: {nums2}")
    nums2.remove(item)
    print(f"List After Remove: {nums2}")

1 Answer

Brandon White
.a{fill-rule:evenodd;}techdegree seal-36
Brandon White
Full Stack JavaScript Techdegree Graduate 34,660 Points

Hi wc93,

I think maybe you're understanding what's going on, but I'm not super sure what you mean by "Does the for in loop get the index length once..."

The first iteration through the loop the item with an index of zero is removed. The second iteration through the loop the item with an index of one is removed. But the item with an index of one is no longer 2, because 1 has been removed. So now 2 is the first item in the list (or the item with the zero-ith index). So now the item with an index of one (or the item in the second position in the list) is 3. So 3 is removed. So now the list is [2, 4, 5, 6], now the item in the third position (or the item with an index of 2) is 5. So 5 will be removed.

wc93
wc93
25,725 Points

Thanks Brandon. That was how I was understanding it but not expressing properly. That is why I wrote the script to give me a better look at what was going on where.