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Ryan Francis
5,290 PointsPython Collections: Removing Items from a List Challenge Task 2
I decided to try to use a loop to solve this issue, but I encountered some strange behavior that I cannot explain. I created a for loop to go through the items in the list and used if to test the type() of the list item and if it != int, then it would remove it. This works until it gets to the list within the messy_list:
messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]
# Your code goes below here
messy_list.insert(0, messy_list.pop(3))
for item in messy_list:
if type(item) != int:
del messy_list[messy_list.index(item)]
However, if I replace the del with a print(item), it will output the items that I would expect the loop to remove from the list:
messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]
# Your code goes below here
messy_list.insert(0, messy_list.pop(3))
for item in messy_list:
if type(item) != int:
print(item)
What I find even more confusing, is if I keep the print and the del in, it will no longer print the list:
messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]
# Your code goes below here
messy_list.insert(0, messy_list.pop(3))
for item in messy_list:
if type(item) != int:
print(item)
del messy_list[messy_list.index(item)]
I am sure that there are far simply ways to solve this, but I'm really hung up on this because I can't explain why this happens. I have tested running the del manually and it works just fine.
>>> messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]
>>> del messy_list[messy_list.index([1, 2, 3])]
>>> messy_list
['a', 2, 3, 1, False]
messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]
# Your code goes below here
messy_list.insert(0, messy_list.pop(3))
for item in messy_list:
if type(item) != int:
print(item)
[MOD: added ```python formatting -cf]
1 Answer
Chris Freeman
Treehouse Moderator 68,423 PointsThe primary issue is you should not modify the iterable of a for loop. Doing so affects the indexes and throws off the looping causing skipped items.
In the early-stage challenge, the purpose is to practice using various indexes on a list. It is OK to simply put integers as the index values. You do not need to search for items using index().
Using your third code attempt, It can pass the challenge if you make a copy of messy_list for the for loop. This way, the deletions do not alter the indexes of the copy.
messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]
# Your code goes below here
messy_list.insert(0, messy_list.pop(3))
for item in messy_list.copy(): # <-- iterate over a copy of messy_list
if type(item) != int:
print(item)
del messy_list[messy_list.index(item)]
Ryan Francis
5,290 PointsRyan Francis
5,290 PointsThank you so much for your explanation! That makes complete sense.
That's what I wound up doing to pass the challenge, just passing the integers to list.remove() and not attempting to automate the process with a loop.
ilya A
3,802 Pointsilya A
3,802 Pointswhat is .copy( ) ?
Chris Freeman
Treehouse Moderator 68,423 PointsChris Freeman
Treehouse Moderator 68,423 PointsFrom the docs:
list.copy() Return a shallow copy of the list. Equivalent to a[:].An example that uses most of the list methods: