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Deleting a List Item While Iterating

Deleting a List Item While Iterating

Let’s trim a long list and list the list.

Python 3.5
list_1 = [1, 2, 3, 4]
list_2 = [1, 2, 3, 4]
list_3 = [1, 2, 3, 4]
list_4 = [1, 2, 3, 4]
for idx, item in enumerate(list_1):
    del item
for idx, item in enumerate(list_2):
    list_2.remove(item)
for idx, item in enumerate(list_3[:]):
    list_3.remove(item)
for idx, item in enumerate(list_4):
    list_4.pop(idx)
print(list_1)
print(list_2)
print(list_3)
print(list_4)

Can you guess why the output is [2, 4]?

Explanation

  • It’s never a good idea to change the object that you’re iterating over. The correct way to do so is to iterate over a copy of the object instead, and list_3[:] does just that.
Python 3.5
some_list = [1, 2, 3, 4]
print(id(some_list))
print(id(some_list[:])) # Notice that python creates new object for sliced list.

The difference between ...

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