Removing Items from a List

There are many ways to remove values from a list, we will be exploring most of them.

With removal, there are a lot of dangerous situations that can occur. It is advised to understand each technique before thinking about value removals.

Removal by a Target Value

.remove(target) method will remove the first occurance of the targetted value if it exists in the list.

If the target_value doesn’t exist, it will produce an error.

# .remove() example

a_list = ['apple', 'oranges', 'kiwi', 'honeydew', 'apple']
a_list.remove('apple')
a_list.remove('kiwi')

print('a_list:', a_list)

a_list: ['oranges', 'honeydew', 'apple']

Removal by Index

.pop(target_index) will remove a value located at the target_index and RETURN THE VALUE.

• If the target_index goes beyond the list size, it will produce an error.

• pop() without an target index returns and removes the last value

# .pop() example

a_list = ['apple', 'oranges', 'kiwi', 'honeydew', 'apple']
removed1 = a_list.pop()
a_list.pop(2)


print('a_list:', a_list)
print('removed1:', removed1)

a_list: ['apple', 'oranges', 'honeydew']
removed1: apple

Removing All Items in a List

.clear() is a method that will empty out its list.

# .clear() example

a_list = ['apple', 'oranges', 'kiwi', 'honeydew', 'apple']
a_list.clear()

print('a_list:', a_list)

a_list: []

Introduction to del

del is used to delete objects in Python. The concepts of “objects” are discussed in a future course!

del can delete an entire instance of a list

# del example 1

a_list = ['apple', 'oranges', 'kiwi', 'honeydew', 'apple']
del a_list

print('a_list:', a_list)

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

<ipython-input-5-eb3269d4b4cb> in <module>
      4 del a_list
      5
----> 6 print('a_list:', a_list)


NameError: name 'a_list' is not defined

Notice that since the variable was deleted, we get an error say that the a_list was never created.

del can delete a single item in a list by indexing

# del example 2

a_list = ['apple', 'oranges', 'kiwi', 'honeydew', 'apple']
del a_list[1]
del a_list[-1]

print('a_list:', a_list)

a_list: ['apple', 'kiwi', 'honeydew']

There are no errors here since we are only deleting single items from the list.

del can delete via slicing

# del example 3

a_list = ['apple', 'oranges', 'kiwi', 'honeydew', 'apple']
del a_list[0:3]

print('a_list:', a_list)

a_list: ['honeydew', 'apple']

Notice that the first three items were deleted from a_list.

MAJOR DANGER: Deletion During Iteration

It is very common for beginner programmers to delete while iterating the targeted list.

Observe:

# Example of bad practice

a_list = ['apple', 'oranges', 'kiwi', 'honeydew', 'apple']
for value in a_list:
    if value == 'kiwi':
        a_list.remove(value)

    print('value is currently:', value)
print('-'*64)

print('a_list:', a_list)

value is currently: apple
value is currently: oranges
value is currently: kiwi
value is currently: apple
----------------------------------------------------------------
a_list: ['apple', 'oranges', 'honeydew', 'apple']

Explanation:

• The print statement inside the iteration prints ‘kiwi’

• The print statement of ‘kiwi’ should have been impossible since ‘kiwi’ was deleted

• This happens because a for loop never re-checks the value of a_list

Recommendations:

1. Use while loops and index-based deletion

2. Make a copy of the list, execute deletion on the copy