Looping into a list to get the previous value of a particular value within that list

We have a list of strings here called list_strings `

list_strings =["john", "sarah", "bianca", "savie", "sandy", "john", "harry", "john", "debra", "john"]

If a specified value is "john" and can we get all the strings before john, so the output should look like this!`

result = ["none", "sandy", "harry", "debra]

>Solution :

You can use a simple for loop:

list_strings =["john", "sarah", "bianca", "savie", "sandy", "john", "harry", "john", "debra", "john"]
result = []
for i, j in enumerate(list_strings):
    if j == "john":
        if i:
            result.append(list_strings[i-1])
        else:
            result.append(None)

[None, 'sandy', 'harry', 'debra']