寻找细分的python替换子字符串脚本

Question

我正在学习如何替换字符串中的子字符串的脚本，我偶然发现了while循环，并且我不了解使用after和after变量的用途和逻辑，以及通常情况下while循环的结果。我在网上检查了一下，发现它们使用了replace方法，但是我想了解这个常规步骤和逻辑。

things = "tree, box, chair, lamp, \n" \
    "desk, cat, dog, grass, \n" \
    "pig, box, lamp, shelf"
print(things)
print()

old_item = input("Old item: ")
new_item = input("New item: ")
len_old_item = len(old_item) # i didn't get why they added this.

i = things.find(old_item) 
while i > 0:
    before = things[:i] # i don't get how this part works
    after = things[i+len_old_item:] # i don't get how this part works
    things = before + new_item +after # i don't get how this part works
    i = things.find(old_item) # i don't get how this part works

print()
print(things)

输出要求用户输入列表中的旧项目，然后用新项目将其替换为事物列表。