更改列表中的第n个项目

Question

For, example if I wanted to make every second uppercase and lowercase like

original_list = ['FisH', 'Dog', 'cow', 'piG']

desired_list = ['fish', 'DOG', 'cow', 'PIG']

my way to go about it was to make a separate lists for upper and lower words and then join them some how but I know this is a bad/ way to do it:

list1 = []
list2 = []

for i in orginal_list[0::2]:
    i = i.lower()
    list1.append(i)


for i in orginal_list[1::2]:
    i = i.upper()
    list2.append(i)

Answer 1

With enumerate() function and modulo division by 2 (to detect an even positions):

original_list = ['FisH', 'Dog', 'cow', 'piG']
result = [w.lower() if not i%2 else w.upper() for i,w in enumerate(original_list)]

print(result)

The output:

['fish', 'DOG', 'cow', 'PIG']

Answer 2

Similar to your own, but assigning the slices right back.

mylist[::2] = map(str.lower, mylist[::2])
mylist[1::2] = map(str.upper, mylist[1::2])

Demo:

>>> mylist= ['FisH', 'Dog', 'cow', 'piG']
>>> mylist[::2] = map(str.lower, mylist[::2])
>>> mylist[1::2] = map(str.upper, mylist[1::2])
>>> mylist
['fish', 'DOG', 'cow', 'PIG']