list = ['abc_apple','abc_orange','cherry','abc_berry','banana']
for i in range(len(list)):
    if 'abc_' in list[i]:
        pass
    else:
        list[i] = 'abc_' + list[i]
        
list

输出:

['abc_apple', 'abc_orange', 'abc_cherry', 'abc_berry', 'abc_banana']

或

list = ['abc_apple','abc_orange','cherry','abc_berry','banana']
for i in range(len(list)):
    if 'abc_' not in list[i]:
        list[i] = 'abc_' + list[i]
        
list

还是更好的答案

list = ['abc_apple','abc_orange','cherry','abc_berry','banana']
for i in range(len(list)):
    if  list[i].startswith('abc_'):
        pass
    else:
        list[i] = 'abc_' + list[i]
        
list