how do I redo the input if data is in incorrect format?

Question

so here is the question. I have 'for in range' for me to input value to the list however, if I input value which is not numbers, it could only tells me I enter the wrong input but I can't re-enter the right value, it just keeps asking me to move forward to enter the next one.
How could I redo the input if I entered the wrong value(ex:english characters)so I could have a right value in certain position?

Thanks a lot for any help ! rookie for python

the code I wrote is:

count=15
student=list()
print('please insert student score:')
for item in range(count):
    line = input('enter score for student:'+str(item+1))
    if line.isdecimal() and 0<=int(line)<=100:
        data=int(line)
        student.append(data)
    else:
        print(f"what you entered:Num{item+1:3d}is not a score")
total=0
for line in student:
    total += line
    totalF = total/15
print('Student Average Score is：','%.2f' % totalF)
print('Finish Input\n')
print("student:",student)    
print('You have', end='--> ')
for item in student:
    print(f'{item:d}', end='  ')

Answer 1

Extract the part of code getting the score into it's own function, and in this function put it in a while loop that will keep on asking for value until the value is valid:

def get_score(num):
    while True:
        value = input(f'enter score for student {num}:')
        try:
            # int(value) will raise a ValueError if value 
            # is not a proper representation of an int
            value = int(value)
            # now we have an int, test it's value,
            # and raise a ValueError if it doesn't match 
            if not 0 <= value <= 100:
                raise ValueError("value should be between 0 and 100")

             # value is ok, done 
             return value

        except ValueError as e:
            # either it's not an int or it's not in the right range
            print(f"Invalid value '{value}': {e}")


# naming tip: use a plural for collections
students = []  

# tip: avoid those useless "item+1" things ;-) 
for num in range(1, count+1):
    score = get_score(num)
    students.append(score)

Answer 2

You have to do your check in a loop and finish it when the score is correct.

Here with a simple boolean variable:

for item in range(count):
    correct_score = False
    while not correct_score:
        line = input('enter score for student:'+str(item+1))
        if line.isdecimal() and 0<=int(line)<=100:
            data=int(line)
            student.append(data)
            correct_score = True  # to get out of the loop
        else:
            print(f"what you entered:Num{item+1:3d}is not a score")
            # we don't change `correct_score` here, so the loop will be executed again