Start numeric variable with null value in Python

I particularly do not like the solution to read the value before the loop and read it back into the loop. They are equal code lines, which become two places to edit if the application changes and the code becomes redundant. Since you do not know how many iterations it will take to read a valid value, nothing is fairer than creating an infinite loop . We also do not know if the read value of the user will be numeric, so we must pay attention to the exception that can be triggered by the float initializer when trying to convert the value. And given that we're going to treat an exception, nothing better than triggering the same exception if the value is not in the desired range, thus managing to focus the error treatment on just one place.

See an example:

while True:
    try:
        nota = float(input("Digite uma nota entre 0 e 10: "))
        if not 0 < nota < 10:
            raise ValueError("A nota deve ser um valor entre 0 e 10")
    except ValueError as error:
        print(error)
    else:
        break

See working at Repl.it

The block else of try will be executed only if no exception is triggered, indicating that the read value is numeric and is in the desired interval, stopping the loop; otherwise, the loop continues to execute until it reads a valid value.

In your code, the error occurs because you try to convert a null value to float . This does not make sense in Python: either it's a null value or it's a float . What you could do is start with a value that you are sure is invalid for the range, ensuring that the loop runs.

nota = 0
while not 0 < nota < 10:
    nota = float(input("Digite uma nota de 0 a 10: "))
    if not 0 < nota < 10:
        print('Nota inválida, digite apenas uma nota de 0 a 10.')

print('Nota: {:.1f}'.format(nota))

See working at Repl.it

Initializing the variable with 0, you guarantee that the loop will be executed by reading the value of the user, however it falls on what I commented at the beginning of the answer: you will need to do the same condition inside the loop, repeating code. Exception capture is paramount to the correct operation of the program, because the way it is, if the user types a text, his program will end with the exception thrown. With block try/except you prevent this.

I was able to resolve the problem by requesting the note before the while and within the while body requesting again, in case the value is outside the specified range. >

nota = float(input("Digite uma nota de 0 a 10: "))
while nota < 0 or nota > 10:

    print('Nota inválida, digite apenas uma nota de 0 a 10.')
    nota = float(input("Digite uma nota de 0 a 10: "))

print('Nota: {:.1f}'.format(nota))

But still my question regarding the variable with null value has repercussions. Can anyone explain me how this works in Python 3?

I removed the while variable and the program reproduced correctly, asking for a note and giving the value of what I typed, it looks like this:

nota = float(input("Digite uma nota de 0 a 10: "))
while nota < 0 or nota > 10:

print('Nota inválida, digite apenas uma nota de 0 a 10.')

print('Nota: {:.1f}'.format(nota))