try 语句中的“except”（在 while 循环内）不起作用

Question

I've written a program which repeatedly reads numbers until the user enters "done" . If the user enters anything other than a number, detect their mistake using try and except and print an error message and skip to the next number.

The problem is the code never gets to the except part of the code (user should get an error for not entering anything or for entering invalid characters (anything not 'done' or a number). What am I doing wrong?

numbers=(input("Enter a number or 'done' to quit: "))
count=1
total_num=int(numbers)
while True:
    numbers=(input("Enter another number or 'done' to quit: "))
    try:
        if (numbers.isalpha() and numbers=='done'):  #elif numbers.isalpha() and numbers=='done':
            break
        elif numbers.isdigit():
            numbers=int(numbers)
            total_num+=int(numbers)
            count+=1
    except:
        print('Valid a number or done to quit:' )

print('sum is', total_num)
print('count is', count)
print('average is', int(total_num/count))

Answer 1

You've misunderstood the point of the try..except block. You don't need to check if the input isdigit , since that precludes any errors from being thrown. Just try to convert the input to int , and if this fails, you'll fall through to the except . There, you can check if the input is == "done" , and break out:

while True:
    numbers = input("Enter another number or 'done' to quit: ")
    try:
        numbers = int(numbers)
        total_num += numbers
        count += 1
    except ValueError:
        if numbers == "done": break
        print('Valid a number or done to quit:' )

Note that I qualified the except clause to only catch ValueError s, since it is a bad practice to catch all errors -- that can hide actual problems with your code, and disable the ability to interrupt the program with Ctrl+C

Answer 2

There is nothing that apparently breaks the code so the Except statement is never executed.

You can put an else statement with same thing as Except:

numbers=(input("Enter a number or 'done' to quit: "))
count=1
total_num=int(numbers)

while True:
    numbers=(input("Enter another number or 'done' to quit: "))
    try:
        if (numbers.isalpha() and numbers=='done'):  #elif numbers.isalpha() and numbers=='done':
            break
        elif numbers.isdigit():
            numbers=int(numbers)
            total_num+=int(numbers)
            count+=1
        else:
            print('Valid a number or done to quit:' )
    except:
        print('Valid a number or done to quit:' )

print('sum is', total_num)
print('count is', count)
print('average is', int(total_num/count))

Unless you find some input that breaks your code I suggest removing it and stay only with the if elif else