python程序中无法追踪的错误（使用无限循环查找最大和最小数字）

Question

我是编程的新手，我编写了这段代码，但在某些方面它出现了故障。因为它没有考虑用于确定最大和最小数量的第一个输入数字。此外，它不会打印错误消息＆＃34;无效输入＆＃34;如果在第一次输入时给出了错误的输入。简而言之，它忽略了第一个输入。我无法在逻辑中找到错误。如果您检查我的代码并告诉我逻辑中的错误，那将是一个很大的帮助。

largest = None
smallest = None
num = input('Enter a number: ')
while True:
    num = input('Enter a number: ')
    if num == 'done':    
        break 
    try:
        fnum=int(num)

    except:
        print ('Invalid input')

        continue
    if largest is None:
        largest=fnum
    else:
        if largest<fnum:
            largest=fnum


    if smallest is None:
        smallest=fnum
    else: 
        if smallest>fnum:
            smallest=fnum   

print('Maximum is ',largest)
print('Minimum is ',smallest)

问题1： Here no error message is displayed as invalid input is given in the first input

Problem2： Here '2'(which is first input) is neglected (as it would have been the minimum of all the numbers) and the error message is displayed

Problem3：Here '10'(which is first input) is neglected (as it would have been the maximum of all the numbers) and the error message is displayed