Python的try-except为何无法抑制错误？仍抛出Traceback问题

尝试确保用户输入数值而非字符串，原本以为try-except能捕获并抑制错误，但程序仍抛出Traceback。

程序输出 #

# python conversion.py 
Select conversion type, 1) inches to mm 2) mm to inches     -1
Invalid response. 

Select conversion type, 1) inches to mm 2) mm to inches     g
Invalid response. 

Select conversion type, 1) inches to mm 2) mm to inches     1

Please specify inches in decimal to convert to mm   1
1.0 inches is equal to 25.4 mm
Traceback (most recent call last):
  File "/home/mcolombo/conversion.py", line 67, in <module>
    select_conversion()
  File "/home/mcolombo/conversion.py", line 58, in select_conversion
    select_conversion()
  File "/home/mcolombo/conversion.py", line 56, in select_conversion
    if conversion_type < 1:
TypeError: '<' not supported between instances of 'str' and 'int'

错误代码片段 #

def select_conversion():
    conversion_type=input("Select conversion type, 1) inches to mm 2) mm to inches 	")
    try:  # checks if input is numeric
        conversion_type = int(conversion_type)
    except:
        print("Invalid response. ")
        pass
        select_conversion()
    if conversion_type < 1:
        print("Invalid response. ")
        select_conversion()
    elif int(conversion_type) > 2:
        print("Invalid response. ")
        select_conversion()
    elif int(conversion_type) == 1:
        in_mm()
    else:
        mm_in()

问题原因 #

当用户输入非数字内容（如"g"），try块转换int失败进入except，执行递归调用select_conversion()后，原函数的代码会继续执行第56行的判断。此时conversion_type仍是字符串类型，和整数1比较触发TypeError，而这个判断不在try-except范围内，因此抛出Traceback。

解决方案 #

方案1：递归调用后立即返回 #

修改except块，在递归调用后添加return，终止当前函数的后续代码执行：

def select_conversion():
    conversion_type=input("Select conversion type, 1) inches to mm 2) mm to inches 	")
    try:  # checks if input is numeric
        conversion_type = int(conversion_type)
    except ValueError:  # 明确捕获转换失败的异常，避免捕获其他意外错误
        print("Invalid response. ")
        select_conversion()
        return  # 终止当前函数，不再执行后续判断
    if conversion_type < 1:
        print("Invalid response. ")
        select_conversion()
    elif conversion_type > 2:
        print("Invalid response. ")
        select_conversion()
    elif conversion_type == 1:
        in_mm()
    else:
        mm_in()

方案2：用循环代替递归（推荐） #

递归不适合这种输入校验场景，改用循环逻辑更清晰，也避免递归深度问题：

def select_conversion():
    while True:
        conversion_type = input("Select conversion type, 1) inches to mm 2) mm to inches 	")
        try:
            conversion_type = int(conversion_type)
            # 校验数值范围，符合条件则跳出循环
            if 1 <= conversion_type <= 2:
                break
            print("Invalid response. ")
        except ValueError:
            print("Invalid response. ")
    # 执行对应转换逻辑
    if conversion_type == 1:
        in_mm()
    else:
        mm_in()

内容的提问来源于stack exchange，提问作者korbdallas