Python 3 中单个变量的意外多次返回

Question

我编写了一些代码，以便人们可以输入日期。停止输入小于 1 或大于 12 的月份的错误检查应仅在其在这些范围内时返回一个值。如果我输入一些“超出范围”的数字，它会正确地重新要求重新输入一个月，但会返回所有值。怎么回事？

# the question asked to get the month input for the xml updater
def month_q():
    try:
        month = int(input('What was the month [MM] which the installers were  updated/created by xxx?:'))
    except:
        print("That was not a valid number. Please re-enter a 2 digit month")
        month_q()
    updatemonth = month_check(month)
    print("Month q returning:", updatemonth)
    return updatemonth


# check the update month is a valid month
def month_check(month):
    if month < 1:
        print("The month must be a number between 01 and 12. Please re-enter")
        month_q()
    elif month > 12:
        print("The month must be a number between 01 and 12. Please re-enter")
        month_q()
    else:
        print("Month_check returning month:", month)
        return month



# this updates the xml file with the date of the last installer    
def xml_updater():
    updatemonth = month_q()
    print("Update month:", updatemonth)


xml_updater()

结果是在输入错误月份“15”、“14”和“13”之前输入正确的“12”是：

What was the month [MM] which the installers were updated/created by xxx?:15
The month must be a number between 01 and 12. Please re-enter
What was the month [MM] which the installers were updated/created by xxx?:14
The month must be a number between 01 and 12. Please re-enter
What was the month [MM] which the installers were updated/created by xxx?:13
The month must be a number between 01 and 12. Please re-enter
What was the month [MM] which the installers were updated/created by xxx?:12
Month_check returning month: 12
Month q returning: 12
Month q returning: None
Month q returning: None
Month q returning: None
Update month: None

发生了什么事？

Answer 1

你的函数没有返回任何东西；它一次又一次地调用问题函数，但忽略了返回值：

def month_check(month):
    if month < 1:
        print("The month must be a number between 01 and 12. Please re-enter")
        month_q()
    elif month > 12:
        print("The month must be a number between 01 and 12. Please re-enter")
        month_q()
    else:
        print("Month_check returning month:", month)
        return month

如果月份小于 1 或大于 12，该函数将返回 None（默认值）。语句updatemonth = month_check(month) 然后将None 分配给updatemonth，当print("Month q returning:", updatemonth) 运行时，它会打印Month q returning: None。但是由于你递归调用了month_q，然后返回到之前的month_check() 调用，它再次返回None，以此类推。

这是您的函数的嵌套跟踪，如 month_q 调用 month_check 调用 month_q 等：

month_q():
    What was the month [MM] which the installers were updated/created by xxx?:15
    month_check(15):
        The month must be a number between 01 and 12. Please re-enter
        month_q():
            What was the month [MM] which the installers were updated/created by xxx?:14
            month_check(15):
                The month must be a number between 01 and 12. Please re-enter
                month_q():
                    What was the month [MM] which the installers were updated/created by xxx?:13
                    month_check(13):
                        The month must be a number between 01 and 12. Please re-enter
                        month_q():
                            What was the month [MM] which the installers were updated/created by xxx?:12
                            month_check(12):
                                Month_check returning month: 12
                                <- 12
                            Month q returning: 12
                            <- 12
                        <- None
                    Month q returning: None
                    <- None
                <- None
            Month q returning: None
            <- None
        <- None
    Month q returning: None
    <- None
Update month: None

相反，让检查函数返回 True 或 False 以指示正确的结果，并将要求重新输入的请求留给 month_q。

使用while 循环而不是递归来做到这一点；如果你必须经常问，你最终会遇到一个递归太深的异常。

def month_q():
    while True:
        try:
            month = int(input('What was the month [MM] which the installers were  updated/created by xxx?:'))
            if not month_check(month):
                raise ValueError()
        except ValueError:
            print("Please re-enter a 2 digit month")
            continue
        print("Month q returning:", month)
        return month


# check the update month is a valid month
def month_check(month):
    if month < 1 or month > 12:
        print("The month must be a number between 01 and 12")
        return False
    return True

您也可以内联该检查：

def month_q():
    while True:
        try:
            month = int(input('What was the month [MM] which the installers were  updated/created by xxx?:'))
            if month < 1 or month > 12:
                raise ValueError()
        except ValueError:
            print("Please re-enter a 2 digit month between 01 and 12.")
            continue
        print("Month q returning:", month)
        return month

使用毯子except: 子句绝不是一个好主意；在上面的代码中，当您输入一个非整数值时，我捕获了int() 引发的ValueError，如果您输入了一个整数但它不是介于1 和 12 包括在内。这大大简化了“不是一个月”的错误处理。

Answer 2

试试这个：

try:
    month = int(input('What was the month [MM] which the installers were  updated/created by xxx?:'))
except:
    print("That was not a valid number. Please re-enter a 2 digit month")
    month_q()
    break

这应该会中断不正确月份的打印

另外，你应该改变

updatemonth = month_check(month)
print("Month q returning:", updatemonth)
return updatemonth

到：

updatemonth = month_check(month)
try:
    updatemonth
except NameError:
    updatemonth = None

if updatemonth is None:
    break
print("Month q returning:", updatemonth)
return updatemonth