Python 3中单变量的意外多个返回
Python 3中单变量的意外多个返回
提问于 2013-01-21 20:07:18
我已经写了一些代码，以便人们可以输入日期。如果错误检查停止输入的月份小于1或大于12，则仅当该值在这些范围内时才应返回值。如果我输入了几个“越界”数字，它会正确地重新请求重新输入一个月，但返回所有的值。怎么一回事？
# 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()结果是输入坏的月份'15‘、'14’和'13‘，然后输入正确的月份'12’：
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怎么一回事？
回答 2
Stack Overflow用户
回答已采纳
发布于 2013-01-21 20:13:13
您的函数没有返回任何内容；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month小于1或大于12，该函数将返回None (默认值)。然后，语句updatemonth = month_check(month)将None赋值给updatemonth，当print("Month q returning:", updatemonth)运行时，它将打印Month q returning: None。但由于您以递归方式调用了month_q，因此它将返回到前面的month_check()调用，该调用再次返回None，依此类推。
下面是您的函数的嵌套跟踪，因为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取而代之的是，让check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。
不过，使用while循环而不是递归可以做到这一点；如果您不得不频繁地询问，您最终会得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except:子句从来都不是一个好主意；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，如果您确实输入了一个整数，但它不是介于1和12之间(包括1和12)的值，则会引发相同的异常。这大大简化了“不是一个月”的错误处理。
Stack Overflow用户
发布于 2013-01-21 20:12:14
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这应该会中断错误月份的打印
另外，你应该改变
updatemonth = month_check(month)
print("Month q returning:", updatemonth)
return updatemonth至：
updatemonth = month_check(month)
try:
updatemonth
except NameError:
updatemonth = None
if updatemonth is None:
break
print("Month q returning:", updatemonth)
return updatemonth页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/14438259
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