Python编程基础：第二十九节 异常Exception

前言

我们在写代码时不可避免地会出错，这时候编译器便会抛出异常并中断程序的执行。针对这种情况，我们可以采用异常处理的方式捕捉程序中的异常信息，并将异常信息打印出来供程序员分析，同时呢还要避免因为程序抛出异常而中断其继续向下执行。异常检测代码结构如下：

try:
	要检测异常信息的代码段
except 异常信息 as e:
	要打印的报错信息供程序员分析
else:
	如果要检测的代码段没有发生异常，那就执行这部分代码
finally:
	无论程序是否有异常，这里的代码均可以执行

实践

我们以分母为0为例学习异常处理的相关内容，众所周知在除法中分母是不能为0的，若分母为0，程序便会抛出异常信息：

numerator = int(input("Enter a number to divide: "))
denominator = int(input("Enter a number to divide by: "))
result = numerator / denominator
print(result)
>>> Enter a number to divide: 5
>>> Enter a number to divide by: 0
>>> ZeroDivisionError: division by zero

可见程序抛出了异常信息ZeroDivisionError并且中断执行。此时我们便可以通过try-except结构来捕捉异常信息：

try:
    numerator = int(input("Enter a number to divide: "))
    denominator = int(input("Enter a number to divide by: "))
    result = numerator / denominator
except ZeroDivisionError as e:
    print("You can't divide by zero! idiot!")
>>> Enter a number to divide: 5
>>> Enter a number to divide by: 0
>>> You can't divide by zero! idiot!

可见此时程序并没有中断，当其发现程序中有ZeroDivisionError时，便执行print("You can't divide by zero! idiot!")这行代码用于提示程序员发生了除零错误。
那么如果我们不小心把除数写成了字符串会发生什么呢？

numerator = int(input("Enter a number to divide: "))
denominator = int(input("Enter a number to divide by: "))
result = numerator / denominator
print(result)
>>> Enter a number to divide: 5
>>> Enter a number to divide by: david
>>> ValueError: invalid literal for int() with base 10: 'david'

可见代码抛出了ValueError，提示程序员这里应该是int型数据而不是字符串，那么我们同样可以使用异常处理方式捕捉这个异常信息：

try:
    numerator = int(input("Enter a number to divide: "))
    denominator = int(input("Enter a number to divide by: "))
    result = numerator / denominator
except ZeroDivisionError as e:
    print("You can't divide by zero! idiot!")
except ValueError as e:
    print("Enter only numbers plz")
>>> Enter a number to divide: 5
>>> Enter a number to divide by: david
>>> Enter only numbers plz

不难发现程序捕捉到了这个异常信息，并打印出Enter only numbers plz，用于提醒用户只能输入数字，不能输入字符串。其实Python中包含了许许多多的异常信息用于帮助程序员分析代码，如果我们想让代码检测所有的异常信息，那么就用到了下述代码：

try:
    numerator = int(input("Enter a number to divide: "))
    denominator = int(input("Enter a number to divide by: "))
    result = numerator / denominator
except ZeroDivisionError as e:
	# 用于打印程序自带的异常信息
    print(e)
    print("You can't divide by zero! idiot!")
except ValueError as e:
    print(e)
    print("Enter only numbers plz")
except Exception as e:
    print(e)
    print("something went wrong :(")

也就是不指定特定的异常信息，用Exception来代替，这样做的好处就是程序即便发现了异常也不会中断会继续执行下去。这里需要注意的是，在判断异常时，一定是先判断是否是某一类异常，最后再判断是否是一个异常信息。那么如果程序没有异常时，我们希望它能执行的代码放在else语句下边：

try:
    numerator = int(input("Enter a number to divide: "))
    denominator = int(input("Enter a number to divide by: "))
    result = numerator / denominator
except ZeroDivisionError as e:
    print(e)
    print("You can't divide by zero! idiot!")
except ValueError as e:
    print(e)
    print("Enter only numbers plz")
except Exception as e:
    print(e)
    print("something went wrong :(")
else:
    print(result)
>>> Enter a number to divide: 5
>>> Enter a number to divide by: 1
>>> 5.0

可见，当我们的输入都合法时，程序就会得到两个数相除的计算结果并打印到控制台。其实我们还可以书写一些代码，这些代码无论程序是否发生异常，他们都会被执行，这些代码需要放在finally的下面：

try:
    numerator = int(input("Enter a number to divide: "))
    denominator = int(input("Enter a number to divide by: "))
    result = numerator / denominator
except ZeroDivisionError as e:
    print(e)
    print("You can't divide by zero! idiot!")
except ValueError as e:
    print(e)
    print("Enter only numbers plz")
except Exception as e:
    print(e)
    print("something went wrong :(")
else:
    print(result)
finally:
    print("This will always execute")
>>> Enter a number to divide: 5
>>> Enter a number to divide by: 1
>>> 5.0
>>> This will always execute

>>> Enter a number to divide: 5
>>> Enter a number to divide by: 0
>>> division by zero
>>> You can't divide by zero! idiot!
>>> This will always execute

以上便是异常的全部内容，感谢大家的收藏、点赞、评论。我们下一节将介绍文件检测（File Detection），敬请期待~