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Python Try Except

Python基础 2022-06-06 12:54:26小码哥的IT人生shichen

Python Try Except

try 块允许您测试代码块以查找错误。

except 块允许您处理错误。

finally 块允许您执行代码,无论 try 和 except 块的结果如何。

异常处理

当我们调用 Python 并发生错误或异常时,通常会停止并生成错误消息。

可以使用 try 语句处理这些异常:

示例代码:

try 块将生成异常,因为 x 未定义:

try:
  print(x)
except:
  print("An exception occurred")

完整实例:

try:
  print(x)
except:
  print("An exception occurred")

由于 try 块引发错误,因此会执行 except 块。

如果没有 try 块,程序将崩溃并引发错误:

示例代码:

该语句将引发错误,因为未定义 x:

print(x)

完整实例:

print(x)
#Traceback (most recent call last):
#  File "<stdin>", line 1, in <module>
#NameError: name 'x' is not defined

多个异常

您可以根据需要定义任意数量的 exception 块,例如,假如您要为特殊类型的错误执行特殊代码块:

示例代码:

如果 try 块引发 NameError,则打印一条消息,如果是其他错误则打印另一条消息:

try:
  print(x)
except NameError:
  print("Variable x is not defined")
except:
  print("Something else went wrong")

完整实例:

#The try block will generate a NameError, because x is not defined:
try:
  print(x)
except NameError:
  print("Variable x is not defined")
except:
  print("Something else went wrong")

Else

如果没有引发错误,那么您可以使用 else 关键字来定义要执行的代码块:

示例代码:

在本例中,try 块不会生成任何错误:

try:
  print("Hello")
except:
  print("Something went wrong")
else:
  print("Nothing went wrong")

完整实例:

#The try block does not raise any errors, so the else block is executed:
try:
  print("Hello")
except:
  print("Something went wrong")
else:
  print("Nothing went wrong")

Finally

如果指定了 finally 块,则无论 try 块是否引发错误,都会执行 finally 块。

示例代码:

try:
  print(x)
except:
  print("Something went wrong")
finally:
  print("The 'try except' is finished")

完整实例:

#The finally block gets executed no matter if the try block raises any errors or not:
try:
  print(x)
except:
  print("Something went wrong")
finally:
  print("The 'try except' is finished")

这对于关闭对象并清理资源非常有用:

示例代码:

试图打开并写入不可写的文件:

try:
  f = open("demofile.txt")
  f.write("Lorum Ipsum")
except:
  print("Something went wrong when writing to the file")
finally:
  f.close()

完整实例:

#The try block will raise an error when trying to write to a read-only file:
try:
  f = open("demofile.txt")
  f.write("Lorum Ipsum")
except:
  print("Something went wrong when writing to the file")
finally:
  f.close()
#The program can continue, without leaving the file object open

程序可以继续,而且不会打开文件对象。

引发异常

作为 Python 开发者,您可以选择在条件发生时抛出异常。

如需抛出(引发)异常,请使用 raise 关键词。

示例代码:

假如 x 小于 0,则引发异常并终止程序:

x = -1
if x < 0:
  raise Exception("Sorry, no numbers below zero")

完整实例:

x = -1
if x < 0:
  raise Exception("Sorry, no numbers below zero")

raise 关键字用于引发异常。

您能够定义所引发异常的类型、以及打印给用户的文本。

示例代码:

如果 x 不是整数,则引发 TypeError:

x = "hello"
if not type(x) is int:
  raise TypeError("Only integers are allowed")

完整实例:

x = "hello"
if not type(x) is int:
  raise TypeError("Only integers are allowed")

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