Python学习笔记（10）：异常-优快云博客

本文详细介绍了Python中的异常处理机制，包括错误处理、try-except语句的使用、自定义异常的创建以及try-finally结构的应用。通过具体示例展示了如何有效地处理程序运行中可能出现的各种异常情况。

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当程序执行的时候，可能会出现一些异常现象，这个可能是会存在的。例如，你在读文件的时候，而那个文件不存在。此时，我们可以用异常来处理。

1. 错误

我们来做个简单的测试，我们故意把print方法写出Print。

>>> Print("Hello world")
Traceback (most recent call last):
File "<pyshell#0>", line 1, in <module>
Print("Hello world")
NameError: name 'Print' is not defined
>>>

系统会抛出NameError异常。

2. try..except

我们可以用try..except处理异常。

try:
    f = open("test.txt")
    f.close()
except(IOError):
    print("The file is not exist.")
except:
    print("Some error occurred.")

print("Done")

运行结果：

The file is not exist.
Done

3. 引发异常

你可以用raise语句来引发异常。我们先自定义一个ShortInputError异常，需要继承Exception类。

class ShortInputError(Exception):
    '''A user-defined exception class.'''
    def __init__(self, length, atleast):
        Exception.__init__(self)
        self.length = length
        self.atleast = atleast

try:
    s = input("Enter something -->")
    if len(s) < 3:
        raise(ShortInputError(len(s), 3))
    #Other work can continue as usual here
except(EOFError):
    print("Why did you do an EOF on me?")
except ShortInputError as e:
    print("ShortInputError: The input was of length %d, \
was expecting at least %d" % (e.length, e.atleast))
else:
    print("No exception was raised.")

输入两个字符运行结果：

>>>
Enter something -->tr
ShortInputError: The input was of length 2, was expecting at least 3
>>>

输入三个字符以上运行结果：

>>>
Enter something -->test
No exception was raised.
>>>

4. try..finally

假如你在读一个文件的时候，希望在无论异常发生与否的情况下都关闭文件，该怎么做呢？这可以使用finally块来完成。注意，在一个try块下，你可以同时使用except从句和finally块。如果你要同时使用它们的话，需要把一个嵌入另外一个。

import time

try:
    f = open("poem.txt")
    while True: # our usual file-reading idiom
        line = f.readline()
        if len(line) == 0:
            break
        time.sleep(2)
        print(line),
finally:
    f.close()
    print("Cleaning up...closed the file")

运行结果：

>>>
Programming is fun

When the work is done

if you wanna make your work also fun:

use Python!

Cleaning up...closed the file
>>>

def divide(x, y):
    try:
        result = x/ y
    except ZeroDivisionError:
        print("Division by zero!")
    else:
        print("result is", result)
    finally:
        print("executing finally clause")

测试结果：

>>> divide(2, 1)
result is 2.0
executing finally clause
>>> divide(2, 0)
Division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
File "<pyshell#13>", line 1, in <module>
divide("2", "1")
File "<pyshell#10>", line 3, in divide
result = x/ y
TypeError: unsupported operand type(s) for /: 'str' and 'str'
>>>