学步园

1.错误

>>> Print 'Hello World'
    File "<stdin>", line 1
      Print 'Hello World'
                        ^
SyntaxError: invalid syntax

>>> print 'Hello World'
Hello World

可以看到有一个SynataxError被引发，并且检测到的错误位置被打印了，这是错误处理器所作的工作。

2.try...except

使用try...except语句来处理异常，把通常的语句放在try块中，而把错误的处理语句放在except块中。

#!/usr/bin/python
# Filename: try_except.py

import sys

try:
    s = raw_input('Enter something --> ')
except EOFError:
    print '\nWhy did you do an EOF on me?'
    sys.exit() # exit the program
except:
    print '\nSome error/exception occurred.'
    # here, we are not exiting the program

print 'Done'

输出：

$ python try_except.py
Enter something -->
Why did you do an EOF on me?

$ python try_except.py
Enter something --> Python is exceptional!
Done

把所有可能引发错误的语句放在try块中，然后在except从句/块中处理所有的错误和异常。except从句可以专门处理单一的错误或异常，或者一组包括在圆括号内的错误/异常。如果没有给出错误或异常的名称，它会处理所有的错误和异常。对于每个try从句，至少都有一个相关联的except从句。

如果某个错误或异常没有被处理，默认的Python处理器就会被调用。它会终止程序的运行，并且打印一个消息。

还可以让try...catch块关联上一个else从句，当没有异常发生的时候，else从句将被执行。

3.引发异常

可以使用raise语句引发异常，指明错误/异常的名称和伴随异常出发的异常对象。引发的错误或异常应分别是Error或Exception类的直接或简介导出类。

#!/usr/bin/python
# Filename: raising.py

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

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

输出：

$ python raising.py
Enter something -->
Why did you do an EOF on me?

$ python raising.py
Enter something --> ab
ShortInputException: The input was of length 2, was expecting at least 3

$ python raising.py
Enter something --> abc
No exception was raised.

4.try...finally

finally从句无论在是否发生异常的情况下都会执行。

#!/usr/bin/python
# Filename: finally.py

import time

try:
    f = file('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'

输出：

$ python finally.py
Programming is fun
When the work is done
Cleaning up...closed the file
Traceback (most recent call last):
  File "finally.py", line 12, in ?
    time.sleep(2)
KeyboardInterrupt