python try/except/finally

稍微总结一下，否则总是忘。

x = 'abc'
def fetcher(obj, index):
    return obj[index]

fetcher(x, 4)

输出：

  File "test.py", line 6, in 
    fetcher(x, 4)
  File "test.py", line 4, in fetcher
    return obj[index]
IndexError: string index out of range

第一： try不仅捕获异常，而且会恢复执行

def catcher():
    try:
        fetcher(x, 4)
    except:
        print "got exception"
    print "continuing"

输出：

got exception
continuing

第二：无论try是否发生异常，finally总会执行

def catcher():
    try:
        fetcher(x, 4)
    finally:
        print 'after fecth'

输出：

after fecth
Traceback (most recent call last):
  File "test.py", line 55, in 
    catcher()
  File "test.py", line 12, in catcher
    fetcher(x, 4)
  File "test.py", line 4, in fetcher
    return obj[index]
IndexError: string index out of range

第三：try无异常，才会执行else

def catcher():
    try:
        fetcher(x, 4)
    except:
        print "got exception"
    else:
        print "not exception"

输出：

got exception

def catcher():
    try:
        fetcher(x, 2)
    except:
        print "got exception"
    else:
        print "not exception"

输出：

not exception

else作用：没有else语句，当执行完try语句后，无法知道是没有发生异常，还是发生了异常并被处理过了。通过else可以清楚的区分开。

第四：利用raise传递异常

def catcher():
    try:
        fetcher(x, 4)
    except:
        print "got exception"
        raise

输出：

got exception
Traceback (most recent call last):
  File "test.py", line 37, in 
    catcher()
  File "test.py", line 22, in catcher
    fetcher(x, 4)
  File "test.py", line 4, in fetcher
    return obj[index]
IndexError: string index out of range

raise语句不包括异常名称或额外资料时，会重新引发当前异常。如果希望捕获处理一个异常，而又不希望

异常在程序代码中消失，可以通过raise重新引发该异常。

第五：except(name1, name2)

def catcher():
    try:
        fetcher(x, 4)
    except(TypeError, IndexError):
        print "got exception"
    else:
        print "not exception"

捕获列表列出的异常，进行处理。若except后无任何参数，则捕获所有异常。

def catcher():
    try:
        fetcher(x, 4)
    except:
        print "got exception"