错误处理

错误处理:

在程序运行的过程中,如果发生了错误,可以事先约定返回一个错误的代码,这样,就可以知道是否有错,

以及出错的原因。

在操作系统提供的调用中,返回错误码非常常见。

def fun1(a):
    return a*3

def foo():
    r = some_function()
    if r==(-1):
        return (-1)
    else:
       # do something
        return r
print fun1('aa')


def fun1(a):
    return a-2

def foo(a):
    r = fun1(a)
    if r==(-1):
        return (-1)
    else:
       # do something
        return r
print fun1(1)
print '--------------'
print foo(30)

C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a3.py
-1
--------------
28

try:

让我们用一个例子来看看try的机制:


------------------------------------------------
try:
    print 'try...'
    r = 10 / 0
    print 'result:', r
except ZeroDivisionError, e:
    print 'except:', e
finally:
    print 'finally...'
print 'END'

C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a3.py
try...
except: integer division or modulo by zero
finally...
END


try:
    print 'try...'
    r = 10 / 5
    print 'result:', r
except ZeroDivisionError, e:
    print 'except:', e
finally:
    print 'finally...'
print 'END'

C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a3.py
try...
result: 2
finally...
END


def foo(s):
    return 10 / int(s)

def bar(s):
    return foo(0) * 2

def main():
    try:
        bar('0')
    except StandardError, xx:
        print 'Error!'
    finally:
        print 'finally...'

main()

C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a3.py
Error!
finally

调用堆栈:

# err.py:
def foo(s):
    return 10 / int(s)

def bar(s):
    return foo(s) * 2

def main():
    bar('0')

main()


C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a3.py
Traceback (most recent call last):
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 11, in <module>
    main()
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 9, in main
    bar('0')
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 6, in bar
    return foo(s) * 2
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 3, in foo
    return 10 / int(s)
ZeroDivisionError: integer division or modulo by zero

Process finished with exit code 1

------------------------------------------------------------------------
# err.py:
def foo(s):
    return 10 / int(s)

def bar(s):
    return foo(s) * 2

def main():
    bar('0')

main()
print 'xxxxxxxxxxxxxx'

C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a3.py
Traceback (most recent call last):
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 11, in <module>
    main()
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 9, in main
    bar('0')
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 6, in bar
    return foo(s) * 2
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 3, in foo
    return 10 / int(s)
ZeroDivisionError: integer division or modulo by zero

Process finished with exit code 1




# err.py:
import logging
def foo(s):
    return 10 / int(s)

def bar(s):
    return foo(s) * 2

def main():
    try:
        bar('0')
    except StandardError, e:
        logging.exception(e)

main()
print 'xxxxxxxxxxxxxx'

C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a3.py
xxxxxxxxxxxxxx
ERROR:root:integer division or modulo by zero
Traceback (most recent call last):
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 11, in main
    bar('0')
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 7, in bar
    return foo(s) * 2
  File "C:/Users/TLCB/PycharmProjects/untitled/a3.py", line 4, in foo
    return 10 / int(s)
ZeroDivisionError: integer division or modulo by zero

Process finished with exit code 0


抛出错误:

因为错误是class,捕获一个错误就是捕获到该class的一个实例。


因此,错误并不是凭空产生的,而是有意创建并抛出的。


如果要抛出错误,首先根据需要,定义一个错误的class,选择好继承关系,

然后,用raise语句抛出一个错误的实例:

# err.py
class FooError(StandardError):
    def __init__(self):

def foo(self,s):
    n = int(s)
    if n==0:
        raise FooError('invalid value: %s' % s)
    return 10 / n

-----------------------------------------------------------
# err.py
class FooError(StandardError):
    def __init__(self):
        print 11
    def foo(self, s):
        n = int(s)
        if n == 0:
            raise FooError('invalid value: %s' % s)
        return 10 / n


from   mycompany.web.FooError import *
s=FooError()
print s.foo(5)

C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a2.py
11
2

C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a2.py
11
Traceback (most recent call last):
  File "C:/Users/TLCB/PycharmProjects/untitled/a2.py", line 3, in <module>
    print s.foo(0)
  File "C:UsersTLCBPycharmProjectsuntitledmycompanywebFooError.py", line 8, in foo
    raise FooError('invalid value: %s' % s)
TypeError: __init__() takes exactly 1 argument (2 given)

最后,我们来看另一种错误处理的方式；

def foo(s):
    n = int(s)
    return 10 / n

def bar(s):
    try:
        return foo(s) * 2
    except StandardError, e:
        print 'Error!'
        raise

def main():
    bar('0')

main()

def foo(s):
    n = int(s)
    return 10 / n

def bar(s):
    try:
        return foo(s) * 2
    except StandardError, e:
        print 'Error!'
        raise

def main():
    bar('0')

main()


C:Python27python.exe C:/Users/TLCB/PycharmProjects/untitled/a4.py
Error!
Traceback (most recent call last):
  File "C:/Users/TLCB/PycharmProjects/untitled/a4.py", line 15, in <module>
    main()
  File "C:/Users/TLCB/PycharmProjects/untitled/a4.py", line 13, in main
    bar('0')
  File "C:/Users/TLCB/PycharmProjects/untitled/a4.py", line 7, in bar
    return foo(s) * 2
  File "C:/Users/TLCB/PycharmProjects/untitled/a4.py", line 3, in foo
    return 10 / n
ZeroDivisionError: integer division or modulo by zero