Python的异常处理

常见错误

1.ZeroDivisionError(零除错误)

In [1]: 10/0
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-1-242277fd9e32> in <module>()
----> 1 10/0

ZeroDivisionError: integer division or modulo by zero

2.IOError（文件错误）

In [2]: open('hello')
---------------------------------------------------------------------------
IOError                                   Traceback (most recent call last)
<ipython-input-2-9834044a9e14> in <module>()
----> 1 open('hello')

IOError: [Errno 2] No such file or directory: 'hello'

3.SyntaxError（语法错误）

In [3]: for i in [1,2,3]
  File "<ipython-input-3-ae71676907af>", line 1
    for i in [1,2,3]
                    ^
SyntaxError: invalid syntax

4.IndexError（索引错误）

In [4]: t = (1,2,3)

In [5]: t[3]
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-5-7d5cf04057c5> in <module>()
----> 1 t[3]

IndexError: tuple index out of range

5.KeyError（字典的key错误）

In [7]: d = {'name':'cici'}

In [8]: d['name']
Out[8]: 'cici'

In [9]: d['age']
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-9-39d5d5cefe61> in <module>()
----> 1 d['age']

KeyError: 'age'

6.NameError（名称错误）

In [10]: print a
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-10-9d7b17ad5387> in <module>()
----> 1 print a

NameError: name 'a' is not defined

错误排查

捕获异常

python异常处理机制: try….except…finally…
如果异常则执行except后的语句，没有异常则直接跳过，也可以用else来提示没有异常
finally后的语句是必然执行的，不管try后的语句是否正确

1.已知错误（跳过错误继续执行）

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"
try:                        #用 try 来运行可能会出错的代码
    print 'starting...'
    li = [1, 2, 3]
    print a
    print li[3]
except IndexError:              #用except来捕获错误,如果异常则执行
    print 'index out of list'
except NameError,e:             #捕获第二个错误，同时给错误赋值
    print e
finally:                        #必执行的
    print 'ending..'

2.未知错误

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"
try:
    print 'starting...'
    li = [1, 2, 3]
    print a
    print li[3]
except BaseException,e:        #BaseException是一个错误的父类，表示所有错误
    print 'Error:%s'%e
else:
    print 'no error'
finally:
    print 'ending..'

3.没有异常的情况

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"
try:
    print 'starting...'
    li = [1, 2, 3]
    li.append(4)
    a = 1
    print a
    print li[3]
except IndexError:              #检测第一个错误
    print 'index out of list'
except NameError:               #检测第二个错误
    print 'name is not defined'
else:                           #没有异常时
    print 'no error'
finally:
    print 'ending..'

4.函数排错
直接在主函数里排错，可以增强排错效率

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"
def func1(s):
    return func2(s) * 2


def func2(s):
    return 20 / s


def main():
    try:
        print func1('10')
    except BaseException, e:
        print e


main()

5.错误日志

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"

import logging                          ##导入日志模块
logging.basicConfig(filename='err.log') #日志文件的定义

def func1(s):
    return func2(s) * 2


def func2(s):
    return 20 / s


def main():
    try:
        print func1('10')
    except Exception as e:
        logging.exception(e)    #所有错误导入到日志文件


main()

抛出异常

except: 捕获异常
raise: 抛出异常

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"

class MyError(BaseException):
    pass
a = 1
if a == 1:
    raise  MyError

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"

class MyError(BaseException):
    pass

def func1(s):
    return func2(s) * 2


def func2(s):
    return 20 / int(s)


def main():
    try:
        print func1('0')
    except ZeroDivisionError as e:
        raise MyError
        #ZeroDivisionError 转换为 MyError

main()

调试

print

用 print 把可能有问题的变量打印出来
用 print 最大的坏处是将来还得删掉它,运行结果也会包含很多垃圾信息。

断言

凡是用 print 来辅助查看的地方,都可以用断言(assert)来替代:;
如果断言失败, assert 语句本身就会抛出 AssertionError

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"

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


def main():
    foo('0')


assert foo(5) == 2
print 'right'

Python 解释器执行时可以用 -O 参数来关闭 assert,把所有的 assert 语句当成 pass。
[[e-mail] date7.15]$ python -O error_4.py

logging

logging 不会抛出错误,而且可以输出到文件
logging.info() 就可以输出一段文本到日志文件中

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"

import logging
logging.basicConfig(filename='logging.log',level=logging.INFO)


def foo(s):
    # 日志级别:debug,info,waring,error
    n = int(s)
    logging.info('n=%d'%n)
    logging.warning('n=%d....warn'%n)
    return 10 / n


def main():
    foo('10')

main()

pdb

[[e-mail] date7.15]$ python -m pdb error_3.py
n – next
p 变量 – 取出变量
q – 退出

#!/usr/bin/env python
# coding:utf-8
_author_ = "lvah"

import pdb
n1 =1
n = int(n1)
print n
pdb.set_trace()     #在可能出错的地方放一个 pdb.set_trace() ,就可以设置一个断点
print 'hello'
pdb.set_trace()
s = 2
print s

python …
程序会自动在 pdb.set_trace() 暂停并进入 pdb 调试环境, p 查看变量, c 继续运行