Python异常处理学习。
2017/7/19
错误和异常
错误
异常
错误和异常区别
编写的代码出现语法错误或者逻辑错误，程序是不能正常运行的。异常是在程序运行后，碰到特殊情况而是程序不能继续运行。
python常见错误
- a: NameError
变量没有定义就被使用
In [1]: a
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
<ipython-input-1-60b725f10c9c> in <module>()
----> 1 a
NameError: name 'a' is not defined
- if True SyntaxError
语法错误。。
In [2]: if a
File "<ipython-input-2-73bf85fc5fba>", line 1
if a
^
SyntaxError: invalid syntax
- f = open(‘1.txt’) : IOError
文件处理错误
In [4]: f = open('test1.txt')
---------------------------------------------------------------------------
IOError Traceback (most recent call last)
<ipython-input-4-2659d8b9b5b7> in <module>()
----> 1 f = open('test1.txt')
IOError: [Errno 2] No such file or directory: 'test1.txt'
- 10/0 ZeroDivisionError
除数为0的错误
In [5]: 10 / 0
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-5-3e699e72a4ab> in <module>()
----> 1 10 / 0
ZeroDivisionError: integer division or modulo by zero
- a = int(‘dd’) ValueError
类型错误
In [8]: a = int('a100')
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-8-c5ee1e4f90b8> in <module>()
----> 1 a = int('a100')
ValueError: invalid literal for int() with base 10: 'a100'
Ctrl+c 错误 KeyboardInterrupt
In [9]: import time
In [10]: for i in range(10):
....: time.sleep(2) ....:^C—————————————————————————
KeyboardInterrupt Traceback (most recent call last)
in () 1 for i in range(10):—-> 2 time.sleep(2)
3KeyboardInterrupt:
try – except 异常处理
使用try捕获要处理的代码，其中except用来处理异常。。
自己可以设置，是那些类型的异常，来进行处理。。Exception可以设置。。
1 #!/usr/bin/python
2
3 a
4 print "come on"
运行结果，可以看到出现了,NameError的错误。
root@Ubuntu:~/show_me_code/XTeach# python error.py
Traceback (most recent call last):
File "error.py", line 3, in <module>
a
NameError: name 'a' is not defined
捕获一下异常
1 #!/usr/bin/python
2 try:
3 a
4 except NameError,e:
5 print "catch Error:", e
6
7 print "come on"
可以看到运行结果，把异常捕获到，进行处理，后面的内容还是可以进行运行的。
其中的e就是捕获到的异常错误信息。
root@Ubuntu:~/show_me_code/XTeach# python error.py
catch Error: name 'a' is not defined
come on
这个异常的是因为a没有被定义，现在定义一个a。
1 #!/usr/bin/python
2 a = 0
3 try:
4 a
5 except NameError,e:
6 print "catch Error:", e
7
8 print "come on"
运行结果，就不会出现捕获的异常了。
root@Ubuntu:~/show_me_code/XTeach# python error.py
come on
- try-except 捕获异常分析
1 try:
2 undef
3 except:
4 print "catch error"
结果
root@Ubuntu:~/show_me_code/XTeach# python error2.py
catch error
1 try:
2 if undef
3 except:
4 print "catch error"
这次不能捕获到异常了。
root@Ubuntu:~/show_me_code/XTeach# python error2.py
File "error2.py", line 2
if undef
^
SyntaxError: invalid syntax
case1： 可以捕获异常，因为是运行时错误
case2：不能捕获异常，因为是语法错误，运行前错误。。
py文件生成字节码，，
- try–except 捕获异常分析
设置错误的类型。。
1 try:
2 undef
3 except NameError, e:
4 print "catch error", e
设置的错误类型是名字没有被定义。。
root@Ubuntu:~/show_me_code/XTeach# python error2.py
catch error name 'undef' is not defined
设置的捕获错误类型为IOError
1 try:
2 undef
3 except IOError, e:
4 print "catch error", e
但是捕获不到。。
root@Ubuntu:~/show_me_code/XTeach# python error2.py
Traceback (most recent call last):
File "error2.py", line 2, in <module>
undef
NameError: name 'undef' is not defined
case 3: 可以捕获异常，因为设置捕获NameError异常
case4: 不能捕获异常，因为设置IOError，不会处理NameError。。
关于异常的处理，首先这个程序能够正常运行，异常捕获需要程序先能运行为前提。捕获的类型也要一一对应，，比如IOError不能处理NameError的异常。。
- try–except 用例
这个例子，会出现问题。
1 #!/usr/bin/python
2
3 import random
4
5 num = random.randint(0,100)
6
7 while True:
8 guess = int(raw_input("Enter 1~100: "))
9 if guess > num:
10 print "guess Bigger:", guess
11 elif guess < num:
12 print "guess Smaller:", guess
13 else:
14 print "Guess OK, Game Over"
15 break
16 print "\n"
输入 89a 的时候
root@Ubuntu:~/show_me_code/XTeach# python error2.py
Enter 1~100: 80
guess Bigger: 80
Enter 1~100: 89a
Traceback (most recent call last):
File "error2.py", line 8, in <module>
guess = int(raw_input("Enter 1~100: "))
ValueError: invalid literal for int() with base 10: '89a'
如何处理这个异常问题。。
加入一个异常处理，还是出现了 guess Bigger 80,为了美观，需要添加continue
1 #!/usr/bin/python
2
3 import random
4
5 num = random.randint(0,100)
6
7 while True:
8 try:
9 guess = int(raw_input("Enter 1~100: "))
10 except ValueError, e:
11 print "Enter 1~100"
12 if guess > num:
13 print "guess Bigger:", guess
14 elif guess < num:
15 print "guess Smaller:", guess
16 else:
17 print "Guess OK, Game Over"
18 break
19 print "\n"
运行结果
root@Ubuntu:~/show_me_code/XTeach# python error2.py
Enter 1~100: 80
guess Bigger: 80
Enter 1~100: 70a
Enter 1~100
guess Bigger: 80
Enter 1~100:
加入continue
1 #!/usr/bin/python
2
3 import random
4
5 num = random.randint(0,100)
6
7 while True:
8 try:
9 guess = int(raw_input("Enter 1~100: "))
10 except ValueError, e:
11 print "Enter 1~100"
12 continue
13 if guess > num:
14 print "guess Bigger:", guess
15 elif guess < num:
16 print "guess Smaller:", guess
17 else:
18 print "Guess OK, Game Over"
19 break
20 print "\n"
加入continue之后。。后面的代码就不执行了。。
root@Ubuntu:~/show_me_code/XTeach# python error2.py
Enter 1~100: 90
guess Bigger: 90
Enter 1~100: 45a
Enter 1~100
Enter 1~100: 45
guess Smaller: 45
- try-except 处理多个异常
1 #!/usr/bin/python
2 try:
3 f = open('1.txt')
4 line = f.read(2)
5 num = int(line)
6 print "read num=%d" % num
7 except IOError,e:
8 print "catch Error:", e
不能存在1.txt，运行。
root@Ubuntu:~/show_me_code/XTeach# python error.py
catch Error: [Errno 2] No such file or directory: '1.txt'
添加一个1.txt，可以运行成功。。
root@Ubuntu:~/show_me_code/XTeach# python error.py
read num=11
把1.txt的内容改变一下。。aa11111
运行时报错。。。
root@Ubuntu:~/show_me_code/XTeach# python error.py
Traceback (most recent call last):
File "error.py", line 5, in <module>
num = int(line)
ValueError: invalid literal for int() with base 10: 'aa'
多个捕获异常
1 #!/usr/bin/python
2 try:
3 f = open('1.txt')
4 line = f.read(2)
5 num = int(line)
6 print "read num=%d" % num
7 except IOError,e:
8 print "catch Error:", e
9 except ValueError, e:
10 print "catch Error:", e
可以看到能捕获到异常
root@Ubuntu:~/show_me_code/XTeach# python error.py
catch Error: invalid literal for int() with base 10: 'aa'
使用多个except可以处理多个异常类
- try-except–else使用
如果没有异常，执行else语句中的代码。。
1 #!/usr/bin/python
2 try:
3 f = open('1.txt')
4 line = f.read(2)
5 num = int(line)
6 print "read num=%d" % num
7 except IOError,e:
8 print "catch Error:", e
9 except ValueError, e:
10 print "catch Error:", e
11 else:
12 print "No Error"
再修改1.txt里面的内容，执行结果。。。
root@Ubuntu:~/show_me_code/XTeach# python error.py
read num=11
No Error
- try–finally 语句
finally语句会被执行的。。
1 try:
2 f = open('1.txt')
3 print int(f.read())
4 finally:
5 print "file close"
6 f.close()
执行结果
root@Ubuntu:~/show_me_code/XTeach# python error.py
1111111
file close
修改 1.txt中的内容 qq11111
再次执行程序。。可以看到finally的语句被执行了。。
root@Ubuntu:~/show_me_code/XTeach# python error.py
file close
Traceback (most recent call last):
File "error.py", line 3, in <module>
print int(f.read())
ValueError: invalid literal for int() with base 10: 'qq1111111\n'
try--finally的作用
- try–except–finally
无论如何都是要执行finally的。
1 try:
2 f = open('1.txt')
3 line = f.read(2)
4 num = int(line)
5 print "read num=%d" % num
6 except IOError, e:
7 print "catch IOError:", e
8 except ValueError, e:
9 print "catch ValueError:", e
10 finally:
11 print "close file"
12 f.close()
运行结果
root@Ubuntu:~/show_me_code/XTeach# python error.py
catch ValueError: invalid literal for int() with base 10: 'qq'
close file
这次运行一下，2.txt不存在。。
1 try:
2 f = open('2.txt')
3 line = f.read(2)
4 num = int(line)
5 print "read num=%d" % num
6 except IOError, e:
7 print "catch IOError:", e
8 except ValueError, e:
9 print "catch ValueError:", e
10 finally:
11 print "close file"
12 f.close()
出现的结果。。捕获到了异常，但是f.close()有异常，因为open('2.txt')异常后，没有对f进行赋值。。
root@Ubuntu:~/show_me_code/XTeach# python error.py
catch IOError: [Errno 2] No such file or directory: '2.txt'
close file
Traceback (most recent call last):
File "error.py", line 12, in <module>
f.close()
NameError: name 'f' is not defined
解决方案，在添加一个异常处理。。
1 try:
2 f = open('2.txt')
3 line = f.read(2)
4 num = int(line)
5 print "read num=%d" % num
6 except IOError, e:
7 print "catch IOError:", e
8 except ValueError, e:
9 print "catch ValueError:", e
10 finally:
11 try:
12 print "close file"
13 f.close()
14 except NameError, e:
15 print "catch Error:", e
对f没有打开情况，进行一个异常处理。。捕获两个错误。。
root@Ubuntu:~/show_me_code/XTeach# python error.py
catch IOError: [Errno 2] No such file or directory: '2.txt'
close file
catch Error: name 'f' is not defined
- try–except–else–finally
1 try:
2 f = open('1.txt')
3 line = f.read(2)
4 num = int(line)
5 print "read num=%d" % num
6 except IOError, e:
7 print "catch IOError:", e
8 except ValueError, e:
9 print "catch ValueError:", e
10 else:
11 print "No Error"
12 finally:
13 try:
14 print "close file"
15 f.close()
16 except NameError, e:
17 print "catch Error:", e
执行结果
root@Ubuntu:~/show_me_code/XTeach# python error.py
read num=11
No Error
close file
- with 语句
with 对一个文件进行操作
实例演习，with和open一个文件
1 try:
2 f = open('1.txt')
3 print "in try f.read():", f.read(2)
4 except IOError, e:
5 print "catch IOError:", e
6 except ValueError, e:
7 print "catch ValueError:", e
8 finally:
9 f.close()
10 print "try-finally:",f.closed
11
12 with open('1.txt') as f1:
13 print "in with f1.read:", f1.read(2)
14
15 print "with :", f1.closed
运行结果，关闭状态。。
root@Ubuntu:~/show_me_code/XTeach# python error.py
in try f.read(): 11
try-finally: True
in with f1.read: 11
with : True
with虽然可以关闭文件，但对关闭文件的错误异常没有处理。。
1 import os
2 try:
3 f = open('1.txt')
4 print "in try f.read():", f.read(2)
5 f.seek(-10,os.SEEK_SET)
6 except IOError, e:
7 print "catch IOError:", e
8 except ValueError, e:
9 print "catch ValueError:", e
10 finally:
11 f.close()
12 print "try-finally:",f.closed
13
14 with open('1.txt') as f1:
15 print "in with f1.read:", f1.read(2)
16 f.seek(-10,os.SEEK_SET)
17
18 print "with :", f1.closed
可以看到运行的异常
root@Ubuntu:~/show_me_code/XTeach# python error.py
in try f.read(): 11
catch IOError: [Errno 22] Invalid argument
try-finally: True
in with f1.read: 11
Traceback (most recent call last):
File "error.py", line 16, in <module>
f.seek(-10,os.SEEK_SET)
ValueError: I/O operation on closed file
虽然with可以关闭文件，但对关闭的异常没有进行处理。。。
import os
try:
f = open('1.txt')
print "in try f.read():", f.read(2)
f.seek(-10,os.SEEK_SET)
except IOError, e:
print "catch IOError:", e
except ValueError, e:
print "catch ValueError:", e
finally:
f.close()
print "try-finally:",f.closed
try:
with open('1.txt') as f1:
print "in with f1.read:", f1.read(2)
f.seek(-10,os.SEEK_SET)
except ValueError, e:
print "in with catch ValueError:", e
print "with :", f1.closed
运行结果。。。
root@Ubuntu:~/show_me_code/XTeach# python error.py
in try f.read(): 11
catch IOError: [Errno 22] Invalid argument
try-finally: True
in with f1.read: 11
in with catch ValueError: I/O operation on closed file
with : True
with语句实质是上下文管理
编写一个类实现with
1 #!/usr/bin/python
2
3 class Mycontex(object):
4 def __init__(self,name):
5 self.name = name
6
7 def __enter__(self):
8 print "__enter__"
9 return self
10
11 def do_self(self):
12 print "do_self"
13
14 def __exit__(self,exc_type,exc_value,traceback):
15 print "__exit__"
16 print "Error:", exc_type, "info:", exc_value
17
18 if __name__ == '__main__':
19 with Mycontex('test context') as f:
20 print f.name
21 f.do_self()
运行结果
root@Ubuntu:~/show_me_code/XTeach# python with.py
__enter__
test context
do_self
__exit__
Error: None info: None
with的使用
- raise 和 assert
assert 语句
- 标准异常和自定义异常
Built-in Exceptions
- 自定义异常
自定义异常示例
In [5]: class FileError(IOError):
...: pass
...:
In [6]: raise FileError, "Test FileError"
---------------------------------------------------------------------------
FileError Traceback (most recent call last)
<ipython-input-6-822ad3f0675d> in <module>()
----> 1 raise FileError, "Test FileError"
FileError: Test FileError
In [7]: try:
...: raise FileError, "Test FileError"
...: except FileError, e:
...: print e
...:
Test FileError
产生异常，抛出异常、捕获异常。。分析异常。。
1 #!/usr/bin/python
2
3 class CustomError(Exception):
4 def __init__(self,info):
5 Exception.__init__(self)
6 self.errorinfo = info
7 print id(self)
8
9 def __str__(self):
10 return "CustionError: %s" % self.errorinfo
11 try:
12 raise CustomError("test CustomError")
13 except CustomError, e:
14 print "ErrorInfo:%d,%s" %(id(e),e)
运行结果
root@Ubuntu:~/show_me_code/XTeach# python except.py
3071267020
ErrorInfo:3071267020,CustionError: test CustomError