95.html ( File view )

  • By 2010-08-21
  • View(s):12
  • Download(s):0
  • Point(s): 1
			






Safari | Python Developer's Handbook -> Raising Exceptions





< BACKMake Note | BookmarkCONTINUE >
152015024128143245168232148039199167010047123209178152124239215162147037129068180064019224

Raising Exceptions

There are several ways to raise exceptions. You can either raise your own exceptions or Python standard exceptions by using any of the four techniques listed as follows:

  • raise class

  • raise exception, argument

  • raise exception, (argument1, argument2, )

  • raise exception (argument1, argument2, )

Note that the second and third forms of raising exceptions use the old form of passing arguments with the exception. I recommended using only the first and fourth forms.

Passing None, as the second argument, to the raise statement is equivalent to omitting it.

raise class, None is equivalent to raise class()

Check the following cases.

					
raise IndexError()
raise IndexError
raise IndexError("x is out of range")
raise IndexError, "x is out of range"

				

In the previous lines, the examples use a standard exception called IndexError. However, you can raise any one of the supported built-in exceptions.

Look at another example that uses a different exception:

					
op = raw_input("Enter an operator: ")
op1 = input("Enter first operand: ")
op2 = input("Enter second operand: ")
if op == "+":
     print op1 + op2
else:
     raise RuntimeError("I don't know this command")

				

In the next chapter, after learning how you can handle classes, you will be able to easily understand this next example. For the present time, take a deep breath and just have some fun.

This example raises an exception that blocks your access to nonexistent members of the c class.

					
 1: >>> class c:
 2:        def __init__(self, name):
 3:                self.name = name
 4:        def __getattr__(self, attr):
 5:                if attr <> "name":
 6:                        raise AttributeError
 7: 
 8: >>> a = c("Andre")
 9: >>> a.name
10: 'Andre'
11: >>> a.age

				

The following traceback message is generated after running the command located at line 11.

					
Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 6, in __getattr__
AttributeError

				

As you can see, line 5 checks the name of the attribute that is being passed to the method. That makes the exception in line 6 to always be raised when the attribute name is not "name".

However, note that if you assign something to a.age, as demonstrated next, getting the value of a.age will no longer cause the error. To handle that, you would need to write a code to deal with the __setattr__ method, but that would be another example.

					
>>> a.age = 32
>>> print a.age
32

				

Raising an Exception to Leave the Interpreter

Raising the SystemExit exception is a generic way to leave the Python interpreter.

						
C:\Program Files\Python>python
Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> raise SystemExit
C:\Program Files\Python>

					

The next example demonstrates how you can trap the SystemExit exception.

						
>>> try:
       raise SystemExit
   except SystemExit:
       print "Sorry. You can not leave."

Sorry. You can not leave.

					

The sys.exit() function raises an exception SystemExit that, if not caught, causes the thread to exit silently.

						
>>> import sys
>>> try:
       sys.exit()
   except SystemExit:
       print "I have already told you. You can not leave."

I have already told you. You can not leave. 

					

Raising an Exception to Leave Nested Loops

Sometimes you are so deeply involved in your data structures that you only want to get out of all your nested loops quickly. Normally, you would have to use break for each level of interaction. The next example demonstrates how to handle this situation by using exceptions.

						
>>> ExitLoop = "ExitLoop"
>>> try:
       i=1
       while i < 10:
               for j in xrange(1,5):
                       print i,j
                       if (i==2) and (j==3):
                               raise ExitLoop
               i = i + 1
   except ExitLoop:
       print "i=2 and j=3 is a special case."

1 1
1 2
1 3
1 4
2 1
2 2
2 3
i=2 and j=3 is a special case. 

					

Raising String Exceptions

Older versions used to support only strings for both Python standard exceptions and user-defined exceptions.

						
>>> NetworkError = "NetworkError"
>>> raise NetworkError, "Bad hostname"

					

Nowadays, Python supports both strings and exception classes. There are costs to using class exceptions because they must be instantiated to be caught. Note that most people don't use exceptions to control the flow of their program, so they don't occur much.

However, classes give you much more flexibility to generalize the type of error that you want to catch.

Tip

Try to define your own exceptions as classes instead of strings.



Instancing an Exception Class

Every time an exception is raised, an instance of the exception class is created. The next syntax demonstrates how to catch a class instance in your program.

						
try:
    <statements>
except exception, instance:
    <statements>

					

The instance variable is an instance of the raised exception. Therefore, it inherits attributes from the exception class.

Each instance has an attribute called args that returns the error string in a tuple format.

						
>>> try:
       a = [1,2]
       print a[4]
   except IndexError, b:
       print b.args

('list index out of range',)

					

Particularly, the EnvironmentError exception has a 2-tuple or 3-tuple structure that can be translated as (error number, string error message, and a ... ... (Not finished, please download and read the complete file)

...
Expand> <Close

Want complete source code? Download it here

Point(s): 1

Download
0 lines left, continue to read
Sponsored links

File list

Tips: You can preview the content of files by clicking file names^_^
Name Size Date
0672319942.html3.37 kB01-06-02 20:06
1.html5.10 kB01-06-02 20:06
10.html3.69 kB01-06-02 20:06
100.html5.41 kB01-06-02 20:06
101.html7.96 kB01-06-02 20:06
102.html3.75 kB01-06-02 20:06
103.html3.75 kB01-06-02 20:06
104.html5.81 kB01-06-02 20:06
105.html16.46 kB01-06-02 20:06
106.html25.87 kB01-06-02 20:06
107.html7.44 kB01-06-02 20:06
108.html20.95 kB01-06-02 20:06
109.html10.02 kB01-06-02 20:06
11.html3.66 kB01-06-02 20:06
110.html9.58 kB01-06-02 20:06
111.html9.96 kB01-06-02 20:06
112.html11.34 kB01-06-02 20:06
113.html7.87 kB01-06-02 20:06
114.html13.56 kB01-06-02 20:06
115.html3.76 kB01-06-02 20:06
116.html3.76 kB01-06-02 20:06
117.html4.27 kB01-06-02 20:06
118.html4.27 kB01-06-02 20:06
119.html9.45 kB01-06-02 20:06
12.html3.66 kB01-06-02 20:06
120.html8.62 kB01-06-02 20:06
121.html65.99 kB01-06-02 20:06
122.html28.42 kB01-06-02 20:06
123.html14.92 kB01-06-02 20:06
124.html7.17 kB01-06-02 20:06
125.html24.21 kB01-06-02 20:06
126.html10.82 kB01-06-02 20:06
127.html10.54 kB01-06-02 20:06
128.html3.86 kB01-06-02 20:06
129.html3.86 kB01-06-02 20:06
13.html12.82 kB01-06-02 20:06
130.html5.85 kB01-06-02 20:06
131.html5.30 kB01-06-02 20:06
132.html29.95 kB01-06-02 20:06
133.html61.54 kB01-06-02 20:06
134.html40.34 kB01-06-02 20:06
135.html9.11 kB01-06-02 20:06
136.html15.53 kB01-06-02 20:06
137.html3.86 kB01-06-02 20:06
138.html3.86 kB01-06-02 20:06
139.html5.80 kB01-06-02 20:06
14.html11.79 kB01-06-02 20:06
140.html14.32 kB01-06-02 20:06
141.html26.46 kB01-06-02 20:06
142.html21.93 kB01-06-02 20:06
143.html17.03 kB01-06-02 20:06
144.html7.95 kB01-06-02 20:06
145.html28.59 kB01-06-02 20:06
146.html52.14 kB01-06-02 20:06
147.html8.37 kB01-06-02 20:06
148.html3.63 kB01-06-02 20:06
149.html3.63 kB01-06-02 20:06
15.html10.93 kB01-06-02 20:06
150.html4.42 kB01-06-02 20:06
151.html16.23 kB01-06-02 20:06
152.html32.55 kB01-06-02 20:06
153.html13.13 kB01-06-02 20:06
154.html28.33 kB01-06-02 20:06
155.html40.15 kB01-06-02 20:06
156.html23.47 kB01-06-02 20:06
157.html7.73 kB01-06-02 20:06
158.html10.61 kB01-06-02 20:06
159.html3.72 kB01-06-02 20:06
16.html11.42 kB01-06-02 20:06
160.html3.72 kB01-06-02 20:06
161.html3.64 kB01-06-02 20:06
162.html3.64 kB01-06-02 20:06
163.html4.70 kB01-06-02 20:06
164.html60.18 kB01-06-02 20:06
165.html42.25 kB01-06-02 20:06
166.html17.91 kB01-06-02 20:06
167.html9.76 kB01-06-02 20:06
168.html13.52 kB01-06-02 20:06
169.html10.35 kB01-06-02 20:06
17.html17.40 kB01-06-02 20:06
170.html9.08 kB01-06-02 20:06
171.html3.61 kB01-06-02 20:06
172.html3.61 kB01-06-02 20:06
173.html6.95 kB01-06-02 20:06
174.html27.86 kB01-06-02 20:06
175.html28.55 kB01-06-02 20:06
176.html16.39 kB01-06-02 20:06
177.html24.60 kB01-06-02 20:06
178.html10.20 kB01-06-02 20:06
179.html3.62 kB01-06-02 20:06
18.html12.04 kB01-06-02 20:06
180.html3.62 kB01-06-02 20:06
181.html7.21 kB01-06-02 20:06
182.html11.83 kB01-06-02 20:06
183.html17.37 kB01-06-02 20:06
184.html87.57 kB01-06-02 20:06
185.html25.23 kB01-06-02 20:06
186.html6.62 kB01-06-02 20:06
187.html3.73 kB01-06-02 20:06
188.html3.73 kB01-06-02 20:06
189.html4.76 kB01-06-02 20:06
19.html5.49 kB01-06-02 20:06
190.html74.34 kB01-06-02 20:06
191.html9.56 kB01-06-02 20:06
192.html31.86 kB01-06-02 20:06
193.html67.73 kB01-06-02 20:06
194.html69.48 kB01-06-02 20:06
195.html32.75 kB01-06-02 20:06
196.html10.74 kB01-06-02 20:06
197.html3.54 kB01-06-02 20:06
198.html3.54 kB01-06-02 20:06
199.html3.81 kB01-06-02 20:06
2.html5.10 kB01-06-02 20:06
20.html9.25 kB01-06-02 20:06
200.html3.81 kB01-06-02 20:06
201.html8.82 kB01-06-02 20:06
202.html7.07 kB01-06-02 20:06
203.html50.32 kB01-06-02 20:06
204.html8.07 kB01-06-02 20:06
205.html7.53 kB01-06-02 20:06
206.html3.60 kB01-06-02 20:06
207.html3.60 kB01-06-02 20:06
208.html7.22 kB01-06-02 20:06
209.html21.63 kB01-06-02 20:06
21.html5.84 kB01-06-02 20:06
210.html24.67 kB01-06-02 20:06
211.html30.17 kB01-06-02 20:06
212.html159.15 kB01-06-02 20:06
213.html18.72 kB01-06-02 20:06
214.html6.54 kB01-06-02 20:06
215.html7.22 kB01-06-02 20:06
216.html6.76 kB01-06-02 20:06
217.html3.11 kB01-06-02 20:06
218.html3.54 kB01-06-02 20:06
219.html4.14 kB01-06-02 20:06
22.html3.69 kB01-06-02 20:06
220.html4.14 kB01-06-02 20:06
221.html4.82 kB01-06-02 20:06
222.html50.92 kB01-06-02 20:06
223.html3.87 kB01-06-02 20:06
224.html57.67 kB01-06-02 20:06
225.html37.66 kB01-06-02 20:06
226.html5.04 kB01-06-02 20:06
227.html3.85 kB01-06-02 20:06
228.html3.85 kB01-06-02 20:06
229.html4.47 kB01-06-02 20:06
23.html3.69 kB01-06-02 20:06
230.html21.41 kB01-06-02 20:06
231.html19.56 kB01-06-02 20:06
232.html26.27 kB01-06-02 20:06
233.html10.07 kB01-06-02 20:06
234.html22.22 kB01-06-02 20:06
235.html36.83 kB01-06-02 20:06
236.html49.23 kB01-06-02 20:06
237.html16.63 kB01-06-02 20:06
238.html6.96 kB01-06-02 20:06
239.html3.09 kB01-06-02 20:06
24.html4.86 kB01-06-02 20:06
240.html3.43 kB01-06-02 20:06
241.html3.59 kB01-06-02 20:06
242.html3.59 kB01-06-02 20:06
243.html17.49 kB01-06-02 20:06
244.html9.38 kB01-06-02 20:06
245.html16.62 kB01-06-02 20:06
246.html9.81 kB01-06-02 20:06
247.html11.50 kB01-06-02 20:06
248.html8.95 kB01-06-02 20:06
249.html8.93 kB01-06-02 20:06
25.html11.48 kB01-06-02 20:06
250.html10.89 kB01-06-02 20:06
251.html8.21 kB01-06-02 20:06
252.html5.14 kB01-06-02 20:06
253.html3.61 kB01-06-02 20:06
254.html3.61 kB01-06-02 20:06
255.html4.61 kB01-06-02 20:06
256.html4.61 kB01-06-02 20:06
257.html43.07 kB01-06-02 20:06
258.html10.12 kB01-06-02 20:06
259.html7.99 kB01-06-02 20:06
26.html25.88 kB01-06-02 20:06
260.html17.17 kB01-06-02 20:06
261.html10.99 kB01-06-02 20:06
262.html15.70 kB01-06-02 20:06
263.html36.19 kB01-06-02 20:06
264.html61.93 kB01-06-02 20:06
265.html39.81 kB01-06-02 20:06
266.html15.13 kB01-06-02 20:06
267.html16.34 kB01-06-02 20:06
268.html3.55 kB01-06-02 20:06
269.html3.55 kB01-06-02 20:06
27.html27.92 kB01-06-02 20:06
270.html14.30 kB01-06-02 20:06
271.html14.21 kB01-06-02 20:06
272.html9.44 kB01-06-02 20:06
273.html8.41 kB01-06-02 20:06
274.html5.45 kB01-06-02 20:06
275.html5.45 kB01-06-02 20:06
276.html8.08 kB01-06-02 20:06
277.html9.78 kB01-06-02 20:06
278.html6.04 kB01-06-02 20:06
279.html6.89 kB01-06-02 20:06
28.html12.02 kB01-06-02 20:06
280.html11.77 kB01-06-02 20:06
281.html10.18 kB01-06-02 20:06
282.html4.98 kB01-06-02 20:06
283.html4.98 kB01-06-02 20:06
284.html4.12 kB01-06-02 20:06
285.html5.78 kB01-06-02 20:06
286.html12.42 kB01-06-02 20:06
287.html6.87 kB01-06-02 20:06
29.html45.69 kB01-06-02 20:06
3.html5.51 kB01-06-02 20:06
30.html10.03 kB01-06-02 20:06
31.html23.06 kB01-06-02 20:06
32.html22.26 kB01-06-02 20:06
33.html18.88 kB01-06-02 20:06
34.html18.25 kB01-06-02 20:06
35.html15.70 kB01-06-02 20:06
36.html5.50 kB01-06-02 20:06
37.html11.83 kB01-06-02 20:06
38.html3.69 kB01-06-02 20:06
39.html3.69 kB01-06-02 20:06
4.html5.51 kB01-06-02 20:06
40.html8.24 kB01-06-02 20:06
41.html12.92 kB01-06-02 20:06
42.html4.51 kB01-06-02 20:06
43.html3.90 kB01-06-02 20:06
44.html3.52 kB01-06-02 20:06
45.html5.99 kB01-06-02 20:06
46.html4.33 kB01-06-02 20:06
47.html4.19 kB01-06-02 20:06
48.html4.01 kB01-06-02 20:06
49.html3.95 kB01-06-02 20:06
5.html4.69 kB01-06-02 20:06
50.html3.63 kB01-06-02 20:06
51.html3.75 kB01-06-02 20:06
52.html5.56 kB01-06-02 20:06
53.html4.41 kB01-06-02 20:06
54.html6.99 kB01-06-02 20:06
55.html3.52 kB01-06-02 20:06
56.html3.95 kB01-06-02 20:06
57.html3.69 kB01-06-02 20:06
58.html4.27 kB01-06-02 20:06
59.html3.88 kB01-06-02 20:06
6.html4.69 kB01-06-02 20:06
60.html3.58 kB01-06-02 20:06
61.html4.17 kB01-06-02 20:06
62.html3.93 kB01-06-02 20:06
63.html3.68 kB01-06-02 20:06
64.html4.16 kB01-06-02 20:06
65.html4.04 kB01-06-02 20:06
66.html4.97 kB01-06-02 20:06
67.html3.91 kB01-06-02 20:06
68.html3.55 kB01-06-02 20:06
69.html3.51 kB01-06-02 20:06
7.html10.67 kB01-06-02 20:06
70.html3.52 kB01-06-02 20:06
71.html3.83 kB01-06-02 20:06
72.html4.02 kB01-06-02 20:06
73.html21.61 kB01-06-02 20:06
74.html12.94 kB01-06-02 20:06
75.html31.26 kB01-06-02 20:06
76.html11.98 kB01-06-02 20:06
77.html4.24 kB01-06-02 20:06
78.html4.26 kB01-06-02 20:06
79.html10.95 kB01-06-02 20:06
8.html10.67 kB01-06-02 20:06
80.html12.11 kB01-06-02 20:06
81.html4.88 kB01-06-02 20:06
82.html6.48 kB01-06-02 20:06
83.html5.64 kB01-06-02 20:06
84.html13.30 kB01-06-02 20:06
85.html7.00 kB01-06-02 20:06
86.html3.90 kB01-06-02 20:06
87.html4.59 kB01-06-02 20:06
88.html5.00 kB01-06-02 20:06
89.html17.71 kB01-06-02 20:06
9.html3.69 kB01-06-02 20:06
90.html6.00 kB01-06-02 20:06
91.html3.68 kB01-06-02 20:06
92.html3.68 kB01-06-02 20:06
93.html12.98 kB01-06-02 20:06
94.html10.60 kB01-06-02 20:06
95.html19.93 kB01-06-02 20:06
96.html10.34 kB01-06-02 20:06
97.html6.27 kB01-06-02 20:06
98.html6.77 kB01-06-02 20:06
99.html16.97 kB01-06-02 20:06
front_matter.html4.16 kB01-06-02 20:06
index.html3.37 kB01-06-02 20:06
new_toc.html15.79 kB01-06-02 20:06
rindex1.html53.02 kB01-06-02 20:05
rindex10.html5.16 kB01-06-02 20:05
rindex11.html3.96 kB01-06-02 20:05
rindex12.html15.07 kB01-06-02 20:05
rindex13.html84.43 kB01-06-02 20:05
rindex14.html10.85 kB01-06-02 20:05
rindex15.html26.04 kB01-06-02 20:05
rindex16.html58.33 kB01-06-02 20:05
rindex17.html3.52 kB01-06-02 20:05
rindex18.html16.83 kB01-06-02 20:05
rindex19.html52.89 kB01-06-02 20:05
rindex2.html13.47 kB01-06-02 20:05
rindex20.html17.55 kB01-06-02 20:05
rindex21.html13.56 kB01-06-02 20:05
rindex22.html10.63 kB01-06-02 20:05
rindex23.html16.92 kB01-06-02 20:05
rindex24.html4.90 kB01-06-02 20:05
rindex25.html2.50 kB01-06-02 20:05
rindex3.html43.59 kB01-06-02 20:06
rindex4.html21.76 kB01-06-02 20:06
rindex5.html22.31 kB01-06-02 20:06
rindex6.html36.57 kB01-06-02 20:06
rindex7.html14.06 kB01-06-02 20:06
rindex8.html11.93 kB01-06-02 20:06
rindex9.html25.33 kB01-06-02 20:06
toc.html24.97 kB01-06-02 20:06
<(ebook>0.00 BPython) O'Reilly
<0672319942>0.00 B25-08-02 17:24
<graphics>0.00 B25-08-02 17:24
<images>0.00 B25-08-02 17:24
<oreillyi>0.00 B25-08-02 17:24
oreillyM.css4.42 kB31-05-02 17:16
oreillyN.css4.42 kB31-05-02 17:16
00.gif41.00 B31-05-02 17:16
01fig01.gif58.03 kB28-01-02 08:54
01fig02.gif39.76 kB28-01-02 08:54
01fig03.gif22.06 kB28-01-02 08:54
01fig04.gif64.75 kB28-01-02 08:54
02fig01.gif54.48 kB28-01-02 08:54
02fig02.gif16.29 kB28-01-02 08:54
02fig03.gif14.11 kB28-01-02 08:54
02fig04.gif47.82 kB28-01-02 08:54
06fig01.gif32.19 kB28-01-02 08:54
06fig02.gif58.39 kB28-01-02 08:54
06fig03.gif48.96 kB28-01-02 08:54
07fig01.gif47.96 kB28-01-02 08:54
07fig02.gif8.99 kB28-01-02 08:54
07fig03.gif9.46 kB28-01-02 08:54
07fig04.gif11.51 kB28-01-02 08:54
07fig05.gif8.27 kB28-01-02 08:54
07fig06.gif31.46 kB28-01-02 08:54
12fig01.gif34.54 kB28-01-02 08:54
15fig01.gif1.30 kB28-01-02 08:54
15fig02.gif1.50 kB28-01-02 08:54
15fig03.gif5.93 kB28-01-02 08:54
15fig04.gif924.00 B28-01-02 08:54
15fig05.gif1.55 kB28-01-02 08:54
15fig06.gif1.62 kB28-01-02 08:54
15fig07.gif1.69 kB28-01-02 08:54
15fig08.gif1.02 kB28-01-02 08:54
15fig09.gif2.54 kB28-01-02 08:54
15fig10.gif1.92 kB28-01-02 08:54
15fig11.gif2.44 kB28-01-02 08:54
15fig12.gif2.48 kB28-01-02 08:54
15fig13.gif2.36 kB28-01-02 08:54
15fig14.gif5.43 kB28-01-02 08:54
15fig15.gif5.51 kB28-01-02 08:54
15fig16.gif2.18 kB28-01-02 08:54
15fig17.gif12.22 kB28-01-02 08:54
16fig01.gif28.99 kB28-01-02 08:54
16fig02.gif23.02 kB28-01-02 08:54
16fig03.gif32.48 kB28-01-02 08:54
16fig04.gif17.27 kB28-01-02 08:54
16fig05.gif23.88 kB28-01-02 08:54
16fig06.gif23.85 kB28-01-02 08:54
16fig07.gif36.76 kB28-01-02 08:54
16fig08.gif20.66 kB28-01-02 08:54
16fig09.gif71.63 kB28-01-02 08:54
16fig10.gif23.23 kB28-01-02 08:54
16fig11.gif22.97 kB28-01-02 08:54
16fig12.gif22.06 kB28-01-02 08:54
16fig13.gif49.73 kB28-01-02 08:54
16fig14.gif60.34 kB28-01-02 08:54
18fig01.gif63.94 kB28-01-02 08:54
18fig02.gif24.07 kB28-01-02 08:54
ccc.gif109.00 B28-01-02 08:54
spacer.gif41.00 B31-05-02 17:16
view.gif41.00 B31-05-02 17:16
0672319942_s.jpg2.91 kB29-01-02 11:29
...
Sponsored links
  • Sent successfully!
  • 1 point

95.html (1.65 MB)

Need 1 point
Your Point(s)

Your Point isn't enough.

Get point immediately by PayPal

More(Debit card / Credit card / PayPal Credit / Online Banking)

Submit your source codes. Get more point

LOGIN

Don't have an account? Register now
Need any help?
Mail to: support@codeforge.com

切换到中文版?

Where are you going?

^_^"Oops ...

Sorry!This guy is mysterious, its blog hasn't been opened, try another, please!
OK

Warm tip!

CodeForge to FavoriteFavorite by Ctrl+D