My code:
class AError(Exception):
print 'error occur'
for i in range(3):
try:
print '---oo'
raise AError
except AError:
print 'get AError'
else:
print 'going on'
finally:
print 'finally'
When I run the above code, the output is this:
error occur
---oo
get AError
finally
---oo
get AError
finally
---oo
get AError
finally
I think the string "error occur" should occur three times, like "---oo", but it only occurs once; why?
To clarify Paul's answer, here's a simple example:
class Test(object):
print "Class being defined"
def __init__(self):
print "Instance being created"
for _ in range(3):
t = Test()
The output from this will be:
Class being defined
Instance being created
Instance being created
Instance being created
Code within the class definition but outside a method definition runs only once, when the class is defined.
If you want code to run whenever an instance is created, it should be in the __init__ instance method (or, occasionally, the __new__ class method). However, note that if you define __init__ for a subclass, you should probably ensure that it also calls the superclass's __init__:
class AError(Exception):
def __init__(self, *args, **kwargs):
Exception.__init__(self, *args, **kwargs) # call the superclass
print 'error occur' # print your message
This ensures that the subclass supports the arguments for the superclass; in the case of Exception, you can e.g. pass an error message:
>>> raise AError("Something went wrong.")
error occur # your message gets printed when the instance is created
Traceback (most recent call last):
File "<pyshell#11>", line 1, in <module>
raise AError("Something went wrong.")
AError: Something went wrong. # the error message passes through to the traceback
For an explanation of the *args, **kwargs syntax, if you aren't familiar with it, see e.g. What does ** (double star) and * (star) do for parameters?. You can also use super to call the superclass methods, see e.g. Understanding Python super() with __init__() methods.