__init__ method and raising exceptions

[NavyJay]

I have a simple for-loop, which instantiates a class object each
iteration. As part of my class constructor, __init__(), I check for
valid input settings. If there is a problem with this iteration, I
want to abort the loop, but equivalently 'continue' on in the for-loop.

I can't use 'break' or 'continue' in a class method, nor can I return a
boolean value from __init__() to check for errors within the for-loop.
How would I be able to stop the current iteration and continue with the
next after reporting an error?

Jay

[Leif K-Brooks]

Re: __init__ method and raising exceptions

NavyJay wrote:[color=blue]
> I have a simple for-loop, which instantiates a class object each
> iteration. As part of my class constructor, __init__(), I check for
> valid input settings. If there is a problem with this iteration, I
> want to abort the loop, but equivalently 'continue' on in the for-loop.
>
> I can't use 'break' or 'continue' in a class method, nor can I return a
> boolean value from __init__() to check for errors within the for-loop.
> How would I be able to stop the current iteration and continue with the
> next after reporting an error?[/color]

You have the right idea in the subject header: raise an exception.
Something along the lines of this:

class Foo(object):
def __init__(self, value):
if value > 10:
raise ValueError("Val ue must be under 10, was %s." % value)
else:
self.value = value

for value in [1, 4, 10, 7, 15, 13, 6, 3]:
try:
obj = Foo(value)
except ValueError, ex:
print str(ex)
continue
# Do stuff with obj here

[Vikram]

Re: __init__ method and raising exceptions

> I can't use 'break' or 'continue' in a class method, nor can I return a[color=blue]
> boolean value from __init__() to check for errors within the for-loop.
> How would I be able to stop the current iteration and continue with the
> next after reporting an error?[/color]

maybe i don't fully understand your qn but why don't you let __init__ of
your class raise an exception and then catch it in the outer for loop and
continue. something like:

for i in ...
try:
foo()
except:
continue

Vikram

[NavyJay]

Re: __init__ method and raising exceptions

Exactly the answer I was looking for! 12 hours of straight programming
tends to fog ones mind. Thanks for making it clear!

Jay

[Scott David Daniels]

Re: __init__ method and raising exceptions

Vikram wrote:[color=blue][color=green]
>>I can't use 'break' or 'continue' in a class method, nor can I return a
>>boolean value from __init__() to check for errors within the for-loop.
>>How would I be able to stop the current iteration and continue with the
>>next after reporting an error?[/color]
>
>
> maybe i don't fully understand your qn but why don't you let __init__ of
> your class raise an exception and then catch it in the outer for loop and
> continue. something like:
>
> for i in ...
> try:
> foo()
> except:[/color]
except (ValueError, TypeError), error:[color=blue]
> continue[/color]

Use something like the above, (always expect certain kinds of errors)
lest you accidentally capture an real attempt to stop the program
such as the exception a Control-C causes.

-Scott David Daniels
Scott.Daniels@A cm.Org

[NavyJay]

Re: __init__ method and raising exceptions

Or better yet, define your own string/class exception to catch your
errors. In my code, things can break in more than a few ways. In each
case I catch the exception(s) specific to that piece of code, print a
warning message to the user at sys.stdout and raise a new exception to
be caught by my "wrapper" exception clause.