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comp.lang.ruby

Help begin/rescue/ensure

S Kanakakorn

2/9/2007 5:21:00 AM

I have this simple code.

require 'rubygems'
require 'net/ssh'

begin
session = Net::SSH.start('aaaa-lnx', :password=>'xyz',
:username=>'root',:timeout=>5)
rescue
puts 'cannot connect '
ensure
puts 'reach ensure point'
end
puts 'at last'

--------------------------------------

If I put in non existing host name (aaaa-lnx), then the statement
"puts 'at last'" won't print out.
The ensure blocks work fine. Somehow, the "rescue" block can't catch
the error. This is my output.

reach ensure point
/Applications/Locomotive2/Bundles/standardRailsJan2007.locobundle/i386/lib/ruby/1.8/timeout.rb:54:in
`open': execution expired (Timeout::Error)
from /Applications/Locomotive2/Bundles/standardRailsJan2007.locobundle/i386/lib/ruby/gems/1.8/gems/net-ssh-1.0.10/lib/net/ssh/transport/session.rb:88:in
`initialize'

If I put in a good hostname with wrong password, the "rescue" block
will catch the error just fine. Here is the output:

cannot connect
reach ensure point
at last


What did I do wrong ?

Thanks,
-Nick
4 Answers

Brian Candler

2/9/2007 9:07:00 AM

0

On Fri, Feb 09, 2007 at 02:20:30PM +0900, S Kanakakorn wrote:
> require 'rubygems'
> require 'net/ssh'
>
> begin
> session = Net::SSH.start('aaaa-lnx', :password=>'xyz',
> :username=>'root',:timeout=>5)
> rescue
> puts 'cannot connect '
> ensure
> puts 'reach ensure point'
> end
> puts 'at last'
...
> What did I do wrong ?

A bare "rescue" doesn't catch all exceptions, only those which are
subclasses of StandardError. Change to:

rescue Exception

HTH,

Brian.

Chris Hulan

2/9/2007 9:17:00 AM

0

On Feb 9, 12:20 am, "S Kanakakorn" <configt...@gmail.com> wrote:
> I have this simple code.
>
...
> rescue
> puts 'cannot connect '
> ensure
> puts 'reach ensure point'
> end
> puts 'at last'
>
> --------------------------------------
>
> If I put in non existing host name (aaaa-lnx), then the statement
> "puts 'at last'" won't print out.
> The ensure blocks work fine. Somehow, the "rescue" block can't catch
> the error. This is my output.
>
> reach ensure point
> /Applications/Locomotive2/Bundles/standardRailsJan2007.locobundle/i386/lib/ruby/1.8/timeout.rb:54:in
> `open': execution expired (Timeout::Error)
> from /Applications/Locomotive2/Bundles/standardRailsJan2007.locobundle/i386/lib/ruby/gems/1.8/gems/net-ssh-1.0.10/lib/net/ssh/transport/session.rb:88:in
> `initialize'
>
> If I put in a good hostname with wrong password, the "rescue" block
> will catch the error just fine. Here is the output:
>
> cannot connect
> reach ensure point
> at last
>
> What did I do wrong ?


rescue without a specified exception defaults to StandardError.
Tiimeout::Error is a subclass of SignalException, which is a sibling
of StandardError, not a subclass. Thus that exception doesn't match
your rescue, so the exception gets passed up the call stack, the
ensure
gets executed, but the code after it is bypassed.

For a picture of the exception class hierarchy see http://
www.insula.cz/dali/material/rubycl/RubyExceptionClasses.jpg

cheers
Chris

Daniel Berger

2/9/2007 4:46:00 PM

0



On Feb 9, 2:06 am, Brian Candler <B.Cand...@pobox.com> wrote:
> On Fri, Feb 09, 2007 at 02:20:30PM +0900, S Kanakakorn wrote:
> > require 'rubygems'
> > require 'net/ssh'
>
> > begin
> > session = Net::SSH.start('aaaa-lnx', :password=>'xyz',
> > :username=>'root',:timeout=>5)
> > rescue
> > puts 'cannot connect '
> > ensure
> > puts 'reach ensure point'
> > end
> > puts 'at last'
> ...
> > What did I do wrong ?
>
> A bare "rescue" doesn't catch all exceptions, only those which are
> subclasses of StandardError. Change to:
>
> rescue Exception

Given how often this issue appears on the mailing list, I'm beginning
to think that a bare rescue should just rescue Exception. That seems
to be what most people expect it to do.

Is there a downside to this idea?

Dan

Brian Candler

2/9/2007 9:15:00 PM

0

On Sat, Feb 10, 2007 at 01:45:54AM +0900, Daniel Berger wrote:
> > A bare "rescue" doesn't catch all exceptions, only those which are
> > subclasses of StandardError. Change to:
> >
> > rescue Exception
>
> Given how often this issue appears on the mailing list, I'm beginning
> to think that a bare rescue should just rescue Exception. That seems
> to be what most people expect it to do.
>
> Is there a downside to this idea?

Well, it would be very confusing if a rescue intended to catch an
invalid-data sort of error also caught an invalid program error (such as a
"method missing" exception caused by a typo in a method name). At best,
these would end up finding their way into a log somewhere. At worst, they
would be silently ignored, making debugging very difficult.