Question
Ryan Davis’s Ruby QuickRef says (without explanation):
Don’t rescue Exception. EVER. or I will stab you.
Why not? What’s the right thing to do?
Answer
TL;DR : Use StandardError
instead for general exception catching. When
the original exception is re-raised (e.g. when rescuing to log the exception
only), rescuing Exception
is probably okay.
Exception
is the root of Ruby's exception
hierarchy,
so when you rescue Exception
you rescue from everything , including
subclasses such as SyntaxError
, LoadError
, and Interrupt
.
Rescuing Interrupt
prevents the user from using CTRL``C
to exit the
program.
Rescuing SignalException
prevents the program from responding correctly to
signals. It will be unkillable except by kill -9
.
Rescuing SyntaxError
means that eval
s that fail will do so silently.
All of these can be shown by running this program, and trying to CTRL``C
or
kill
it:
loop do
begin
sleep 1
eval "djsakru3924r9eiuorwju3498 += 5u84fior8u8t4ruyf8ihiure"
rescue Exception
puts "I refuse to fail or be stopped!"
end
end
Rescuing from Exception
isn't even the default. Doing
begin
# iceberg!
rescue
# lifeboats
end
does not rescue from Exception
, it rescues from StandardError
. You should
generally specify something more specific than the default StandardError
,
but rescuing from Exception
broadens the scope rather than narrowing it,
and can have catastrophic results and make bug-hunting extremely difficult.
If you have a situation where you do want to rescue from StandardError
and
you need a variable with the exception, you can use this form:
begin
# iceberg!
rescue => e
# lifeboats
end
which is equivalent to:
begin
# iceberg!
rescue StandardError => e
# lifeboats
end
One of the few common cases where it’s sane to rescue from Exception
is for
logging/reporting purposes, in which case you should immediately re-raise the
exception:
begin
# iceberg?
rescue Exception => e
# do some logging
raise # not enough lifeboats ;)
end