Why is it bad style to `rescue Exception => e` in Ruby?

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 CTRLC 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 evals that fail will do so silently.

All of these can be shown by running this program, and trying to CTRLC or kill it:

loop do  begin    sleep 1    eval "djsakru3924r9eiuorwju3498 += 5u84fior8u8t4ruyf8ihiure"  rescue Exception    puts "I refuse to fail or be stopped!"  endend

Rescuing from Exception isn't even the default. Doing

begin  # iceberg!rescue  # lifeboatsend

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  # lifeboatsend

which is equivalent to:

begin  # iceberg!rescue StandardError => e  # lifeboatsend

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

The real rule is: Don't throw away exceptions. The objectivity of the author of your quote is questionable, as evidenced by the fact that it ends with

or I will stab you

Of course, be aware that signals (by default) throw exceptions, and normally long-running processes are terminated through a signal, so catching Exception and not terminating on signal exceptions will make your program very hard to stop. So don't do this:

#! /usr/bin/rubywhile true do  begin    line = STDIN.gets    # heavy processing  rescue Exception => e    puts "caught exception #{e}! ohnoes!"  endend

No, really, don't do it. Don't even run that to see if it works.

However, say you have a threaded server and you want all exceptions to not:

1. be ignored (the default)
2. stop the server (which happens if you say thread.abort_on_exception = true).

Then this is perfectly acceptable in your connection handling thread:

begin  # do stuffrescue Exception => e  myLogger.error("uncaught #{e} exception while handling connection: #{e.message}")    myLogger.error("Stack trace: #{backtrace.map {|l| "  #{l}\n"}.join}")end

The above works out to a variation of Ruby's default exception handler, with the advantage that it doesn't also kill your program. Rails does this in its request handler.

Signal exceptions are raised in the main thread. Background threads won't get them, so there is no point in trying to catch them there.

This is particularly useful in a production environment, where you do not want your program to simply stop whenever something goes wrong. Then you can take the stack dumps in your logs and add to your code to deal with specific exception further down the call chain and in a more graceful manner.

Note also that there is another Ruby idiom which has much the same effect:

a = do_something rescue "something else"

In this line, if do_something raises an exception, it is caught by Ruby, thrown away, and a is assigned "something else".

Generally, don't do that, except in special cases where you know you don't need to worry. One example:

debugger rescue nil

The debugger function is a rather nice way to set a breakpoint in your code, but if running outside a debugger, and Rails, it raises an exception. Now theoretically you shouldn't be leaving debug code lying around in your program (pff! nobody does that!) but you might want to keep it there for a while for some reason, but not continually run your debugger.

Note:

1. If you've run someone else's program that catches signal exceptions and ignores them, (say the code above) then:

   • in Linux, in a shell, type pgrep ruby, or ps | grep ruby, look for your offending program's PID, and then run kill -9 <PID>.
   • in Windows, use the Task Manager (CTRL-SHIFT-ESC), go to the "processes" tab, find your process, right click it and select "End process".

2. If you are working with someone else's program which is, for whatever reason, peppered with these ignore-exception blocks, then putting this at the top of the mainline is one possible cop-out:

   %W/INT QUIT TERM/.each { |sig| trap sig,"SYSTEM_DEFAULT" }

   This causes the program to respond to the normal termination signals by immediately terminating, bypassing exception handlers, with no cleanup. So it could cause data loss or similar. Be careful!

3. If you need to do this:

   begin  do_somethingrescue Exception => e  critical_cleanup  raiseend

   you can actually do this:

   begin  do_somethingensure  critical_cleanupend

   In the second case, critical cleanup will be called every time, whether or not an exception is thrown.

TL;DR

Don't rescue Exception => e (and not re-raise the exception) - or you might drive off a bridge.

Let's say you are in a car (running Ruby). You recently installed a new steering wheel with the over-the-air upgrade system (which uses eval), but you didn't know one of the programmers messed up on syntax.

You are on a bridge, and realize you are going a bit towards the railing, so you turn left.

def turn_left  self.turn left:end

oops! That's probably Not Good™, luckily, Ruby raises a SyntaxError.

The car should stop immediately - right?

Nope.

begin  #...  eval self.steering_wheel  #...rescue Exception => e  self.beep  self.log "Caught #{e}.", :warn  self.log "Logged Error - Continuing Process.", :infoend

beep beep

Warning: Caught SyntaxError Exception.

Info: Logged Error - Continuing Process.

You notice something is wrong, and you slam on the emergency breaks (^C: Interrupt)

beep beep

Warning: Caught Interrupt Exception.

Info: Logged Error - Continuing Process.

Yeah - that didn't help much. You're pretty close to the rail, so you put the car in park (killing: SignalException).

beep beep

Warning: Caught SignalException Exception.

Info: Logged Error - Continuing Process.

At the last second, you pull out the keys (kill -9), and the car stops, you slam forward into the steering wheel (the airbag can't inflate because you didn't gracefully stop the program - you terminated it), and the computer in the back of your car slams into the seat in front of it. A half-full can of Coke spills over the papers. The groceries in the back are crushed, and most are covered in egg yolk and milk. The car needs serious repair and cleaning. (Data Loss)

Hopefully you have insurance (Backups). Oh yeah - because the airbag didn't inflate, you're probably hurt (getting fired, etc).

But wait! There's more reasons why you might want to use rescue Exception => e!

Let's say you're that car, and you want to make sure the airbag inflates if the car is exceeding its safe stopping momentum.

 begin     # do driving stuff rescue Exception => e    self.airbags.inflate if self.exceeding_safe_stopping_momentum?    raise end

Here's the exception to the rule: You can catch Exception only if you re-raise the exception. So, a better rule is to never swallow Exception, and always re-raise the error.

But adding rescue is both easy to forget in a language like Ruby, and putting a rescue statement right before re-raising an issue feels a little non-DRY. And you do not want to forget the raise statement. And if you do, good luck trying to find that error.

Thankfully, Ruby is awesome, you can just use the ensure keyword, which makes sure the code runs. The ensure keyword will run the code no matter what - if an exception is thrown, if one isn't, the only exception being if the world ends (or other unlikely events).

 begin     # do driving stuff ensure    self.airbags.inflate if self.exceeding_safe_stopping_momentum? end

Boom! And that code should run anyways. The only reason you should use rescue Exception => e is if you need access to the exception, or if you only want code to run on an exception. And remember to re-raise the error. Every time.

Note: As @Niall pointed out, ensure always runs. This is good because sometimes your program can lie to you and not throw exceptions, even when issues occur. With critical tasks, like inflating airbags, you need to make sure it happens no matter what. Because of this, checking every time the car stops, whether an exception is thrown or not, is a good idea. Even though inflating airbags is a bit of an uncommon task in most programming contexts, this is actually pretty common with most cleanup tasks.