Why is it bad style to rescue Exception e in Ruby

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

User · Answer

TL DR Don t rescue Exception   gt  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   gt  e   self beep   self log  quot Caught   e   quot    warn   self log  quot Logged Error - Continuing Process  quot    info end   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   gt  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   gt  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   gt  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

User · Answer

This blog post explains it perfectly  Ruby s Exception vs StandardError  What s the difference   Why you shouldn t rescue Exception The problem with rescuing Exception is that it actually rescues every exception that inherits from Exception  Which is    all of them  That s a problem because there are some exceptions that are used internally by Ruby  They don t have anything to do with your app  and swallowing them will cause bad things to happen  Here are a few of the big ones   SignalException  Interrupt - If you rescue this  you can t exit your app by hitting control-c   ScriptError  SyntaxError - Swallowing syntax errors means that things like puts  quot Forgot something  will fail silently   NoMemoryError - Wanna know what happens when your program keeps running after it uses up all the RAM  Me neither    begin   do something   rescue Exception   gt  e     Don t do this  This will swallow every single exception  Nothing gets past it   end  I m guessing that you don t really want to swallow any of these system-level exceptions  You only want to catch all of your application level errors  The exceptions caused YOUR code  Luckily  there s an easy way to to this  Rescue StandardError Instead All of the exceptions that you should care about inherit from StandardError  These are our old friends  NoMethodError - raised when you try to invoke a method that doesn t exist TypeError - caused by things like 1    quot  quot  RuntimeError - who could forget good old RuntimeError  To rescue errors like these  you ll want to rescue StandardError  You COULD do it by writing something like this  begin   do something   rescue StandardError   gt  e     Only your app s exceptions are swallowed  Things like SyntaxErrror are left alone   end  But Ruby has made it much easier for use  When you don t specify an exception class at all  ruby assumes you mean StandardError  So the code below is identical to the above code  begin   do something   rescue   gt  e     This is the same as rescuing StandardError end

User · 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 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     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   gt  e     lifeboats end   which is equivalent to   begin     iceberg  rescue StandardError   gt  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   gt  e     do some logging   raise   not enough lifeboats    end

User · Answer

Because this captures all exceptions  It s unlikely that your program can recover from any of them    You should handle only exceptions that you know how to recover from  If you don t anticipate a certain kind of exception  don t handle it  crash loudly  write details to the log   then diagnose logs and fix code   Swallowing exceptions is bad  don t do this

User · Answer

That s a specific case of the rule that you shouldn t catch any exception you don t know how to handle  If you don t know how to handle it  it s always better to let some other part of the system catch and handle it

User · Answer

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 ruby  while true do   begin     line   STDIN gets       heavy processing   rescue Exception   gt  e     puts  caught exception   e   ohnoes     end end   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    be ignored  the default  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 stuff rescue Exception   gt  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    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  lt PID gt      in Windows  use the Task Manager  CTRL-SHIFT-ESC   go to the  processes  tab  find your process  right click it and select  End process    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  If you need to do this   begin   do something rescue Exception   gt  e   critical cleanup   raise end   you can actually do this   begin   do something ensure   critical cleanup end   In the second case  critical cleanup will be called every time  whether or not an exception is thrown

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

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