Project: Ruby-style-guide

Publisher: Womply

exceptions

Signal exceptions using the fail method. Use raise only when
catching an exception and re-raising it (because here you're not
failing, but explicitly and purposefully raising an exception).
```
begin
  fail 'Oops'
rescue => error
  raise if error.message != 'Oops'
end
```

Don't specify RuntimeError explicitly in the two argument version of fail/raise.

# bad
fail RuntimeError, 'message'

# good - signals a RuntimeError by default
fail 'message'

Prefer supplying an exception class and a message as two separate
arguments to fail/raise, instead of an exception instance.

# bad
fail SomeException.new('message')
# Note that there is no way to do `fail SomeException.new('message'), backtrace`.

# good
fail SomeException, 'message'
# Consistent with `fail SomeException, 'message', backtrace`.

Never return from an ensure block. If you explicitly return from a
method inside an ensure block, the return will take precedence over
any exception being raised, and the method will return as if no
exception had been raised at all. In effect, the exception will be
silently thrown away.
```
def foo
  begin
    fail
  ensure
    return 'very bad idea'
  end
end
```

Use implicit begin blocks where possible.

# bad
def foo
  begin
    # main logic goes here
  rescue
    # failure handling goes here
  end
end

# good
def foo
  # main logic goes here
rescue
  # failure handling goes here
end

Mitigate the proliferation of begin blocks by using
contingency methods (a term coined by Avdi Grimm).

# bad
begin
  something_that_might_fail
rescue IOError
  # handle IOError
end

begin
  something_else_that_might_fail
rescue IOError
  # handle IOError
end

# good
def with_io_error_handling
   yield
rescue IOError
  # handle IOError
end

with_io_error_handling { something_that_might_fail }

with_io_error_handling { something_else_that_might_fail }

Don't suppress exceptions.

# bad
begin
  # an exception occurs here
rescue SomeError
  # the rescue clause does absolutely nothing
end

# bad
do_something rescue nil

Avoid using rescue in its modifier form.

# bad - this catches exceptions of StandardError class and its descendant classes
read_file rescue handle_error($!)

# good - this catches only the exceptions of Errno::ENOENT class and its descendant classes
def foo
  read_file
rescue Errno::ENOENT => ex
  handle_error(ex)
end

Don't use exceptions for flow of control.

# bad
begin
  n / d
rescue ZeroDivisionError
  puts 'Cannot divide by 0!'
end

# good
if d.zero?
  puts 'Cannot divide by 0!'
else
  n / d
end

Avoid rescuing the Exception class. This will trap signals and calls to
exit, requiring you to kill -9 the process.

# bad
begin
  # calls to exit and kill signals will be caught (except kill -9)
  exit
rescue Exception
  puts "you didn't really want to exit, right?"
  # exception handling
end

# good
begin
  # a blind rescue rescues from StandardError, not Exception as many
  # programmers assume.
rescue => e
  # exception handling
end

# also good
begin
  # an exception occurs here

rescue StandardError => e
  # exception handling
end

Put more specific exceptions higher up the rescue chain, otherwise
they'll never be rescued from.

# bad
begin
  # some code
rescue Exception => e
  # some handling
rescue StandardError => e
  # some handling
end

# good
begin
  # some code
rescue StandardError => e
  # some handling
rescue Exception => e
  # some handling
end

Release external resources obtained by your program in an ensure block.

f = File.open('testfile')
begin
  # .. process
rescue
  # .. handle error
ensure
  f.close unless f.nil?
end

Favor the use of exceptions for the standard library over
introducing new exception classes.

Last published over 6 years ago by David Kariuki.

exceptions

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