python - equivalent of Python's "with" in Ruby

Question

In Python, the with statement is used to make sure that clean-up code always gets called, regardless of exceptions being thrown or function calls returning. For example:

with open("temp.txt", "w") as f:
    f.write("hi")
    raise ValueError("spitespite")

Here, the file is closed, even though an exception was raised. A better explanation is here.

Is there an equivalent for this construct in Ruby? Or can you code one up, since Ruby has continuations?

Answer 1

Ruby has syntactically lightweight support for literal anonymous procedures (called blocks in Ruby). Therefore, it doesn't need a new language feature for this.

So, what you normally do, is to write a method which takes a block of code, allocates the resource, executes the block of code in the context of that resource and then closes the resource.

Something like this:

def with(klass, *args)
  yield r = klass.open(*args)
ensure
  r.close
end

You could use it like this:

with File, 'temp.txt', 'w' do |f|
  f.write 'hi'
  raise 'spitespite'
end

However, this is a very procedural way to do this. Ruby is an object-oriented language, which means that the responsibility of properly executing a block of code in the context of a File should belong to the File class:

File.open 'temp.txt', 'w' do |f|
  f.write 'hi'
  raise 'spitespite'
end

This could be implemented something like this:

def File.open(*args)
  f = new(*args)
  return f unless block_given?
  yield f
ensure
  f.close if block_given?
end

This is a general pattern that is implemented by lots of classes in the Ruby core library, standard libraries and third-party libraries.

---

A more close correspondence to the generic Python context manager protocol would be:

def with(ctx)
  yield ctx.setup
ensure
  ctx.teardown
end

class File
  def setup; self end
  alias_method :teardown, :close
end

with File.open('temp.txt', 'w') do |f|
  f.write 'hi'
  raise 'spitespite'
end

Note that this is virtually indistinguishable from the Python example, but it didn't require the addition of new syntax to the language.

Answer 2

The equivalent in Ruby would be to pass a block to the File.open method.

File.open(...) do |file|
  #do stuff with file
end  #file is closed

This is the idiom that Ruby uses and one that you should get comfortable with.

Answer 3

You could use Block Arguments to do this in Ruby:

class Object  
    def with(obj)  
        obj.__enter__  
        yield  
        obj.__exit__  
    end  
end

Now, you could add __enter__ and __exit__ methods to another class and use it like this:

with GetSomeObject("somefile.text") do |foo|  
    do_something_with(foo)
end  

Answer 4

I'll just add some more explanations for others; credit should go to them.

Indeed, in Ruby, clean-up code is as others said, in ensure clause; but wrapping things in blocks is ubiquitous in Ruby, and this is how it is done most efficiently and most in spirit of Ruby. When translating, don't translate directly word-for-word, you will get some very strange sentences. Similarly, don't expect everything from Python to have one-to-one correspondence to Ruby.

From the link you posted:

class controlled_execution:
    def __enter__(self):
        set things up
        return thing
    def __exit__(self, type, value, traceback):
        tear things down

with controlled_execution() as thing:
     some code

Ruby way, something like this (man, I'm probably doing this all wrong :D ):

def controlled_executor
  begin
    do_setup
    yield
  ensure
    do_cleanup
  end
end

controlled_executor do ...
  some_code
end

Obviously, you can add arguments to both controlled executor (to be called in a usual fashion), and to yield (in which case you need to add arguments to the block as well). Thus, to implement what you quoted above,

class File
  def my_open(file, mode="r")
    handle = open(file, mode)
    begin
      yield handle
    ensure
      handle.close
    end
  end
end

File.my_open("temp.txt", "w") do |f|
  f.write("hi")
  raise Exception.new("spitesprite")
end

Answer 5

It's possible to write to a file atomically in Ruby, like so:

File.write("temp.txt", "hi")
raise ValueError("spitespite")

Writing code like this means that it is impossible to accidentally leave a file open.

Answer 6

You could always use a try..catch..finally block, where the finally section contains code to clean up.

Edit: sorry, misspoke: you'd want begin..rescue..ensure.

Answer 7

I believe you are looking for ensure.