How do open a file for writing only if it doesn't already exist in ruby

Question

I want to open a file for writing but only if it doesn't already exist. If the file exists I want to raise an exception. Is this the best way to do it?

filename = 'foo'
raise if File.exists? filename
File.open(filename, 'w') do |file| 
  file.write contents
end

What is the most idiomatic way to do this without the race condition?

Answer 1

After doing some further research, it seems you can use the File::CREAT and File::EXCL mode flags.

filename = 'foo'
File.open(filename, File::WRONLY|File::CREAT|File::EXCL) do |file|
  file.write contents
end

In this case, open will raise an exception if the file exists. After running once, this program succeeds without error, creating a file named foo. On the second run, the program emits this:

foo.rb:2:in `initialize': File exists - foo (Errno::EEXIST)
    from foo.rb:2:in `open'
    from foo.rb:2

From man open:

       O_WRONLY        open for writing only
       O_CREAT         create file if it does not exist
       O_EXCL          error if O_CREAT and the file exists