Currently, opening a file with open(file, 'w') overwrites existing files.  It would be useful for open() to raise an error when the file exists.  This proposal is to add a 'c' mode to open, which has the effect to creating a file and opening it for writing, but raising an IOError if it already exists (i.e. the same as os.open(file, os.O_EXCL|os.O_CREAT).

The attached patch implements this, including tests and documentation.