Python 3.3 added a new file mode that allows you to create a new file and open it for write only if it does not already exist.

>>> with open('new_file', 'x') as f:
...   pass
... 
>>> with open('new_file', 'x') as f:
...   pass
... 
Traceback (most recent call last):
  File "", line 1, in 
FileExistsError: [Errno 17] File exists: 'new_file'