gistfile1.txt

>>> with open('test', 'w') as a:
...   with open('test', 'w') as b:
...     a.write('a')
...     b.write('b')
...
>>>
[1]  + 42274 suspended  python
[mwhooker@Wolfe:~/test]$ cat test
a

with open("test", "w") as f:
  for x in range(10):
      f.write("%s\n" %(x, ))
      if x > 6:
          raise Exception()

The file test will be left half-finished… Which is a problem if, somewhere, you make the assumption that the file is complete. https://github.com/shazow/unstdlib.py/pull/15/files#L2R9 ensures that either the file is completely written, or completely not written.