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Comment by makecheck | Hacker News Reader

Not only is it overly-verbose and ultimately unnecessary (as shown by the alternative that follows in the article) but this mechanism is actually broken. For instance if "freqs" were data given to you from somewhere else and "freqs[c]" happened to have a type that cannot legally have 1 added to it, you'd want to see this error. The "except:" however will absorb any and all exception types and respond to all of them by setting the value to 1.

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Also, raising exceptions used to be quite slow, which could hurt for a sparse counting set. Don't know whether that's still the case.

Also Exceptions should be used in "exceptional" circumstances and not as part of normal flow.

One exception (teehee!) to the rule: file operations and other things where atomicity matters. Example code:

  if not os.path.exists("foo"):
      os.mkdir("foo")

That introduces a race condition. If foo does not exist on the first line but is created by something else on the second line then this will raise an exception. The proper code is:

  import errno
  try:
      os.mkdir("foo")
  except OSError as exc:
      if exc.errno != errno.EEXIST:
          raise

That doesn't have the race condition.

(Yes, that's wordy. Eventually they plan to add to Python 3 a fancier exception hierarchy described at http://www.python.org/dev/peps/pep-3151/ , which lets you filter at a more fine-grained level, as below.)

  try:
      os.mkdir("foo")
  except FileExistsError:
      pass

It's good to point out the race condition.

To solve this particular problem in future code more compactly however, note that Python 3.2 (finally) adds an "exist_ok" Boolean keyword parameter to the multi-directory variant, os.makedirs(). In other words, calling os.makedirs("mydir", exist_ok=True) will silently ignore existing directories and only raise if other errors occur.

Thank you for pointing that out, I've just changed some code :)

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