Fixing Python's string .join()

February 21, 2007

The thing that has always irritated me about string .join() is that it doesn't stringify its arguments; if one of the things in the sequence to be joined isn't a string, .join() doesn't call str() on it, it just pukes. This is periodically annoying and inconvenient.

It recently occurred to me that this can be fixed, like so:

class StrifyingStr(str):
  def join(self, seq):
    s2 = [str(x) for x in seq]
    return super(StrifyingStr, self).join(s2)

def str_join(js, seq):
  return StrifyingStr(js).join(seq)

(A similar version for Unicode strings is left as an exercise for the reader.)

You might think that a generator expression would be more efficient than a list comprehension here; in fact, that's what my first version used. Then I actually timed it, and found out that regardless of whether or not .join() was passed a list or an iterator, and for sizes of the list (or iterator) from 10 elements to 10,000, doing the list comprehension was slightly faster.

Now that I have this I can think of a number of places where I may wind up using it, which kind of makes me wish I'd scratched this irritation before now.

Written on 21 February 2007.
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Last modified: Wed Feb 21 22:51:33 2007
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