Building unique comma-delimited list?

Bengt Richter bokr at
Thu Jan 6 20:43:30 CET 2005

On 5 Jan 2005 17:05:40 -0500, roy at (Roy Smith) wrote:

>I've got a silly little problem that I'm solving in C++, but I got to
>thinking about how much easier it would be in Python.  Here's the
>You've got a list of words (actually, they're found by searching a
>data structure on the fly, but for now let's assume you've got them as
>a list).  You need to create a comma-delimited list of these words.
>There might be duplicates in the original list, which you want to
>eliminate in the final list.  You don't care what order they're in,
>except that there is a distinguised word which must come first if it
>appears at all.
>Some examples ("foo" is the distinguised word): 
>["foo"] => "foo"
>["foo", "bar"] => "foo, bar"
>["bar", "foo"] => "foo, bar"
>["bar", "foo", "foo", "baz", "bar"] => "foo, bar, baz" or "foo, baz, bar"
>The best I've come up with is the following.  Can anybody think of a
>simplier way?
(Not tested beyond what you see ;-)
python 2.4:
 >>> words = ["foo", "bar", "baz", "foo", "bar", "foo", "baz"]
 >>> w2 = list(t[1] for t in sorted((w!='foo', w) for w in set(words)))
 >>> w2
 ['foo', 'bar', 'baz']

Gets you a sort in the bargain ;-)

>words = ["foo", "bar", "baz", "foo", "bar", "foo", "baz"]
># Eliminate the duplicates; probably use set() in Python 2.4
Yup, but 2.3 can be a one-liner too:

 >>> words = ["foo", "bar", "baz", "foo", "bar", "foo", "baz"]
 >>> w2 = ('foo' in words and ['foo'] or []) + [w for w in dict(zip(words,words)) if w!='foo']
 >>> w2
 ['foo', 'baz', 'bar']

Not sorted, but foo is out front.

>d = dict()
>for w in words:
>        d[w] = w
>if d.has_key ("foo"):
>        newWords = ["foo"]
>        del (d["foo"])
>        newWords = []
>for w in d.keys():
>        newWords.append (w)
>s = ', '.join (newWords)
>print s

Bengt Richter

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