Joining strings in Python

11. Jun. 2009 at 12:58 under Python, Strings, English

There is already a lot of good material to read about performance. Therefore, it is best to use the following method:

>>> print ' '.join(['The', 'fox', 'jumped', 'over', 'the', 'dog.'])
    The fox jumped over the dog.

Perfect. We have very fast string contatenation that even allows us to choose separator. Now, here is the problem:

>>> print ', '.join(['apples', 'oranges', '', 'cocos'])
    apples, oranges, , cocos

So, if we dynamically generate string and we have a separator for a reason, we do not want this to happen. This is basically the idea:

>>> print ', '.join(filter(None, ['apples', 'oranges', '', 'cocos']))
    apples, oranges, cocos

As you can see, filter(None, iterable) returns list of elements that evalue to True. This way we have somewhat a safe string contatenation. Real world example:

class StringBuffer:

    def __init__(self, sep=''):
            self.sep = sep
            self.output = list()

    def write(self, content):
            self.output.extend(content)

    def getvalue(self):
            return self.sep.join(filter(None, self.output))


>>> sb = StringBuffer(', ')
>>> sb.write(['apples'])
>>> sb.write(['', None])
>>> sb.write(['oranges', 'cocos'])
>>> print sb.getvalue()
apples, oranges, cocos