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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def join(items, separator=None):
 """Join the items into a string using the separator Converts items to strings if needed '1,2,3' """

    if not items:
        return ''
    if separator is None:
        separator = _default_separator()
    return separator.join([str(item) for item in items])