def concatenate(list_to_summarize, separator=', '):
    """Concatenate strings in a list.

    List items that are not strigns will be converted to strings.
    """
    for i, x in enumerate(list_to_summarize):
        if not isinstance(x, str):
            list_to_summarize[i] = str(x)
    return separator.join(list_to_summarize)