2.7.2. 查找contents中第一个NavigableString: findFirstNavigableString

#------------------------------------------------------------------------------
# find the first BeautifulSoup.NavigableString from soup contents
def findFirstNavigableString(soupContents):
    firstString = None;
    for eachContent in soupContents:
        # note here must import NavigableString from BeautifulSoup
        if(isinstance(eachContent, NavigableString)): 
            firstString = eachContent;
            break;

    return firstString;