python 代码中函数中定义函数，是为什么，在什么地方会用到呢？

首先 python 代码中函数中定义函数的一种用法是修饰器，这个在我的上个提问中，已经知道了，也详细的了解了一下装饰器的作用，主要的就是动态的增加函数的功能，在代码测试，日志处理等处理中，作用明显.

今天在读 sqlmap 的源码时，看到了一个crawler.py的爬虫文件，其中的一部分代码就是在函数中定义函数，而且不是用的修饰器，我看了代码，这段代码完全可以在外部定义，这是没有问题的，我想知道的是为什么要这么定义呢?主要的作用是什么呢?比如，共享变量，隐藏函数？

下面贴一部分代码

def crawl(target):
    try:
        visited = set()
        threadData = getCurrentThreadData()
        threadData.shared.value = oset()

        def crawlThread():
            threadData = getCurrentThreadData()

            while kb.threadContinue:
                with kb.locks.limit:
                    if threadData.shared.unprocessed:
                        current = threadData.shared.unprocessed.pop()
                        if current in visited:
                            continue
                        elif conf.crawlExclude and re.search(conf.crawlExclude, current):
                            dbgMsg = "skipping '%s'" % current
                            logger.debug(dbgMsg)
                            continue
                        else:
                            visited.add(current)
                    else:
                        break

                content = None
                try:
                    if current:
                        content = Request.getPage(url=current, crawling=True, raise404=False)[0]
                except SqlmapConnectionException, ex:
                    errMsg = "connection exception detected (%s). skipping " % ex
                    errMsg += "URL '%s'" % current
                    logger.critical(errMsg)
                except SqlmapSyntaxException:
                    errMsg = "invalid URL detected. skipping '%s'" % current
                    logger.critical(errMsg)
                except httplib.InvalidURL, ex:
                    errMsg = "invalid URL detected (%s). skipping " % ex
                    errMsg += "URL '%s'" % current
                    logger.critical(errMsg)

                if not kb.threadContinue:
                    break

                if isinstance(content, unicode):
                    try:
                        match = re.search(r"(?si)<html[^>]*>(.+)</html>", content)
                        if match:
                            content = "<html>%s</html>" % match.group(1)

                        soup = BeautifulSoup(content)
                        tags = soup('a')

                        if not tags:
                            tags = re.finditer(r'(?si)<a[^>]+href="(?P<href>[^>"]+)"', content)

                        for tag in tags:
                            href = tag.get("href") if hasattr(tag, "get") else tag.group("href")

                            if href:
                                if threadData.lastRedirectURL and threadData.lastRedirectURL[0] == threadData.lastRequestUID:
                                    current = threadData.lastRedirectURL[1]
                                url = urlparse.urljoin(current, href)

                                # flag to know if we are dealing with the same target host
                                _ = reduce(lambda x, y: x == y, map(lambda x: urlparse.urlparse(x).netloc.split(':')[0], (url, target)))

                                if conf.scope:
                                    if not re.search(conf.scope, url, re.I):
                                        continue
                                elif not _:
                                    continue

                                if url.split('.')[-1].lower() not in CRAWL_EXCLUDE_EXTENSIONS:
                                    with kb.locks.value:
                                        threadData.shared.deeper.add(url)
                                        if re.search(r"(.*?)\?(.+)", url):
                                            threadData.shared.value.add(url)
                    except UnicodeEncodeError:  # for non-HTML files
                        pass
                    finally:
                        if conf.forms:
                            findPageForms(content, current, False, True)

                if conf.verbose in (1, 2):
                    threadData.shared.count += 1
                    status = '%d/%d links visited (%d%%)' % (threadData.shared.count, threadData.shared.length, round(100.0 * threadData.shared.count / threadData.shared.length))
                    dataToStdout("\r[%s] [INFO] %s" % (time.strftime("%X"), status), True)

        threadData.shared.deeper = set()
        threadData.shared.unprocessed = set([target])

        if not conf.sitemapUrl:
            message = "do you want to check for the existence of "
            message += "site's sitemap(.xml) [y/N] "
            test = readInput(message, default="n")
            if test[0] in ("y", "Y"):
                found = True
                items = None
                url = urlparse.urljoin(target, "/sitemap.xml")
                try:
                    items = parseSitemap(url)
                except SqlmapConnectionException, ex:
                    if "page not found" in getSafeExString(ex):
                        found = False
                        logger.warn("'sitemap.xml' not found")
                except:
                    pass
                finally:
                    if found:
                        if items:
                            for item in items:
                                if re.search(r"(.*?)\?(.+)", item):
                                    threadData.shared.value.add(item)
                            if conf.crawlDepth > 1:
                                threadData.shared.unprocessed.update(items)
                        logger.info("%s links found" % ("no" if not items else len(items)))

        infoMsg = "starting crawler"
        if conf.bulkFile:
            infoMsg += " for target URL '%s'" % target
        logger.info(infoMsg)

        for i in xrange(conf.crawlDepth):
            threadData.shared.count = 0
            threadData.shared.length = len(threadData.shared.unprocessed)
            numThreads = min(conf.threads, len(threadData.shared.unprocessed))

            if not conf.bulkFile:
                logger.info("searching for links with depth %d" % (i + 1))

            runThreads(numThreads, crawlThread, threadChoice=(i>0))
            clearConsoleLine(True)

            if threadData.shared.deeper:
                threadData.shared.unprocessed = set(threadData.shared.deeper)
            else:
                break

    except KeyboardInterrupt:
        warnMsg = "user aborted during crawling. sqlmap "
        warnMsg += "will use partial list"
        logger.warn(warnMsg)

    finally:
        clearConsoleLine(True)

        if not threadData.shared.value:
            warnMsg = "no usable links found (with GET parameters)"
            logger.warn(warnMsg)
        else:
            for url in threadData.shared.value:
                kb.targets.add((url, None, None, None, None))