UnboundLocalError问题：赋值之前引用了局部变量

Question

因此，我正在通过编写此小脚本来测试有关网络抓取的知识（我知道有些事情是多余的，只是我在玩弄事情）：

def programstart():
    import re
    from selenium import webdriver
    from bs4 import BeautifulSoup

    ff=webdriver.Firefox()
    ff.get('https://www.ozbargain.com.au/deals')

    linkpattern = r'https://www.ozbargain.com.au/node/.*'

    donotpassthislink = 'https://www.ozbargain.com.au/node/415763'

    #list of found lnks
    titlelist = []

    #loop to find links
    while True:
        soup = BeautifulSoup(ff.page_source)
        raw_links = soup.find_all(name='a', href=re.compile(linkpattern))
        for links in raw_links:
            titles = ff.find_element_by_xpath("//h2[@class='title']").text
            extract = links
            if donotpassthislink == extract:
                break
            else:
                pass
            titlelist.append(titles)
        # Load next page
        if donotpassthislink == extract:     # stop loop at first movie link from last time.
            break
        else:
            ff.get(ff.find_element_by_xpath("//a[@class='pager-next active']").get_attribute('href'))


programstart()

我收到此错误：

Traceback (most recent call last):
  File "F:/Dropbox/Funfile/ben.py", line 35, in <module>
    programstart()
  File "F:/Dropbox/Funfile/ben.py", line 29, in programstart
    if donotpassthislink == extract:     # stop loop at first movie link from last time.
UnboundLocalError: local variable 'extract' referenced before assignment

我发现这很奇怪，因为此循环已在另一个运行良好的脚本中使用。

所以我写了一个简化的脚本：

test = 'lets break this sentence up'

list = []

def func():
    while True:
        for i in test:
            extract = i
            print(extract)
            if extract == 'u':
                break
            else:
                pass
            list.append(extract)
        if extract == 'u':
            print('done')
            break

func()

它执行没有任何问题。 发生了什么事？