刮擦Python脚本：循环和更新输出

Question

所以我有这个python脚本。现在，我运行脚本，它给我一个CSV输出文件。

我想要的是：当它重新启动并检查这些输出值的更改时（在重新启动时不刷新输出文件并擦除所有以前收集的数据）

同样，每行数据大约需要3秒才能获取。有谁知道如何快速处理大型数据集？

import urllib2,re,urllib,urlparse,csv,sys,time,threading,codecs
from bs4 import BeautifulSoup

def extract(url):
    try:
        sys.stdout.write('0')
        global file
        page = urllib2.urlopen(url).read()

        soup = BeautifulSoup(page, 'html.parser')

        product = soup.find("div", {"class": "js-product-price"})
        price = product.findNext('div',{'class':'js-price-display'}).getText().strip()
        oos = product.findNext('p', attrs={'class': "price-oos"})

        if oos is None:
            oos = 'In Stock'
        else:
            oos = oos.getText()

        val = url + "," + price + "," + oos + "," + time.ctime() + '\n'
        ifile.write(val)
        sys.stdout.write('1')
    except Exception as e:
        print e
        #pass
    return

ifile = open('output.csv', "a", 0)
ifile.write('URL' + "," + 'Price' + "," + 'Stock' + "," + "Time" + '\n')
inputs = csv.reader(open('input.csv'))
#inputs = csv.reader(codecs.open('input.csv', 'rU', 'utf-16'))
for i in inputs:
    extract(i[0])
ifile.close()
print("finished")