import urllib.request
from bs4 import BeautifulSoup
soup = BeautifulSoup(urllib.request.urlopen('https://www.zoopla.co.uk/for-sale/property/uk/?identifier=uk&pn=1').read(), 'html.parser')

ids = []
for i in range(0,21):
    ids.append(soup.findAll(attrs={"class" : "srp clearfix "})[i]['data-listing-id'])

print(ids)