您的GET请求似乎返回有效的HTML，但#timeline元素中没有tweet元素。但是，将用户代理添加到请求标头似乎可以解决此问题。
from bs4 import BeautifulSoup
import requests
url = 'https://twitter.com/search?q=%23bangkokbombing%20since%3A2015-08-10%20until%3A2015-09-30&src=typd&lang=en'
headers = {'User-Agent': 'Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36'}
r = requests.get(url, headers=headers)
data = r.text
soup = BeautifulSoup(data, "lxml")
all_likes = soup.find_all('span', class_='ProfileTweet-actionCountForPresentation')
print(all_likes)
602392714
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