Unable to get the date out of feedparser RSS return in Python

I am trying to get the date of rss and I can see it in the tag of 'published'. But I couldn't get it out. Any suggestion?

import feedparser
df = feedparser.parse('http://www.newslookup.com/rss/business/bloomberg.rss')
for post in df.entries:
        posts.append((df.feed.title, post.title, post.links, post.published))
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