Extracting first sentence from a paragraph using Python?

Let's say there's a big paragraph with lines like the following,

"A.B.C is an example sentence. D.E.F is an example second sentence."

Most answers on StackOverflow mention the basic way of extracting first sentence by splitting those paragraphs like the following,

paragraph.split('. ')[0]+"."

But that doesn't work for the mentioned example. So what's the best way to extract the first sentence from the string using python?