Explanation

I hope they will not have left the city before I reach there.

مجھے امید ہے کہ میرے وہاں پہنچنے سے پہلے وہ شہر چھوڑ کر نہیں گئے ہوں گے۔

The sentence talks about a future action ("before I reach there") 

Future completed action is the future perfect:

will not have + past participle

"Will not have left" shows:

A completed action in the future

That you hope they are still there when you arrive