Flight attendant reported that the gate agent closed the main cabin door before fueling was complete and then reopened door without warning. Escape slide did not deploy.

Narrative

The gate agent at the ZZZ airport didn't tell me they were still filling the gas for the plane. (Or maybe it's because she didn't know either). The pilots didn't tell me either. Nobody told me. I didn't know. We were not supposed to close the door if the plane is filling the gas due to safety reason. I told her we are ready to close the door whenever she is ready. She said ok; and then she closed the door. So I made the arming announcement and then I armed both doors on the airbus A319. And then I did the all call. After that; I asked the pilots if I can close the flight deck door now; they said ok. So I closed the flight deck door with their permission. And just before I was ready to do the live safety demo announcement; the gate agent suddenly opened the door without any warning; without letting me know. I was so shocked. I told her she was not supposed to do that; that was very dangerous. If it's a Boeing 737; the slide would inflate and she might get injured. They're not supposed to open the door if the door is armed. She told me she didn't know. So I don't know if she was really a gate agent or not. How can a gate agent not know? They were taught and trained during training. I ended up doing the disarming door announcement and disarmed another door (1R door) and then I made the all call to make sure all four doors are disarmed. After that; the pilots called me and told me they finished the gas filling. So I told the gate agent that; and then she closed the door again; I made the arming door announcement and armed both doors and made the all call.