One Swallow Does Not a Summer Make(one piece of evidence is not enough to prove something)
Hey, Rebecca. How was your tennis match the other day?
Great! I beat out Garcia and made the final play-offs. I've got the championship in the bag!
Hold on, there. I can understand your excitement, but remember that one swallow does not a summer make. The final outcome won't be known until the last game is played.
You're right. I know I shouldn't form my opinion based on a single victory, but I've got a good feeling about this tournament.
More power to you. I hope you don't end up by being disappointed.
have ~ in the bag = ~ を手に入れたも同然
outcome = 結果