The below picture shoes a part of the notes from Mozart's Alla Turca. It occurs quite early on my note sheet, so it is likely to be found on the first page of whatever note sheet you may have it on.
The question is, how am I supposed to read this part? Am I supposed to repeat the entire part before it, but with three sharps added?
Yes, you are, according to these dots !!! However, they're not exactly right, in that the part of the piece after the two note pick-up is in A major, but the repeat is still in A minor.The music is not really written well. You probably realised that the sharps relate to the next bit, and they should be in the next bar after the repeat, or possibly bracketed in the bar they're in.
The meaning of this notation is to indicate that you are to switch from the current key signature back to A major for the repetition (since the start of the repetition will not in general announce the key signature if it is not changed from before the beginning of the repetition).
If the start of the repeat is not actually in A major, this is a notation error. There is no proper notation for "repeat but in a different key". If that were desired it had to be spelled out explicitly, and it would be the kind of thing one does in a manuscript but never in a printed edition.
Printed editions need to cater for the "perfect sightreader" playing off material in a brainless manner. "Substitute an entirely different key signature and reinterpret all accidentals accordingly" is not in the skill set pianists spend decades on perfecting. So if you change the key for a repeated passage, you reprint it in the changed key.
