This is motivated by a confusion surrounding the specifics of how isothermal processes actually occur.
In an isobaric process, if the gas is free to expand and heat is added, the pressure will stay the same (because the piston will move to the point where the pressure on either side is equal, so the net force is equal).
In an isothermal process, if the gas is free to expand and heat is added (but temperature is constant?), the gas will stay the same but the pressure will decrease.
Suppose you have a gas in a container with a frictionless piston on one end, so it can expand freely. On the other end is a heat reservoir. Is this isothermal or isobaric? On the one hand, the pressure MUST stay the same, or the piston would have a net force on it and the pressure would equalize again. On the other hand, the TEMPERATURE must stay the same, because the heat reservoir is keeping it heated to $T_h$.
All clarification is appreciated.
EDIT: If this process is neither strictly isobaric nor strictly isothermal, how does one achieve an isothermal or isobaric process? I came across this scenario as a example of both a fundamental isobaric process AND a fundamental isothermal process.