Let's make things a little bit clear !!

 

Work done by what ?

 

If  we consider a gas is expanding against a force ( say the wt of a piston )

 

Then work done BY the gas is +ve , BY the wt is -ve .

 

Work done  ON the gas ( BY the wt ) is clearly -ve & Work done on the mass ( BY the gas ) is +ve .

 

There is no arbitrariness ( no sign convension , dW = always F . ds )

 

Hope it is clear now !

 

So from the context try to grasp whose work  done is concerned .