Pretty_Print / show seems to delete trailing spaces:
reset() var('x t') Psi = function('Psi')(x, t) V = function('V')(x) show('Plug ', Psi, ', ', diff(Psi, t), ', ', diff(Psi, x, x), ' into ', V)
See after "Plug" and after "into" (screenshot taken from CoCalc Jupyter Sage 8.9 Kernel):