Comment by torlok

It's just a way to keep the code size in check, make sure it can be read and understood relatively easily. Don't overthink it. I doubt much, if any, research went into picking the limit. The line width is over 120 in many places, and the code inevitably ends up looking like

  cache_key = (device, st, dtype, op, arg, tuple(ref(x) for x in srcs)) if base is None else (st, ref(base))