In [6]: class MyDerivative(Derivative):
   ...:     pass
   ...:

In [7]: MyDerivative(x,x).subs(y,y)
Out[7]:
d
──(x)
dx

In [8]: type(_)
Out[8]: __main__.MyDerivative

In [9]: MyDerivative(x,x).subs(y,y).__class__
Out[9]: __main__.MyDerivative