I agree. In fact, this printing has only been in place since https://trac.sagemath.org/ticket/21286
. Before we'd get D(f)(x). That's in fact also not so hard to parse. With something along the lines of:
from sage.symbolic.operators import FDerivativeOperator
return lambda f: FDerivativeOperator(f,L)
in place, we can just write D(sin)(x) .
If you're going to make diff(...) parseable, perhaps we can make the above as well. Whether we can bind this to the toplevel letter D is another matter.