eval() is only available for Function subclasses. For Expr subclasses,
you have to define __new__, which unfortunately isn't as convenient as
eval(). A basic version would look something like
def __new__(cls, m, n, l):
m, n, l = _sympify(m), _sympify(n), _sympify(l)
if n == 0:
return S.Zero
return Expr.__new__(cls, m, n, l)
Note that you need to manually handle making sure inputs and outputs
are SymPy types. I would suggest looking at the source code for
Integral.__new__ and ExprWithLimits.__new__.
Aaron Meurer
On Tue, Jun 11, 2024 at 6:01 AM Michael Gfrerer <mhgfre...@gmail.com> wrote:
>
> Thanks for your answers. I was thinking of making a custom class, but wanted
> to be sure I wasn't reinventing the wheel.
> I made a first attempt to define a custom class, but I stumbled upon an issue
> with eval and simplify. If the integrand is 0 the whole integral can be set
> to 0.
> So I tried the followiung code without success:
>
> import sympy as sp
> class LebesgueIntegral(sp.Expr):
> def _latex(self, printer, exp=1):
> m, n, l = self.args
> _m, _n, _l = printer._print(m), printer._print(n), printer._print(l)
> return r'\int_{%s} %s \,d%s' % (_m, _n, _l)
>
> @classmethod
> def eval(cls, m, n, l):
> if n == 0:
> return 0
>
> def _eval_simplify(self, **kwargs):
> if self.args[1] == 0:
> return 0
> return self
>
> u = LebesgueIntegral(sp.Symbol('\Omega'), sp.sympify(0),sp.Symbol('x'))
> print(sp.latex(u))
> print(sp.simplify(u))
> print(sp.simplify(2*u))
>
> The above code gives me the output:
> \int_{\Omega} 0 \,dx
> 0
> 2*LebesgueIntegral(\Omega, 0, x)
>
> Why is the method eval not called in the first print statement?
> I had a look at the simplify module but it is very hard for me to understand.
> Is it intended that the simplify method of the class is not called in the
> third print statement?
>
> Michael Gfrerer
>
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