deriving from sympy.Function

[Nico]

This question is connected to my previous question [1].

I'd like to derive from `sympy.Function` and add some extra information to the class. This works:

```

import sympy

class MyTest(sympy.Function):

    def __init__(self, name):

        return

a = MyTest('a')

a.fun = lambda x: x

```

It'd be easier to add the function definition straight in the initializer. However

```

class MyTest(sympy.Function):

    def __init__(self, name, fun):

        self.fun = fun

        return

a = MyTest('a', lambda x: x)

```

does *not* work.

Any idea why that is and how to fix it?

Cheers,

Nico

[1] https://groups.google.com/forum/#!topic/sympy/Iks7BQU_VgA

[Aaron Meurer]

Generally of Function should override the class method eval, like

class MyClass(Function):

    @classmethod

    def eval(cls, *args):

        return an alternate object, or None to remain unevaluated

This automatically puts the *args in the object's .args.  If you want to handle non-SymPy objects like Python functions, you'll have to handle those outside of .args.

I think for what you want, you need to use __new__. Don't forget to use super() to call the superclass __new__ in your subclass.

Aaron Meurer

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