deriving from sympy.Function: object not callable

[Nico]

Following up on question [1], I'm wondering how one can derive from sympy.Function in a way that keeps all the original functionality in place. Something as naive as

```

import sympy

x = sympy.Symbol('x')

a = sympy.Function('a')

a(x)  # no problem

class MyTest(sympy.Function):

    pass

b = MyTest('b')

b(x)  # object is not callable

```

doesn't work: `b` is reported not to be callable here.

Any hints why this might be and how to fix it?

Cheers,

Nico

[1] https://groups.google.com/forum/#!topic/sympy/5mLEq4Gbyfk

[Nico]

Got it:

As outlined in [1], what is mathematically perceived as a "function" is a class in terms of sympy, not an instance thereof.

--Nico

[1] http://docs.sympy.org/latest/modules/functions/index.html

[Aaron Meurer]

Yes, unfortunately, Function acts differently when subclassed. 

What you want to do, I believe, is create a custom subclass of Function with the metaclass UndefinedFunction.  It's worth taking a look at the source code of Function to see how it works.

Aaron Meurer

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