defining custom functions [again]

[Felipe Vieira]

He all,

I've read the posts. And the tutorial. And the code.

Still I'm having trouble defining my own functions.

Let's start with the very basics: I would like to define f(x) = 2 * x:

from sympy import *

from sympy.abc import x, y

import sympy

class Double(Function):

    u"""A simple function that doubles the input."""

    @classmethod

    def eval(cls, arg):

        return None

    def fdiff(self, argindex = 1):

        return Integer(2) * self.args[0]

    def __new__(cls, *args, **kwargs):

        return super(Double, cls).__new__(cls, *args, **kwargs)

    def __init__(self, *args, **kwargs):

        super(Double, self).__init__()

d = Double('d')

d(2)

When I run the code I get: 

TypeError: 'Double' object is not callable

How can I pythonically/sympythonically define a Double inheriting from function? Notice that this is just a first step in defining more 'complex' functions (so don't tell me to use Lambda).

Grea work with the project! Thank you!

[Aaron Meurer]

Function is a bit confusing, because Function('f') creates a subclass
called f, but this property is not inherited when you inherit from
Function. So your object just works like

Double(2)

Also, your overriding of __new__ and __init__ is unnecessary, since
they don't do anything (and in general, anything you'd want to do in
those methods for subclasses of Function you should do in eval()).

Aaron Meurer

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[Felipe Vieira]

Let me try again:

class Double(Function):

    u"""A simple function that doubles the input."""

    nargs = 1

    @classmethod

    def eval(cls, arg):

        pass

        if arg.is_Number:

            return sympy.Mul(2, arg)

    def fdiff(self, argindex = 1):

        return sympy.Integer(2)

This generates:

In [47]: Double(x)

Out[47]: Double(x)

In [48]: Double(2*x)

Out[48]: Double(2*x)

In [49]: Double(2*x).fdiff()

Out[49]: 2

In [50]: Double(2*x).diff()

Out[50]: 4

So now it looks like it is working as expected.

I thought the same way I could do: f = Function('f') I could also do d = Double('d') then d(2).

As mentioned above, is this the expected way of doing things?

[Aaron Meurer]

Yes, this implementation looks correct.

Aaron Meurer

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