distinguishing own from intrinsic functions

[Nico]

To get all function from an expression, one can do

```

from sympy import *

f = Function('f')

x = Symbol('x')

y = f(x) + 2 * sin(x)

for fun in y.atoms(Function):

    print(fun.func)

```

which in this case will print out `f` and `sin`.

I would now like to distinguish between function that I defined myself (`f`) the ones that come from elsewhere. I though about subclassing `sympy.Function`, but apparently it's not exactly meant to be used that way [1].

Any hints?

Cheers,

Nico

[1] https://groups.google.com/forum/#!topic/sympy/pU81Trc_Xr8

[Nico Schlömer]

I guess one thing I could do is taint the function class object as

```

f = Function('f')

f.my_custom_function = True

```

[Aaron Meurer]

sympy.Function is absolutely designed to be subclassed.

If f = Function('f'), then f is a subclass of UndefinedFunction, and
f(x) is an instance of AppliedUndef (both can be imported from
sympy.core.function). You can find custom functions with
y.atoms(AppliedUndef).

Aaron Meurer

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[Nico Schlömer]

Thanks!