Both are special methods defined of subclasses of Function. eval is a
classmethod that tells what explicit values should be evaluated. If
eval is undefined, or returns None, the function will remain
unevaluated. For instance, with
class MyFunc(Function):
@classmethod
def eval(cls, arg):
if arg == 0:
return 0
then MyFunc(0) will return 0, but any other argument, like MyFunc(1),
will remain unevaluated. eval also tells Function how many arguments
the function takes.
fdiff is the derivative of *just* the function, without using the
chain rule. According to a comment in the code for
Function._eval_derivative, f(x).diff(s) -> x.diff(s) * f.fdiff(1)(s).
In fact, if you read the code for that method you should completely
understand how fdiff works. I believe the argindex argument handles
the case when the function has multiple arguments (like atan2).
Aaron Meurer
On Sat, Dec 27, 2014 at 6:07 AM, utpal kumar Jha <[email protected]> wrote:
> i went through all of the docs and then found a link to test interface which
> i am unable to figure out as to how the test_function_series(1/2/3) are
> implementing sin,cos and tan functions respectively . if someone could
> provide an insight of fdiff() and eval() functions i would appreciate it .
>
> https://github.com/sympy/sympy/blob/master/sympy/functions/elementary/tests/test_interface.py
>
>
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