f(x).subs() substitution in sympy (python)

Question

I defined symbols a and f. I'm expecting to use "a(3).subs(a,f)" to get f(3), but instead I got a(3). What's wrong with it?

a, f = symbols('a f')

a(3).subs(a,f)

Answer 1

您已将f定义为一个函数，然后将其替换为symbols()的返回值。

Answer 2

You can use replace to change the function. Here are some examples.

import sympy as sp

f = sp.Function('f')
g = sp.Function('g')
x,y = sp.symbols('x, y')

f(x).replace(f, g)

g(x)

f(x).replace(f, sp.sin)

sin(x)

f(x,y).replace(f, lambda *args: sp.exp(-5 * args[0] + args[1] ))

exp(-5*x + y)

The documentation provides an extensive list of additional examples.