Q:

Python How to get the value of one specific point of derivative?

from sympy import *
x = Symbol('x')
y = x ** 2
dx = diff(y, x)

This code can get the derivative of y.
It's easy dx = 2 * x
Now I want to get the value of dx for x = 2.
Clearly, dx = 2 * 2 = 4 when x = 2
But how can I realize this with python codes? 
Thanks for your help!

A:

Probably the most versatile way is to lambdify:
sympy.lambdify creates and returns a function that you can assign to a name, and call, like any other python callable.
from sympy import *

x = Symbol('x')
y = x**2
dx = diff(y, x)
print(dx, dx.subs(x, 2))  # this substitutes 2 for x as suggested by @BugKiller in the comments

ddx = lambdify(x, dx)     # this creates a function that you can call
print(ddx(2))

