Your guess is right: the code is trying to evaluate x**3+2*x-4 immediately. Unfortunately you can't really prevent it from doing so. The good news is that in Python, functions are first-class objects, by which I mean that you can treat them like any other variable. So to fix your function, we could do:
import numpy as np
import matplotlib.pyplot as plt
def graph(formula, x_range):
x = np.array(x_range)
y = formula(x) # <- note now we're calling the function 'formula' with x
plt.plot(x, y)
plt.show()
def my_formula(x):
return x**3+2*x-4
graph(my_formula, range(-10, 11))
If you wanted to do it all in one line, you could use what's called a lambda function, which is just a short function without a name where you don't use def or return:
graph(lambda x: x**3+2*x-4, range(-10, 11))
And instead of range, you can look at np.arange (which allows for non-integer increments), and np.linspace, which allows you to specify the start, stop, and the number of points to use.