How to execute a * .PY file from a * .IPYNB file on the Jupyter notebook?

Maybe not very elegant, but it does the job:

exec(open("script.py").read())

the below lines would also work

!python script.py

In the %run magic documentation you can find:

-i run the file in IPython’s namespace instead of an empty one. This is useful if you are experimenting with code written in a text editor which depends on variables defined interactively.

Therefore, supplying -i does the trick:

%run -i 'script.py'

---

The "correct" way to do it

Maybe the command above is just what you need, but with all the attention this question gets, I decided to add a few more cents to it for those who don't know how a more pythonic way would look like.
The solution above is a little hacky, and makes the code in the other file confusing (Where does this x variable come from? and what is the f function?).

I'd like to show you how to do it without actually having to execute the other file over and over again.
Just turn it into a module with its own functions and classes and then import it from your Jupyter notebook or console. This also has the advantage of making it easily reusable and jupyters contextassistant can help you with autocompletion or show you the docstring if you wrote one.
If you're constantly editing the other file, then autoreload comes to your help.

Your example would look like this:
script.py

import matplotlib.pyplot as plt

def myplot(f, x):
    """
    :param f: function to plot
    :type f: callable
    :param x: values for x
    :type x: list or ndarray

    Plots the function f(x).
    """
    # yes, you can pass functions around as if
    # they were ordinary variables (they are)
    plt.plot(x, f(x))
    plt.xlabel("Eje $x$",fontsize=16)
    plt.ylabel("$f(x)$",fontsize=16)
    plt.title("Funcion $f(x)$")

Jupyter console

In [1]: import numpy as np

In [2]: %load_ext autoreload

In [3]: %autoreload 1

In [4]: %aimport script

In [5]: def f(x):
      :     return np.exp(-x ** 2)
      :
      :

In [6]: x = np.linspace(-1, 3, 100)

In [7]: script.myplot(f, x)

In [8]: ?script.myplot
Signature: script.myplot(f, x)
Docstring:
:param f: function to plot
:type f: callable
:param x: x values
:type x: list or ndarray
File:      [...]\script.py
Type:      function