当我 append 我的空列表时，为什么我的列表长度为 1？

Question

I am trying to append an empty list with outputs of a function with a range of input values. I then want to plot the inputs against the outputs. However, the dimension of the resulting list is said to be 1, but when I print the list itself, it seems to show many entries. I therefore can not plot the input vs output due to differences in dimension. Any help would be greatly appreciated.

PS - I'm very much in the early stages of learning Python again so I'm sure this is a rudimentary problem.

import math
import numpy as np
import pylab
from sympy import *

Um,x,x0,alpha=symbols('Um x x0 alpha', real=True)
U = Um*((1-exp(-alpha*(x-x0)))**2-1)

F=-U.diff(x)

x = np.arange(0,30,0.1)
y=[]
y.append(-2*Um*alpha*(1 - exp(-alpha*(x - x0)))*exp(-alpha*(x - x0)))

alpha = 2.287 
x0=1.2
Um=7.37
exp=math.exp


print(len(y))
print(y)
pylab.plot(x,y)
pylab.show()

Answer 1

Since the element that you want to append to y is a list, it is appended as single element, which contains several items. That's why the length of y is 1.

In order to add the elements of list into another list, you can use extend , which is basically a list concatenation.

Change append to extend :

y.extend(-2*Um*alpha*(1 - exp(-alpha*(x - x0)))*exp(-alpha*(x - x0)))