'NoneType' object has no attribute 'insert' Python List append insert
<p>I have the following problem: 
I create an array and then I turned it to  a list, to which I want to add two more values. I have tried with append and with insert but I becoma the error message: 'NoneType' object has no attribute 'insert'. That means that my list is not a list. Here is what I am trying to do:</p>

<pre><code>f = 25
bi = np.arange(-f, f + 5, 5)
beta = bi.tolist()
print "beta:", beta

d = np.arange(-f, f + 5, f / 3)
di = d.tolist()
print "di:", di

dj = di.insert(1, -f / 2)
print "dj:", dj 

dk = dj.insert(5, f / 2)
dw = sorted(dk)
delta = [round(elem, 0) for elem in dw]
print "delta:", delta   
</code></pre>

<p>Has anyone an idea what I am doing wrong or how can Imake it work? Moreover the "sorted" seems also not to be working. </p>