[FIXED] Why am I getting none? I am trying to have one list of standard deviations

Issue

My goal is to have one list of standard deviations from the data. I have 13 columns (features) in trainx and three labels(1,2,3) in trainy array corresponding to each row of trainx. My purpose is to find a feature which has minimum standard deviation.

I thought that first I'll calculate standard deviation for each feature for label 1, append it in a list and then will find the minimum std deviation. I tried to write below code blocks but have been unsuccessful so far:

a=[]
for i in range(0,13):
    b=[np.std(trainx[trainy==1,i])]
    print(a.append(b))

It is returning this output:

None
None
None
None
None
None
None
None
None
None
None
None
None

If I try below code:

a=[]
for i in range(0,13):
    b=[np.std(trainx[trainy==1,i])]
    a=a.append(b)
print(a)

it returns:

AttributeError                            Traceback (most recent call last)
<ipython-input-78-6b02e93115a0> in <module>
      3 for i in range(0,13):
      4     b=[np.std(trainx[trainy==1,i])]
----> 5     a=a.append(b)
      6 print(a)

AttributeError: 'NoneType' object has no attribute 'append'

Please help me. Other ways are welcome too.

Solution

list.append doesn't return the list. It only appends the object to the list. To get the list after you append, just access the list.

a=[]
for i in range(0,13):
    b=[np.std(trainx[trainy==1,i])]
    a.append(b)
    print(a)

Answered By - noteness