> lines = open('test.txt').readlines()
> lines
['1\n', '2\n', '3\n', '4,5\n']
> for line in lines:
...  ll.extend(line.strip().split(','))
... 
> ll
['1', '2', '3', '4', '5']

> ll = []
> lines = open('test.txt').readlines()
> lines
['1\n', '2\n', '3\n', '4,5\n']
> for line in lines:
...  ll = ll + line.strip().split(',')
... 
> ll
['1', '2', '3', '4', '5']

> ll = []
> lines = open('test.txt').readlines()
> for line in lines:
...  ll.append(line.strip().split(','))
... 
> ll = np.array(ll)
> np.hstack(ll.flat)
array(['1', '2', '3', '4', '5'], 
  dtype='|S1')
> list(np.hstack(ll.flat))
['1', '2', '3', '4', '5']

> a = [1,'a']
> a.extend(np.array([2,'b']))
> a
[1, 'a', '2', 'b']
> a.extend([3,['c']])
> a
[1, 'a', '2', 'b', 3, ['c']]
> a = [1,'a']
> a.extend(np.array([2,'b']))
> a
[1, 'a', '2', 'b']
> a.extend([3,['c']])
> a
[1, 'a', '2', 'b', 3, ['c']]
> a = [1,'a']
> a.append(np.array([2,'b']))
> a
[1, 'a', array(['2', 'b'], 
  dtype='|S21')]
> a.append([3,['c']])
> a
[1, 'a', array(['2', 'b'], 
  dtype='|S21'), [3, ['c']]]

> a = np.array([[1,2],[3,4]])
> a.dtype
dtype('int64')
> a.flatten()
array([1, 2, 3, 4])
> 
> a = np.array([[1,2],[3,4],[5]])
> a.flatten()
array([[1, 2], [3, 4], [5]], dtype=object)

> ll = []
> lines = open('test.txt').readlines()
> lines
['1\n', '2\n', '3\n', '4,5\n']
> for line in lines:
...  ll.append(map(int,line.strip().split(',')))
... 
> ll
[[1], [2], [3], [4, 5]]