Why does list.append() return None?

Just replace a_list = a_list.append(r) with a_list.append(r).

Most functions, methods that change the items of sequence/mapping does return None: list.sort, list.append, dict.clear ...

Not directly related, but see Why doesn’t list.sort() return the sorted list?.

The method append does not return anything:

>>> l=[]
>>> print l.append(2)
None

You must not write:

l = l.append(2)

But simply:

l.append(2)

In your example, replace:

a_list = a_list.append(r)

to

a_list.append(r)