You can use os.listdir() to list both the files and directories.
To list only files you may use os.path:
from os import listdir
from os.path import isfile, join
onlyfiles = [f for f in listdir(mypath) if isfile(join(mypath, f))]
os.walk() may also be used in case you want two lists: one for files and another for directories.
from os import walk
f = 
for (dirpath, dirnames, filenames) in walk(mypath):
To add one list with another you can use .extend() or,
>>> a = [10,11,12]
>>> b= [13, 14, 15]
>>> a = a + b
[10,11, 12, 13, 14, 15]