Python - AttributeError: '_io.TextIOWrapper' object has no attribute 'append'

You got your append code all mixed up; the append() method is on the filelines object:

ClassFile=open(CN+'.txt','r')
line=ClassFile.readline()
while line!='':
    filelines.append(line)
ClassFile.close()

Note that I also moved the close() call out of the loop.

You don't need to use a while loop there; if you want a list with all the lines, you can simply do:

ClassFile=open(CN+'.txt','r')
filelines = list(ClassFile)
ClassFile.close()

To handle file closing, use the file object as a context manager:

with open(CN + '.txt', 'r') as openfile:
    filelines = list(openfile)

ClassFile is an object of type _io.TextIOWrapper which does not has any attribute append. You are mistaking it to be an object of type List. It seems in place of ClassFile.append(filelines) you want something like filelines.append(line). If you want to write something into a file, open it in write or append mode (depending on your need) and write into it the string you want.