ΘΕΜΑ: Προβλημα με λιστα στη Python

>>> thelist = ['A', 'B', 'C']
>>> thelist2 = ['A', 'B', 'C','D', 'E', 'Z']
>>> thelist[-1]
'C'
>>> 
>>> thelist2[-2]
'E'
>>> new_list = []
>>> new_list.append(thelist[-1]- thelist2[-2])
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    new_list.append(thelist[-1]- thelist2[-2])
TypeError: unsupported operand type(s) for -: 'str' and 'str'
>>> for item in thelist2:
	if item not in thelist:
		new_list.append(item)
 
 
>>> new_list
['D', 'E', 'Z']
>>> new_list2 = []
>>> new_list2 = list(set(thelist2) - set(thelist))
>>> new_list2
['E', 'Z', 'D']

Open the db connection
conn= sqlite3.connect('my.db')
cursor_con=conn.cursor()
# Insert some data
cursor_con.execute('''INSERT INTO my_table (datetime, values)
                  VALUES(?,?)''', (dt_ob, values))
 
# Save data to database
conn.commit()
 
cursor_con.execute(""" SELECT  datetime, values FROM my_table""")
data = cursor_con.fetchall()
 
my_list = []
rythmos = []
 
for row in data:
      my_list.append(int(row[1]))
 
n=len(my_list)-1
 
if n==0:
  print error
 
else:
     if n<len(my_list):
       rythmos=[my_list[-1]-my_list[-2]]
 
print my_list
print rythmos

from __future__ import print_function
my_list = range(11)
rythmos = [i - j for i, j in zip(my_list[1:-1], my_list[0:-2])]
print(rythmos)