python list and tuple

　　1. convertion between list and tuple

            tuple(<list>)  list(<tuple>)

      2. ('aa',) means a tuple containing only one element 'aa'

          ('aa') means a tuple containing two elemetns: 'a' and 'a'

          so, ('aa') != ('aa',), you can convert them to list to check the difference

In [17]: list(('aa'))
Out[17]: ['a', 'a']

In [18]: list(('aa',))
Out[18]: ['aa']

3. iterate a list or tuple

for e in mylist:

　　print(e)

length = len(mylist)

for i in range(length)

　　print(mylist[i])

for ( i,val)  in enumerate ( list ):

     print (i,  val)