1. operators that are somewhat different: // provides integer division, and will return an integer that is closest to it yet lower (it’s different when one of the numbers is float );
   **acts as ‘raise to the power of’;

   >>> 11 // 2   
   5    
   >>> −11 // 2   
   −6   
   >>> 11.0 // 2   
   5.0    
   >>> 11 ** 2   
   121
   

2. fractions module. can be import to calculate with fraction, To define a fraction, create a Fraction object and pass in the numerator and denominator;
   eg:x = fractions.Fraction(1, 3) math moduleis also a helpful one to use constnts like pi;

3. in a list a_list[-n] == a_list[len(a_list) - n];
4. four ways to add items in a list:
   a_list = a_list + [2.0, 3]:creates a second list in memory and assigned to the existing variable a_list; append() adds one new item and extend() method takes a list as the argument, and adds each one of the item to the original list;insert( , )insert an item into a indexed position.
   note that

   >>>a_list+['g','h']
   >>>a_list
   ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']   
   >>> a_list.append(['g', 'h', 'i'])    
   >>> a_list    
   ['a', 'b', 'c', 'd', 'e', 'f', ['g', 'h', 'i']]
   >>> len(a_list)
   7
   

5. useful list methods include count(),in list_name and index(),the last one returns the index of first occurence and will raise an exception (ValueError) if there is no such item;del (takes the index) and remove() (takes the index) is available, while pop() removes the item and returns it.
6. tuple in use:
   in sqlite, if you want to use values from Python variables,Python request that

   Put ? as a placeholder wherever you want to use a value, and then provide a tuple of values as the second argument to the cursor’s execute() method.

example here:

 purchases = [('2006-03-28', 'BUY', 'IBM', 1000, 45.00),
              ('2006-04-05', 'BUY', 'MSFT', 1000, 72.00),
              ('2006-04-06', 'SELL', 'IBM', 500, 53.00),
             ]       
c.executemany('INSERT INTO stocks VALUES (?,?,?,?,?)', purchases)

1. to create a tuple with one item, we need a comma after that item.
   asign name to a number range of values: (MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY) = range(7)
2. an empty set should be >>> a_set = set() , rather than a pair of {}.
3. The update() method takes one argument, a set (also list and tuple), and adds all its members to the original set. And a tuple has no duplicates.
4. If you call the discard() method with a value that doesn’t exist in the set, it does nothing. No error; it’s just a no-op. But for the remove(), such operation would raise a KeyError exception.
5. The union() method returns a new set containing all the elements that are in either set.
   The intersection() method returns a new set containing all the elements that are in both sets.
   The difference() method returns a new set containing all the elements that are in a_set but not b_set.