[Tutor] remove blank list items

[Kent Johnson]

Michael Langford wrote:
> While the keys() method returns an object of type list, the keys of a 
> dict (when you leave them in the dict) behave as a "set", as in the 
> mathematical/computer science idea of a collection of objects where no 
> two objects have the same value. ( http://en.wikipedia.org/wiki/Set)

Before Python had a real set type, dicts were commonly used as a 
substitute for a set.

> The keys of the dictionary may or may not be implemented with a python 
> object that has a class of "set".

They are not, they are implemented as part of the hashtable that is a dict.

In fact set was originally implemented in Python as a wrapper around 
dict. When it proved to be useful it was re-implemented in C. The 
original implementation still survives in the set module.

  The keys() method definitely returns a
> list, and I'm sure it does this as that's much more useful than 
> returning a set object.

Actually the designers of Python believe that for the value of 
dict.keys() to have set-like behaviour is very useful. In Python 3 that 
is how it will be.

http://www.python.org/dev/peps/pep-3106/