newbie question - iterating through dictionary object

[mirandacascade at yahoo.com]

I am attempting to understand the difference between two techniques
that use a for/in loop to iterate through entries in a dictionary
object.  Copy/paste of interactive window illustrates.

>>> a = {}
>>> a.update({1:'a'})
>>> a.update({2:'b'})
>>> a
{1: 'a', 2: 'b'}
>>> for x in a:
... 	print x
... 	print a[x]
...
1
a
2
b
>>> y = a.keys()
>>> for z in y:
... 	print z
... 	print a[z]
...
1
a
2
b

The techniques appear to return the same results.  In the first
example, for each loop iteration, x appears to have the value of the
key of the dictionary object.  In the second example, z appears to have
the value of the key in the dictionary object.  Assuming that I want to
iterate through each dictionary entry, and I don't care about the order
in which the code iterates (i.e. I don't need to sort the list that
gets returned to y when it is assigned the value of the expression
a.keys()), my questions are:

1) Is there any advantage to use the

y = a.keys()
for z in y:

looping technique rather than the

for x in a:

looping technique?

2) What are the tradeoffs for using each of the techniques?