Author Dusty Phillips Reviewers

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As we've seen in previous examples, we can then look up values in the dictionary
by requesting a key inside square brackets. If the key is not in the dictionary, it will 
raise an exception:
>>> stocks["GOOG"]
(613.3, 625.86, 610.5)
>>> stocks["RIM"]
Traceback (most recent call last):
File "", line 1, in
KeyError: 'RIM'
We can, of course, catch the 
KeyError
and handle it. But we have other options. 
Remember, dictionaries are objects, even if their primary purpose is to hold other 
objects. As such, they have several behaviors associated with them. One of the most 
useful of these methods is the 
get
method; it accepts a key as the first parameter 
and an optional default value if the key doesn't exist:
>>> print(stocks.get("RIM"))
None
>>> stocks.get("RIM", "NOT FOUND")
'NOT FOUND'
For even more control, we can use the 
setdefault
method. If the key is in the 
dictionary, this method behaves just like 
get
; it returns the value for that key. 
Otherwise, if the key is not in the dictionary, it will not only return the default value 
we supply in the method call (just like 
get
does), it will also set the key to that same 
value. Another way to think of it is that 
setdefault
sets a value in the dictionary only 
if that value has not previously been set. Then it returns the value in the dictionary, 
either the one that was already there, or the newly provided default value.
>>> stocks.setdefault("GOOG", "INVALID")
(613.3, 625.86, 610.5)
>>> stocks.setdefault("BBRY", (10.50, 10.62, 10.39))
(10.50, 10.62, 10.39)
>>> stocks["BBRY"]
(10.50, 10.62, 10.39)
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Python Data Structures