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11.4. Dictionary methods¶
Dictionaries have a number of useful built-in methods. The following table provides a summary and more details can be found in the Python Documentation.
Returns a view of the keys in the dictionary
Returns a view of the values in the dictionary
Returns a view of the key-value pairs in the dictionary
Returns the value associated with key; None otherwise
Returns the value associated with key; alt otherwise
As we saw earlier with strings and lists, dictionary methods use dot notation, which specifies the name of the method
to the right of the dot and the name of the object on which to apply the method immediately to the left of the dot.
For example, if
x is a variable
whose value is a dictionary,
x.keys is the method object, and
x.keys() invokes the method, returning a view of
11.4.1. Iterating over Dictionaries¶
There are three ways to iterate over the contents of a dictionary. Let’s take a moment to examine them.
The first technique involves iterating over the keys of the dictionary using the
keys method returns a collection of the keys in the dictionary.
Note the first line of the for loop:
for akey in inventory.keys():
Each time through the loop, the loop variable
akey is assigned a different key in the dictionary. In the loop body,
the value associated with the key is accessed by indexing the dictionary with
akey using the expression
inventory[akey]. Note that the order in which the keys are assigned in the loop is not predictable. If you want to
visit the keys in alphabetic order, you must use the
sorted function to produce a sorted collection of keys, like this:
for akey in sorted(inventory.keys()):
It’s so common to iterate over the keys in a dictionary that you can
keys method call in the
for loop — iterating over
a dictionary implicitly iterates over its keys.
values method returns a collection of the values in the dictionary. Here’s an example
that displays a list of the values:
items method returns a collection of tuples containing each key and its associated value.
Take a look at this example that iterates over the dictionary using the
Take a close look at the first line of the for loop:
for k, v in inventory.items():
Each time through the loop,
k receives a key from the dictionary, and
v receives its associated
value. That avoids the need to index the dictionary inside the loop body to access the value associated
with the key.
You may have noticed in the examples above that, to print the result of the
items() methods, we used lines like this:
instead of this:
items() don’t return actual lists. Like the
range function described
previously, they return objects that produce the items one at a time, rather than producing and
storing all of them in advance as a list. If you need to perform an operation on the result of one of these methods such as
extracting the first item, you must convert the result to a list using the
list conversion function. For example, if you want to get the first key,
this won’t work:
inventory.keys(). You need to make the collection of keys into a real list before using
 to index into it:
11.4.2. Safely Retrieving Values¶
Looking up a value in a dictionary is a potentially dangerous operation. When using the
 operator to access
a key, if the key is not present, a runtime error occurs. There are two ways to deal with this problem.
The first approach is to use the
not in operators, which can test if a key is in the dictionary:
The second approach is to use the
get retrieves the value associated with a key, similar to the
 operator. The important
difference is that
get will not cause a runtime error if the key is not present. It will instead return the value
There exists a variation of
get that allows a second parameter that serves as an alternative return value in the
case where the key is not present. This can be seen in the final example below. In this case, since “cherries” is not
get returns 0 (instead of None).
Check your understanding
5. We have a dictionary of the specific events that Italy has won medals in and the number of medals they have won for each event. Assign to the variable
events a list of the keys from the dictionary
medal_events. Use a dictionary method and cast to a list; do not hard code or accumulate a list via iteration.
Add the following line:
events = list(medal_events.keys())