The document provides an introduction to dictionaries in Python, explaining their structure as key-value pairs and how to create, access, modify, and remove items. It covers various operations such as using the subscript operator, handling default values, and avoiding errors during removal. Additionally, it outlines key differences between dictionaries, lists, and sets.

1. An Introduction To Software
Development Using Python
Spring Semester, 2015
Class #22:
Dictionaries

2. What Is A Dictionary?
• A dictionary is a container that keeps
associations between keys and values.
• Keys are unique, but a value may be
associated with several keys.
John
Mike
Ann
Mary
$15,000
$12,000
$30,000
Keys Values
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3. What Is A Dictionary?
• The dictionary structure is also known as a
map because it maps a unique key to a value.
• It stores the keys, values, and the associations
between them.
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4. Creating Dictionaries
• Each key/value pair is separated by a colon.
• You enclose the key/value pairs in braces,
just as you would when forming a set.
• When the braces contain key/value pairs, they
denote a dictionary, not a set.
• The only ambiguous case is an empty {}. By convention, it denotes an
empty dictionary, not an empty set.
• You can create a duplicate copy of a dictionary using the dict function:
oldSalaries = dict(salaries)
salaries = { “John": 15000, “Ann": 30000, “Mike": 12000, “Mary": 15000 }
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5. Accessing Dictionary Values
• The subscript operator [] is used to return the value associated with a key.
• The statement:
print(“Ann’s salary is", salaries[“Ann"])
prints 30000
• Note that the dictionary is not a sequence-type container like a list.
• Even though the subscript operator is used with a dictionary, you cannot
access the items by index or position.
• A value can only be accessed using its associated key
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6. Searching For Keys
• The key supplied to the subscript operator must be a valid key
in the dictionary or a KeyError exception will be raised.
• To find out whether a key is present in the dictionary, use the
in (or not in) operator:
if “Ann" in salaries :
print(“Ann’s salary is", salaries[“Ann”])
else :
print(“Ann’s salary is not in my list.”)
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7. Default Value
• Often, you want to use a default value if a key is not present.
For example, if there is no salary for Mike, you want to get an
average salary instead.
• Instead of using the in operator, you can simply call the get
method and pass the key and a default value. The default
value is returned if there is no matching key.
number = salaries.get(“Mike", 17000)
print(“Salary: " + number)
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8. Adding and Modifying Items
• You can change a dictionary’s contents after it has been
created.
• You can add a new item using the subscript operator [] much
as you would with a list:
salaries["Lisa"] = 25000
• To change the value associated with a given key, set a new
value using the [] operator on an existing key:
salaries["Lisa"] = 17000
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9. Another Way To Create
A Dictionary
• Sometimes you may not know which items will be contained
in the dictionary when it’s created.
• You can create an empty dictionary like this:
salaries = {}
• and add new items as needed:
salaries["John"] = 15000
salaries["Ann"] = 30000
salaries["Mike"] = 12000
salaries["Mary"] = 15000
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10. Removing Items
• To remove an item from a dictionary, call the pop method
with the key as the argument:
salaries.pop(“Mike")
• This removes the entire item, both the key and its associated
value.
• The pop method returns the value of the item being removed,
so you can use it or store it in a variable:
mikesSalary = salaries.pop(“Mike")
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11. Avoiding Removal Errors
• If the key is not in the dictionary, the pop method raises a
KeyError exception.
• To prevent the exception from being raised, you can test for
the key in the dictionary:
if "Mike" in salaries :
contacts.pop("Mike")
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12. Traversing a Dictionary
• You can iterate over the individual keys in a dictionary using a
for loop:
print(“Salaries:")
for key in salaries :
print(key)
• The result of this code fragment is shown below:
Salaries:
John
Ann
Mike
Mary
• Note that the dictionary stores its items in an order that is optimized for
efficiency, which may not be the order in which they were added.
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13. Different Ways Of Doing
The Same Thing
• Lists
– prerequisites = [“COP 2271c”, “Introduction to Computation
and Programming”, 3]
– print(values[5]) # Prints the element at index 5
• Sets
– cheesePizza = {“Creamy garlic”, “Parmesan sauce”,
“Cheese”, “Toasted Parmesan”}
– if "Toasted Parmesan" in cheesePizza :
• Dictionaries
– salaries = {"John": 15000, "Ann": 30000, "Mike": 12000,
"Mary": 15000 }
– print("Ann’s salary is", salaries["Ann"])
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14. What’s In Your Python Toolbox?
print() math strings I/O IF/Else elif While For
DictionaryLists And/Or/Not Functions Files ExceptionSets

15. What We Covered Today
1. Creating Dictionaries
2. Accessing Dictionaries
3. Searching Dictionaries
4. Removing Items
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16. What We’ll Be Covering Next Time
1. External Libraries
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