Python Dictionaries and Sets

Copyright © 2015 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
C HA PT E R 9
Dictionaries
and Sets

Topics
Dictionaries
Sets
Serializing Objects

Dictionaries
Dictionary: object that stores a
collection of data
Each element consists of a key and a value
Often referred to as mapping of key to value
Key must be an immutable object
To retrieve a specific value, use the key
associated with it
Format for creating a dictionary
dictionary =
{key1:val1, key2:val2}

Retrieving a Value from a
Dictionary
Elements in dictionary are unsorted
General format for retrieving value from
dictionary: dictionary[key]
If key in the dictionary, associated value is
returned, otherwise, KeyError exception is
raised
Test whether a key is in a dictionary
using the in and not in operators
Helps prevent KeyError exceptions

Adding Elements to an
Existing Dictionary
Dictionaries are mutable objects
To add a new key-value pair:
dictionary[key] = value
If key exists in the dictionary, the value
associated with it will be changed

Deleting Elements From an
Existing Dictionary
To delete a key-value pair:
del dictionary[key]
If key is not in the dictionary, KeyError
exception is raised

Getting the Number of Elements
and Mixing Data Types
len function: used to obtain number of
elements in a dictionary
Keys must be immutable objects, but
associated values can be any type of
object
One dictionary can include keys of several
different immutable types
Values stored in a single dictionary can
be of different types

Creating an Empty Dictionary and
Using for Loop to Iterate Over a
Dictionary
To create an empty dictionary:
Use {}
Use built-in function dict()
Elements can be added to the dictionary as
program executes
Use a for loop to iterate over a
dictionary
General format: for key in dictionary:

Some Dictionary Methods
clear method: deletes all the elements
in a dictionary, leaving it empty
Format: dictionary.clear()
get method: gets a value associated
with specified key from the dictionary
Format: dictionary.get(key, default)
default is returned if key is not found
Alternative to [] operator
Cannot raise KeyError exception

(cont’d.)
items method: returns all the
dictionaries keys and associated
values
Format: dictionary.items()
Returned as a dictionary view
Each element in dictionary view is a tuple which
contains a key and its associated value
Use a for loop to iterate over the tuples in the
sequence
Can use a variable which receives a tuple, or can use
two variables which receive key and value

(cont’d.)
keys method: returns all the
dictionaries keys as a sequence
Format: dictionary.keys()
pop method: returns value associated
with specified key and removes that
key-value pair from the dictionary
Format: dictionary.pop(key, default)
default is returned if key is not found

(cont’d.)
popitem method: returns a randomly
selected key-value pair and removes
that key-value pair from the dictionary
Format: dictionary.popitem()
Key-value pair returned as a tuple
values method: returns all the
dictionaries values as a sequence
Format: dictionary.values()
Use a for loop to iterate over the values

(cont’d.)

Sets
Set: object that stores a collection of
data in same way as mathematical set
All items must be unique
Set is unordered
Elements can be of different data types

Creating a Set
set function: used to create a set
For empty set, call set()
For non-empty set, call set(argument)
where argument is an object that contains
iterable elements
e.g., argument can be a list, string, or tuple
If argument is a string, each character becomes a
set element
For set of strings, pass them to the function as a list
If argument contains duplicates, only one of the
duplicates will appear in the set

Getting the Number of and
Adding Elements
len function: returns the number of
elements in the set
Sets are mutable objects
add method: adds an element to a set
update method: adds a group of
elements to a set
Argument must be a sequence containing
iterable elements, and each of the elements is
added to the set

Deleting Elements From a Set
remove and discard methods: remove
the specified item from the set
The item that should be removed is passed to
both methods as an argument
Behave differently when the specified item is
not found in the set
remove method raises a KeyError exception
discard method does not raise an exception
clear method: clears all the elements
of the set

Using the for Loop, in, and
not in Operators With a Set
A for loop can be used to iterate over
elements in a set
General format: for item in set:
The loop iterates once for each element in the
set
The in operator can be used to test
whether a value exists in a set
Similarly, the not in operator can be used to
test whether a value does not exist in a set

Finding the Union of Sets
Union of two sets: a set that contains
all the elements of both sets
To find the union of two sets:
Use the union method
Format: set1.union(set2)
Use the | operator
Format: set1 | set2
Both techniques return a new set which
contains the union of both sets

Finding the Intersection of
Sets
Intersection of two sets: a set that
contains only the elements found in
both sets
To find the intersection of two sets:
Use the intersection method
Format: set1.intersection(set2)
Use the & operator
Format: set1 & set2
Both techniques return a new set which
contains the intersection of both sets

Finding the Difference of Sets
Difference of two sets: a set that
contains the elements that appear in
the first set but do not appear in the
second set
To find the difference of two sets:
Use the difference method
Format: set1.difference(set2)
Use the - operator
Format: set1 - set2

Finding the Symmetric
Difference of Sets
Symmetric difference of two sets: a set
that contains the elements that are not
shared by the two sets
To find the symmetric difference of two
sets:
Use the symmetric_difference method
Format: set1.symmetric_difference(set2)
Use the ^ operator
Format: set1 ^ set2

Finding Subsets and
Supersets
Set A is subset of set B if all the
elements in set A are included in set B
To determine whether set A is subset
of set B
Use the issubset method
Format: setA.issubset(setB)
Use the <= operator
Format: setA <= setB

Finding Subsets and
Supersets (cont’d.)
Set A is superset of set B if it contains
all the elements of set B
To determine whether set A is superset
of set B
Use the issuperset method
Format: setA.issuperset(setB)
Use the >= operator
Format: setA >= setB

Serializing Objects
Serialize an object: convert the object
to a stream of bytes that can easily be
stored in a file
Pickling: serializing an object

Serializing Objects (cont’d.)
To pickle an object:
Import the pickle module
Open a file for binary writing
Call the pickle.dump function
Format: pickle.dump(object, file)
Close the file
You can pickle multiple objects to one
file prior to closing the file

Serializing Objects (cont’d.)
Unpickling: retrieving pickled object
To unpickle an object:
Import the pickle module
Open a file for binary writing
Call the pickle.load function
Format: pickle.load(file)
Close the file
You can unpickle multiple objects from
the file

Summary
This chapter covered:
Dictionaries, including:
Creating dictionaries
Inserting, retrieving, adding, and deleting key-value
pairs
for loops and in and not in operators
Dictionary methods

Summary (cont’d.)
This chapter covered (cont’d):
Sets:
Creating sets
Adding elements to and removing elements from
sets
Finding set union, intersection, difference and
symmetric difference
Finding subsets and supersets
Serializing objects
Pickling and unpickling objects

Python Dictionaries and Sets

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