Python Programming/Dictionaries

This lesson introduces Python dictionaries.

Objectives and Skills

Objectives and skills for this lesson include:

• Data Structures
  • Dictionary

Readings

1. Wikipedia: Dictionary (data structure)
2. PythonLearn:
   • Python 2: Dictionaries
   • Python 3: Dictionaries

Multimedia

1. YouTube: Python for Informatics - Chapter 9 - Dictionaries

Examples

Dictionaries

Dictionaries are collections that are indexed by keys, which can be any immutable type. It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary). Placing a comma-separated list of key:value pairs within braces adds initial key:value pairs to the dictionary.

dictionary = {
    'cat': 'Frisky',
    'dog': 'Spot',
    'fish': 'Bubbles',
}

print("dictionary:", dictionary)
print("dictionary['cat']:", dictionary['cat'])
print("dictionary['dog']:", dictionary['dog'])
print("dictionary['fish']:", dictionary['fish'])

Output:
dictionary: {'dog': 'Spot', 'fish': 'Bubbles', 'cat': 'Frisky'}
dictionary['cat']: Frisky
dictionary['dog']: Spot
dictionary['fish']: Bubbles

Dictionary Updates

A pair of braces creates an empty dictionary: {}. Dictionary items may be added, updated, and deleted by key.

dictionary = {}
dictionary['cat'] = 'Frisky'
dictionary['dog'] = 'Spot'
dictionary['fish'] = 'Bubbles'
print("dictionary:", dictionary)

dictionary['dog'] = 'Rover'
print("dictionary:", dictionary)

del dictionary['fish']
print("dictionary:", dictionary)

Output:
dictionary: {'cat': 'Frisky', 'fish': 'Bubbles', 'dog': 'Spot'}
dictionary: {'cat': 'Frisky', 'fish': 'Bubbles', 'dog': 'Rover'}
dictionary: {'cat': 'Frisky', 'dog': 'Rover'}

Multidimensional Dictionaries

Dictionaries may be nested.

dictionary = {
    'John': {'name': 'John Smith', 'phone': '123-555-0101'}, 
    'Jose': {'name': 'Jose Garcia', 'phone': '123-555-0102'}
}

print("John's name:", dictionary['John']['name'])
print("Jose's phone:",dictionary['Jose']['phone'])

Output:
John's name: John Smith
Jose's phone: 123-555-0102

Looping

Dictionary items may be accessed with a for loop.

dictionary = {
    'cat': 'Frisky',
    'dog': 'Spot',
    'fish': 'Bubbles',
}

for key in dictionary:
    print("dictionary[%s]: %s" % (key, dictionary[key]))

Output:
dictionary[dog]: Spot
dictionary[fish]: Bubbles
dictionary[cat]: Frisky

Sorting by Key

Dictionaries are unordered. Dictionary items may be displayed in key order by sorting a list of keys using the sorted() function and the dictionary.keys() method.

dictionary = {
    'cat': 'Frisky',
    'dog': 'Spot',
    'fish': 'Bubbles',
}

for key in sorted(dictionary.keys()):
    print("dictionary[%s]: %s" % (key, dictionary[key]))

Output:
dictionary[cat]: Frisky
dictionary[dog]: Spot
dictionary[fish]: Bubbles

Sorting by Value

Dictionaries are unordered. Dictionary items may be displayed in value order by sorting a list of key-value pairs using the sorted() function and the dictionary.get() method.

dictionary = {
    'cat': 'Frisky',
    'dog': 'Spot',
    'fish': 'Bubbles',
}

for key in sorted(dictionary, key=dictionary.get):
    print("dictionary[%s]: %s" % (key, dictionary[key]))

Output:
dictionary[fish]: Bubbles
dictionary[cat]: Frisky
dictionary[dog]: Spot

Dictionary as Return Value

Functions may return multiple values using a dictionary.

def function(x, y):
    ...
    return {'x': x, 'y': y}

Activities

Tutorials

1. Complete one or more of the following tutorials:
   • LearnPython
     • Dictionaries
   • TutorialsPoint
     • Dictionary
   • Codecademy
     • Lists and Dictionaries
   • Wikiversity
     • Python/Dictionaries
   • Wikibooks
     • Wikibooks: A Beginner's Python Tutorial/Tuples, Lists, Dictionaries

Practice

1. Create a Python program that asks the user to enter grade scores. Start by asking the user how many scores they would like to enter. Then use a loop to request each score and add it to a dictionary, counting how many times each score is entered. Finally, use a function to display the dictionary of entered scores sorted in descending order and a histogram showing how many times each score was entered. Include try and except to handle input errors. For example:
       100: *
    90: ***
    80: *****
    70: **
2. Create a Python program that asks the user to enter a line of comma-separated grade scores. Use the string split() method to parse the line and add each score to a dictionary, counting how many times each score is entered. Finally, use a function to display the list of entered scores sorted in descending order and a histogram showing how many times each score was entered. Include try and except to handle input errors. For example:
       100: *
    90: ***
    80: *****
    70: **
3. Create a Python program that asks the user to enter a line of text. Use the string split() method to parse the line and add each word to a dictionary, counting how many times each word is entered. Convert words to lower case and remove any punctuation to prevent duplicate or invalid results. Finally, use a function to display the list of entered words sorted in alphabetical order and a histogram showing how many times each word was entered. Include try and except to handle input errors. For example:
       cat: *
    hat: *
    in: *
    the: **
4. Create a Python program that asks the user for a line of text that contains HTML tags, such as:
       <p><strong>This is a bold paragraph.</strong></p>
   Use string methods to search for and remove all HTML tags from the text, saving each removed tag in a dictionary. Print the untagged text and then use a function to display the list of removed tags sorted in alphabetical order and a histogram showing how many times each tag was used. Include error handling in case an HTML tag isn't entered correctly (an unmatched < or >). Use a user-defined function for the actual string processing, separate from input and output. For example:
       </p>: *
    </strong>: *
    <p>: *
    <strong>: *

Lesson Summary

Dictionary Concepts

• A dictionary or associative array is a data type composed of a collection of (key: value) pairs, such that each possible key appears at most once in the collection.
• Dictionary operations include:
  • the addition of a pair to the collection
  • the removal of a pair from the collection
  • the modification of an existing pair
  • the lookup of a value associated with a particular key
• Dictionaries are typically implemented in programming languages as either a hash table or a search tree.
• A hash table uses a hash function to compute an index into an array of buckets or slots, from which the desired value can be found.
• A hash function is any function that can be used to map data of arbitrary size to data of fixed size.
• A good hash function should map the expected inputs as evenly as possible over its output range.

Python Dictionaries

• Dictionaries are collections that are indexed by keys, which can be any immutable type.
• It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary).
• Placing a comma-separated list of key:value pairs within braces adds initial key:value pairs to the dictionary using the syntax dictionary = {key: value, key: value}.
• A pair of braces creates an empty dictionary: {}. Dictionary items may be added, updated, and deleted by key using the syntax dictionary[key] = value and del dictionary[key].
• Dictionaries may be nested and are accessed using the syntax dictionary[key][subkey]
• Dictionary items may be accessed with a for loop.
• Dictionaries are unordered. Dictionary items may be displayed in key order by sorting a list of keys using the sorted() function and the dictionary.keys() method.
• Dictionaries are unordered. Dictionary items may be displayed in value order by sorting a list of key-value pairs using the sorted() function and the dictionary.get() method.

Key Terms

dictionary

A mapping from a set of keys to their corresponding values.

hashtable

The algorithm used to implement Python dictionaries.

hash function

A function used by a hashtable to compute the location for a key.

histogram

A set of counters.

implementation

A way of performing a computation.

item

Another name for a key-value pair.

key

An object that appears in a dictionary as the first part of a key-value pair.

key-value pair

The representation of the mapping from a key to a value.

lookup

A dictionary operation that takes a key and finds the corresponding value.

nested loops

When there are one or more loops “inside” of another loop. The inner loop runs to completion each time the outer loop runs once.

value

An object that appears in a dictionary as the second part of a key-value pair. This is more specific than our previous use of the word “value”.

Review Questions

1. A dictionary or associative array is _____.

   A dictionary or associative array is a data type composed of a collection of (key: value) pairs, such that each possible key appears at most once in the collection.
2. Dictionary operations include:

   Dictionary operations include:
   the addition of a pair to the collection
   the removal of a pair from the collection
   the modification of an existing pair
   the lookup of a value associated with a particular key
3. Dictionaries are typically implemented in programming languages as _____.

   Dictionaries are typically implemented in programming languages as either a hash table or a search tree.
4. A hash table uses _____.

   A hash table uses a hash function to compute an index into an array of buckets or slots, from which the desired value can be found.
5. A hash function is _____.

   A hash function is any function that can be used to map data of arbitrary size to data of fixed size.
6. A good hash function should _____.

   A good hash function should map the expected inputs as evenly as possible over its output range.
7. Python dictionaries are _____.

   Python dictionaries are collections that are indexed by keys, which can be any immutable type.
8. It is best to think of a dictionary as _____.

   It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary).
9. Placing a comma-separated list of key:value pairs within braces adds initial key:value pairs to the dictionary using the syntax _____.

   {{{2}}}
10. A pair of braces creates an empty dictionary: {}. Dictionary items may be added, updated, and deleted by key using the syntax _____.

    {{{2}}}
11. Dictionaries may be nested and are accessed using the syntax _____

    Dictionaries may be nested and are accessed using the syntax dictionary[key][subkey]
12. Dictionary items may be accessed with a _____ loop.

    Dictionary items may be accessed with a for loop.
13. Dictionaries are unordered. Dictionary items may be displayed in key order by _____.

    Dictionaries are unordered. Dictionary items may be displayed in key order by sorting a list of keys using the sorted() function and the dictionary.keys()method.
14. Dictionaries are unordered. Dictionary items may be displayed in value order by _____.

    Dictionaries are unordered. Dictionary items may be displayed in value order by sorting a list of key-value pairs using the sorted() function and the dictionary.get() method.

Assessments

• Flashcards: Quizlet: Python Dictionaries
• Flashcards: Quizlet: Python Dictionary Methods
• Quiz: Quizlet: Python Dictionaries
• Quiz: Quizlet: Python Dictionary Methods

See Also

• Python.org: Data Structures
• SoloLearn: Python