Published

2025-03-31

Open In Colab

Dictionaries

Recall our plan: Go through common data types and

  • Learn how to create them
  • Consider commonly used functions and methods
  • See control flow and other tricks along the way

This topic, compound objects: - Dictionaries

Dictionaries are data objects that have key-value pairs associated with it

  • keys must be unique
  • keys can be any immutable object (containing no mutable elements)

Dictionaries are a flexible data type that is

  • unordered
  • mutable

Note: These types of webpages are built from Jupyter notebooks (.ipynb files). You can access your own versions of them by clicking here. It is highly recommended that you go through and run the notebooks yourself, modifying and rerunning things where you’d like!

Creating a Dictionary

Creating a dictionary using dict() or {}

  • Use dict(supply_key_value_pairs) or {supply_key_value_pairs}
{} #empty dictionary
mydict = {
  "key1": [12, -10, "value1"],  #key is specified first (must be immutable)
  "key2": [11, "value2"],       #value is then given after a : (can be anything, doesn't need to match across keys)
  "key3": "value3"
  }
mydict
{'key1': [12, -10, 'value1'], 'key2': [11, 'value2'], 'key3': 'value3'}
mydict2 = dict([
  (1, ['hee', 'haw']),        #passing key value pairs as a tuple
  (2, 'fa')
  ])
mydict2
{1: ['hee', 'haw'], 2: 'fa'}

Creating a dictionary using lists

  • You can create a dictionary using two lists and the zip() function
keys = [x for x in "abcdefgh"]
values = [y for y in range(0,8)]
print(keys)
print(values)
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
[0, 1, 2, 3, 4, 5, 6, 7]
dict(zip(keys, values))
{'a': 0, 'b': 1, 'c': 2, 'd': 3, 'e': 4, 'f': 5, 'g': 6, 'h': 7}

Creating a dictionary with dictionary comprehensions

  • Can create a dictionary using dictionary comprehensions!
  • Smilar to list comprehensions but we use { instead of [
mydict = {i: i for i in range(0, 6)}
mydict
{0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5}
mydict = {"abcdef"[i]: i**2 for i in range(0, 6)}
mydict
{'a': 0, 'b': 1, 'c': 4, 'd': 9, 'e': 16, 'f': 25}

Dictionary Operations

Indexing a Dictionary

  • Index with a [key] (remember unordered!)
AFCDivisions = {
  "North": ["Steelers", "Browns", "Ravens", "Bengals"],
  "East" : ["Patriots", "Jets", "Dolphins", "Bills"],
  "West" : ["Raiders", "Chiefs", "Chargers", "Broncos"]
  }
AFCDivisions["North"]
['Steelers', 'Browns', 'Ravens', 'Bengals']
  • You can access the returned object in the same line of code
AFCDivisions["North"][0]
'Steelers'
  • We can add new key/value pairs by simply referencing a key that doesn’t exist
  • Here we add the “South” key with a silly value
AFCDivisions = {
  "North": ["Steelers", "Browns", "Ravens", "Bengals"],
  "East" : ["Patriots", "Jets", "Dolphins", "Bills"],
  "West" : ["Raiders", "Chiefs", "Chargers", "Broncos"]
  }
AFCDivisions["South"] = [1, 2]
AFCDivisions
{'North': ['Steelers', 'Browns', 'Ravens', 'Bengals'],
 'East': ['Patriots', 'Jets', 'Dolphins', 'Bills'],
 'West': ['Raiders', 'Chiefs', 'Chargers', 'Broncos'],
 'South': [1, 2]}
  • You can iterate over the keys in a dictionary
for key in AFCDivisions:              #keys are what are iterated over
    print(key, ' : ', AFCDivisions[key]) #we access the value associated with the key
North  :  ['Steelers', 'Browns', 'Ravens', 'Bengals']
East  :  ['Patriots', 'Jets', 'Dolphins', 'Bills']
West  :  ['Raiders', 'Chiefs', 'Chargers', 'Broncos']
South  :  [1, 2]
  • You can overwrite the values similar to how you can add a key/value pair after the fact. Here we overwrite the “South” value.
AFCDivisions = {
  "North": ["Steelers", "Browns", "Ravens", "Bengals"],
  "East" : ["Patriots", "Jets", "Dolphins", "Bills"],
  "West" : ["Raiders", "Chiefs", "Chargers", "Broncos"]
  }
AFCDivisions["South"] = [1, 2]
AFCDivisions["South"] = ["Texans", "Colts", "Jaguars", "Titans"]
for key in AFCDivisions:
    print(key, ' : ', AFCDivisions[key])
North  :  ['Steelers', 'Browns', 'Ravens', 'Bengals']
East  :  ['Patriots', 'Jets', 'Dolphins', 'Bills']
West  :  ['Raiders', 'Chiefs', 'Chargers', 'Broncos']
South  :  ['Texans', 'Colts', 'Jaguars', 'Titans']

Dictionary Packing & Unpacking

  • We can pack dictionaries using ** similar to how we packed a list!
  • Here we create a dictionary called Divisions where we pack two dictionaries inside it
AFCDivisions = {
  "AFCNorth": ["Steelers", "Browns", "Ravens", "Bengals"],
  "AFCEast" : ["Patriots", "Jets", "Dolphins", "Bills"],
  "AFCWest" : ["Raiders", "Chiefs", "Chargers", "Broncos"],
  "AFCSouth": ["Texans", "Colts", "Jaguars", "Titans"]
  }

NFCDivisions = {
  "NFCNorth" : ["Lions", "Bears", "Packers", "Vikings"],
  "NFCEast"  : ["Giants", "Cowboys", "Eagles", "Admirals"]
}

Divisions = {**AFCDivisions, **NFCDivisions}
for key in Divisions:
    print(key, ' : ', Divisions[key])
AFCNorth  :  ['Steelers', 'Browns', 'Ravens', 'Bengals']
AFCEast  :  ['Patriots', 'Jets', 'Dolphins', 'Bills']
AFCWest  :  ['Raiders', 'Chiefs', 'Chargers', 'Broncos']
AFCSouth  :  ['Texans', 'Colts', 'Jaguars', 'Titans']
NFCNorth  :  ['Lions', 'Bears', 'Packers', 'Vikings']
NFCEast  :  ['Giants', 'Cowboys', 'Eagles', 'Admirals']

Dictionary Methods

Many useful methods

  • We saw how to index with [] similar to lists/tuples
  • We can index with .get() instead
    • Advantage is that it doesn’t throw an error if the key doesn’t exist
AFCDivisions = {
  "North": ["Steelers", "Browns", "Ravens", "Bengals"],
  "East" : ["Patriots", "Jets", "Dolphins", "Bills"],
  "West" : ["Raiders", "Chiefs", "Chargers", "Broncos"],
  "South": ["Texans", "Colts", "Jaguars", "Titans"]
  }
AFCDivisions.get("South")
['Texans', 'Colts', 'Jaguars', 'Titans']
AFCDivisions.get("Northeast") #doesn't throw an error
AFCDivisions["Northeast"] #throws an error
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-15-4c30b3284c03> in <cell line: 1>()
----> 1 AFCDivisions["Northeast"] #throws an error

KeyError: 'Northeast'
  • Return keys with .keys(); values with .values()
AFCDivisions = {
  "North": ["Steelers", "Browns", "Ravens", "Bengals"],
  "East" : ["Patriots", "Jets", "Dolphins", "Bills"],
  "West" : ["Raiders", "Chiefs", "Chargers", "Broncos"],
  "South": ["Texans", "Colts", "Jaguars", "Titans"]
  }
AFCDivisions.keys()
dict_keys(['North', 'East', 'West', 'South'])
AFCDivisions.values()
dict_values([['Steelers', 'Browns', 'Ravens', 'Bengals'], ['Patriots', 'Jets', 'Dolphins', 'Bills'], ['Raiders', 'Chiefs', 'Chargers', 'Broncos'], ['Texans', 'Colts', 'Jaguars', 'Titans']])
  • Return and remove a specified key with .pop() (similar to the list method)
AFCDivisions = {
  "North": ["Steelers", "Browns", "Ravens", "Bengals"],
  "East" : ["Patriots", "Jets", "Dolphins", "Bills"],
  "West" : ["Raiders", "Chiefs", "Chargers", "Broncos"],
  "South": ["Texans", "Colts", "Jaguars", "Titans"]
  }
AFCDivisions.pop("North") #modifies the dictionary
['Steelers', 'Browns', 'Ravens', 'Bengals']
for key in AFCDivisions:
    print(key, ' : ', AFCDivisions[key]) #North no longer exists!
East  :  ['Patriots', 'Jets', 'Dolphins', 'Bills']
West  :  ['Raiders', 'Chiefs', 'Chargers', 'Broncos']
South  :  ['Texans', 'Colts', 'Jaguars', 'Titans']
  • Merge in another dictionary with .update()
  • Very similar to dictionary packing done above but it modifies one of the dictionaries
Divisions = {
  "AFCNorth": ["Steelers", "Browns", "Ravens", "Bengals"],
  "AFCEast" : ["Patriots", "Jets", "Dolphins", "Bills"],
  "AFCWest" : ["Raiders", "Chiefs", "Chargers", "Broncos"],
  "AFCSouth": ["Texans", "Colts", "Jaguars", "Titans"]
  }
NFCNorth = {
  "NFCNorth": ["Lions", "Packers", "Bears", "Vikings"]
  }
Divisions.update(NFCNorth) #combine the dictionaries
for key in Divisions.keys():
    print(key, " : ", Divisions[key])
AFCNorth  :  ['Steelers', 'Browns', 'Ravens', 'Bengals']
AFCEast  :  ['Patriots', 'Jets', 'Dolphins', 'Bills']
AFCWest  :  ['Raiders', 'Chiefs', 'Chargers', 'Broncos']
AFCSouth  :  ['Texans', 'Colts', 'Jaguars', 'Titans']
NFCNorth  :  ['Lions', 'Packers', 'Bears', 'Vikings']

Video Demo

This quick video demo shows an example of creating dictionaries with lists as the values, creating with tuples as the keys, and checking if some values occur in the dictionary using in and/or not in. Remember to pop the video out into the full player.

The notebook written in the video is available here.

from IPython.display import IFrame
IFrame(src="https://ncsu.hosted.panopto.com/Panopto/Pages/Embed.aspx?id=b270f285-6973-4e6b-aeae-b0f800f38b96&autoplay=false&offerviewer=true&showtitle=true&showbrand=true&captions=false&interactivity=all", height="405", width="720")

Recap

  • Dictionaries have key value pairs

    • Keys can be any immutable object

    • Values can be pretty generic using something like a list

  • Mutable

  • Unordered - access via [key] or using .get() method

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