In this Python dictionary tutorial, you will learn how to use dictionary methods to write some cool programs such as a phone book program.

Also, you will get the source code of all the programs in this tutorial.

Please note,

Let’s get to business now.

What is a dictionary in Python?

Python dictionary associates a collection of keys with data values.

Unlike list in Python, a dictionary organizes information by association, not position.

That’s to say that, the key-value pairs are arranged in no particular order.

The key-value pairs in a dictionary are separated by a comma.

A colon (:) separates a key and its associated value.

I would love to refer these key-value pairs as entries.

The key-value pairs or entries are enclosed in curly braces ({ }).

Python dictionary example

In a new file editor window, type the following code snippet and save it as Dictionary_example.py

Dictionary example Python

my_profile = {"name":"Godson","gender":"male","status":"single"}
constants={"k":1.3805e-23,"h":6.625e-34,"c":2.998e08}
student_info={"name":"Oscar","dept":"computer science","level":300}
empty_dict={}
print("My profile:",my_profile,"Engineering constants:",constants,sep="\n")
print("Student detail:",student_info,"Empty dictionary:",empty_dict,sep="\n")

123456

my_profile={"name":"Godson","gender":"male","status":"single"}constants={"k":1.3805e-23,"h":6.625e-34,"c":2.998e08}student_info={"name":"Oscar","dept":"computer science","level":300}empty_dict={}print("My profile:",my_profile,"Engineering constants:",constants,sep="\n")print("Student detail:",student_info,"Empty dictionary:",empty_dict,sep="\n")

Running Dictionary_example.py you will get:

Python

>>> 
 RESTART: C:\Users\RAPTURE C. GODSON\Documents\Python Dictionary\Dictionary_example.py 
My profile:
{'name': 'Godson', 'gender': 'male', 'status': 'single'}
Engineering constants:
{'k': 1.3805e-23, 'h': 6.625e-34, 'c': 299800000.0}
Student detail:
{'name': 'Oscar', 'dept': 'computer science', 'level': 300}
Empty dictionary:
{}
>>>

1234567891011

>>>RESTART:C:\Users\RAPTUREC.GODSON\Documents\Python Dictionary\Dictionary_example.py My profile:{'name':'Godson','gender':'male','status':'single'}Engineering constants:{'k':1.3805e-23,'h':6.625e-34,'c':299800000.0}Student detail:{'name':'Oscar','dept':'computer science','level':300}Empty dictionary:{}>>>

Note the following:

The keys in a dictionary can be any immutable Python data type(tuple)
Python dictionary keys can be strings or integers.
The associated value can be any Python data type.

In a new file editor window, type the following code snippet and save it as Dictionary_example2.py

Dictionary_example2 Python

x1={('chemistry','physics'):'science_subjects',('french','literature'):'art_subjects'}
x={'science_subjects':['chemistry','physics'],'art_subjects':['french','literature']}
s={'fruits':('apple','banana','strawberry'),'drinks':('coke','pepsi','fanta')}
num={6:'six',7:'seven',8:'eight'}
print ("subjects:",x,"My favorite fruits and drinks:",s,sep="\n")
print("Numbers:",num)
print (x1)

1234567

x1={('chemistry','physics'):'science_subjects',('french','literature'):'art_subjects'}x={'science_subjects':['chemistry','physics'],'art_subjects':['french','literature']}s={'fruits':('apple','banana','strawberry'),'drinks':('coke','pepsi','fanta')}num={6:'six',7:'seven',8:'eight'}print("subjects:",x,"My favorite fruits and drinks:",s,sep="\n")print("Numbers:",num)print(x1)

Running Dictionary_example2.py you will get:

Python

>>> 
 RESTART: C:\Users\RAPTURE C. GODSON\Documents\Python Dictionary\Dictionary_example2.py 
subjects:
{'science_subjects': ['chemistry', 'physics'], 'art_subjects': ['french', 'literature']}
My favorite fruits and drinks:
{'fruits': ('apple', 'banana', 'strawberry'), 'drinks': ('coke', 'pepsi', 'fanta')}
Numbers: {6: 'six', 7: 'seven', 8: 'eight'}
{('chemistry', 'physics'): 'science_subjects', ('french', 'literature'): 'art_subjects'}
>>>

123456789

>>>RESTART:C:\Users\RAPTUREC.GODSON\Documents\Python Dictionary\Dictionary_example2.py subjects:{'science_subjects':['chemistry','physics'],'art_subjects':['french','literature']}My favorite fruits anddrinks:{'fruits':('apple','banana','strawberry'),'drinks':('coke','pepsi','fanta')}Numbers:{6:'six',7:'seven',8:'eight'}{('chemistry','physics'):'science_subjects',('french','literature'):'art_subjects'}>>>

Python will raise a TypeError error if your keys are in list form.

Take a look:

Python

>>> x={['chemistry','physics']:'science_subjects',['french','literature']:'art_subjects'}
Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    x={['chemistry','physics']:'science_subjects',['french','literature']:'art_subjects'}
TypeError: unhashable type: 'list'
>>>

123456

>>>x={['chemistry','physics']:'science_subjects',['french','literature']:'art_subjects'}Traceback(most recent call last):File"<pyshell#4>",line1,in<module>x={['chemistry','physics']:'science_subjects',['french','literature']:'art_subjects'}TypeError:unhashable type:'list'>>>

How to know the number of entries in a Python dictionary.

Python has a built-in function for this operation.

The len() can be used to know the number of entries in a Python dictionary.

Observe the following code snippet. You can type them in Python’s interactive shell.

Python

>>> student_details={'name': 'Oscar', 'dept': 'computer science', 'level': 300}
>>> len(student_details)
3
>>> print("There are",len(student_details),"entries in student details")
There are 3 entries in student details
>>>

123456

>>>student_details={'name':'Oscar','dept':'computer science','level':300}>>>len(student_details)3>>>print("There are",len(student_details),"entries in student details")There are3entries instudent details>>>

Adding keys to an existing Python Dictionary

You add a new key/value pair to a dictionary by using the subscript operator [].

Here’s the format:

“The variable name of your dictionary”[“key ”]=”Value”

Remember, that is just a format.

You use quotes appropriately.

The next code snippet creates an empty dictionary and adds three new entries:

(Please type them in Python’s interactive shell.)

Python

>>> my_profile={}
>>> my_profile["name"]="Godson"
>>> my_profile["gender"]="male"
>>> my_profile["status"]="single"
>>> my_profile
{'name': 'Godson', 'gender': 'male', 'status': 'single'}
>>>

1234567

>>>my_profile={}>>>my_profile["name"]="Godson">>>my_profile["gender"]="male">>>my_profile["status"]="single">>>my_profile{'name':'Godson','gender':'male','status':'single'}>>>

Can you do one for me following these instructions?

Create an empty dictionary and assign it to a variable.
Let that variable be student_info
Add the following entries (key-value pair).

name-Oscar”
dept-computer science
level-300

Now use a print statement to print your dictionary.

If you had problem on the way, don’t worry,

You are in a process, all you need is practice.

Here’s is our program student_details.py

Python

student_info={}
student_info["name"]="Oscar"
student_info["dept"]="computer science"
student_info["level"]=300
print("Student details:",student_info)

12345

student_info={}student_info["name"]="Oscar"student_info["dept"]="computer science"student_info["level"]=300print("Student details:",student_info)

Running student_details.py, we will have:

Python

>>>
RESTART: C:\Users\RAPTURE C. GODSON\Documents\Python Dictionary\student_details.py
Student details: {'name': 'Oscar', 'dept': 'computer science', 'level': 300}
>>>

1234	>>>RESTART:C:\Users\RAPTUREC.GODSON\Documents\Python Dictionary\student_details.pyStudent details:{'name':'Oscar','dept':'computer science','level':300}>>>

Replacing the values of an existing Python dictionary

Now, we have a program containing the details of Oscar.

What if Oscar is now in 400 level?

What can we do?

We can change the value associated with a particular key in a dictionary by using a subscript operator [].

Let’s change Oscar’s level with the code snippet below:

(Please type them in Python’s interactive shell.)

Python

>>> student_details={'name': 'Oscar', 'dept': 'computer science', 'level': 300}
>>> student_details["level"]=400
>>> student_details
{'name': 'Oscar', 'dept': 'computer science', 'level': 400}
>>>

12345

>>>student_details={'name':'Oscar','dept':'computer science','level':300}>>>student_details["level"]=400>>>student_details{'name':'Oscar','dept':'computer science','level':400}>>>

Accessing the values of a Python Dictionary.

Imagine a program, with numerous entries (key/value pairs) and we need just one value associated with a key.

We can print the whole dictionary using the print statement but that will be more work for us to look for that particular entry we need.

To save us time we can use the subscript operator [] to have access to values in a dictionary.

Here’s an illustration:

(Please type them in Python’s interactive shell.)

Python

>>> student_details={'name': 'Oscar', 'dept': 'computer science', 'level': 300}
>>> student_details["name"]
'Oscar'
>>> print("student's name:",student_details["name"])
student's name: Oscar
>>> print("student's dept:",student_details["dept"])
student's dept: computer science
>>> print("student's gender:",student_details["gender"])
Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    print("student's gender:",student_details["gender"])
KeyError: 'gender'
>>>

12345678910111213

>>>student_details={'name':'Oscar','dept':'computer science','level':300}>>>student_details["name"]'Oscar'>>>print("student's name:",student_details["name"])student's name: Oscar>>> print("student'sdept:",student_details["dept"])student's dept: computer science>>> print("student's gender:",student_details["gender"])Traceback (most recent call last): File "<pyshell#4>", line 1, in <module> print("student'sgender:",student_details["gender"])KeyError:'gender'>>>

Note: If a key is not in the dictionary, Python raises a KeyError. You can observe that in the last print statement in the code snippet above.

Project 1:

Write a Python program to prompt the user to enter the position of a chemical element and output the element. Your program should cover the first 20 elements.

The first 20 elements are:

Hydrogen, Helium, Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine, Neon, Sodium, Magnesium, Aluminum, Silicon, Phosphorus, Sulphur, Chlorine, Argon, Potassium, Calcium.

Save the program as elements.py

elements.py

print("To quit, press Enter")
elements={1:"Hydrogen",2:"Helium",3:"Lithium",4:"Beryllium",5:"Boron",
          6:"Carbon",7:"Nitrogen",8:"Oxygen",9:"Flourine",10:"Neon",
          11:"Sodium",12:"Magnesium",13:"Aluminum",14:"Silicon",
          15:"Phosphorus",16:"Sulphur",17:"Chlorine",18:"Argon",
          19:"Potassium",20:"Calcium"}
while True:
    element_position=int(input("Enter the element's position(1-20):"))
    if element_position =="":
        break
    else:
        print(element_position,": ",elements[element_position])

123456789101112

print("To quit, press Enter")elements={1:"Hydrogen",2:"Helium",3:"Lithium",4:"Beryllium",5:"Boron",6:"Carbon",7:"Nitrogen",8:"Oxygen",9:"Flourine",10:"Neon",11:"Sodium",12:"Magnesium",13:"Aluminum"

Write Cool Programs Using Python Dictionary.

What is a dictionary in Python?

Python dictionary example

How to know the number of entries in a Python dictionary.

Adding keys to an existing Python Dictionary

Replacing the values of an existing Python dictionary

Accessing the values of a Python Dictionary.

Project 1:

Write Cool Programs Using Python Dictionary.

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What is a dictionary in Python?

Python dictionary example

How to know the number of entries in a Python dictionary.

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Replacing the values of an existing Python dictionary

Accessing the values of a Python Dictionary.

Project 1:

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