Python dictionary

Dictionary is a set of ordered ( version 3.7 ), changeable and indexed key value pairs.

Each key and value are separated by colon : and each pair of key value is separated by comma ,

Declaring a dictionary

Without any elements and using constructor.

my_dict=dict()
print(type(my_dict))

my_dict={}
print(type(my_dict))

With key value pairs ( data )

my_dict={1:'Alex',2:'Ronald'}
print(my_dict)

Output

{1: 'Alex', 2: 'Ronald'}

Python dictionary creating and modifying key value pairs using various methods and functions

Add , remove , update elements

Methods of dictionary object

Method	Description
clear()	Delete all elements of the dictionary
copy()	Copy and create new dictionary
fromkeys()	Creating dictionary using keys
get()	Collect the value of the input key
items()	View object with key value pair
keys()	View object with keys
popitem()	remove latest added item as a tuple of key value pair
pop()	remove the value of input key
setdefault()	Getting or adding key with value
update()	Adding or updating key and value
values()	View object listing the values

Number of keys present by len()

my_dict={'a':'Alex','b':'Ronald'}
print(len(my_dict))

Output

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
print(len(my_dict))

Output

Length of elements within the key

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
print(len(my_dict['a']))

Output

Displaying elements based on position is not possible as dictionary is unindexed. This will give error message.

my_dict={'a':'Alex','b':'Ronald'}
print(my_dict[0])

Above lines will generate error

Displaying value through key

my_dict={'a':'Alex','b':'Ronald'}
print(my_dict['b'])

Output

Ronald

Using get()

my_dict={'a':'Alex','b':'Ronald'}
print(my_dict.get('a'))

Output

Alex

Creating a list using the values

my_dict={'a':'One','b':'Two','c':'Three'}
x=list(my_dict.values()) # list of values 
print(x) # ['One', 'Two', 'Three']

Displaying all items by looping

We used for loop to display all keys of the dictionary.

my_dict={'a':'Alex','b':'Ronald'}
for i in my_dict:
 print(i)

Output is here

a
b

To display values by using for loop

my_dict={'a':'Alex','b':'Ronald'}
for i in my_dict:
 print(my_dict[i])

Output is here

Alex
Ronald

Searching key by if condition check

my_dict={'a':'Alex','b':'Ronald'}
if 'b' in my_dict:
 print("Yes, it is there ")
else:
 print("No, not there")

Output

Yes, it is there

Searching value by using values()

my_dict={'a':'Alex','b':'Ronald'}
if 'Ronald' in my_dict.values():
 print("Yes, it is there ")
else:
 print("No, not there")

Output is here

Yes, it is there

Getting Key by using Value of a dictionary

But this requirement is not for which dictionary is usually used. If possible then while creating the dictionary, key and value can be changed.

However here is the solution for getting key by using value.

my_dict={'a':'Alex','b':'Ronald'}
for i,j in my_dict.items():
    if j=='Ronald':
         print(i)

Output

When we use list as value of a dictionary, we can get other values by using any one element of the list. Here by using matching name we get the connected mark ( integer value inside list ) and the key.

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
# display the mark of the matching name along with id 
for i,j in my_dict.items():
    if j[0]=='Ronald':
         print("Mark:",j[1], ", Key or id :",i)

Output

Mark: 40 , Key or id : b

Searching values for matching element

for i,j in my_dict.items():
    if 'Ronn' in j:
      print(i,">>",j)

Add , remove , update elements

Add item

my_dict={'a':'Alex','b':'Ronald'}
my_dict['c']='Ronn'
print(my_dict)

Output is here

{'a': 'Alex', 'b': 'Ronald', 'c': 'Ronn'}

Add item by using update() method

Note: We can't keep duplicate keys in a dictionary, however duplicate values are allowed.

del with dictionary

The keyword del is used to remove any element or the complete dictionary object.

Removing a particular key value pair by using del() and the key

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
del my_dict['b']
print(my_dict)

Output

{'a': 'Alex', 'c': 'Ronn'}

If we don’t mention the key in above code then dictionary will be deleted. This code will generate error message.

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
del my_dict
print(my_dict)

Error message is here

name 'my_dict' is not defined

Copying a dictionary by using dict() method

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict1=dict(my_dict)
print(my_dict1)

Output

{'a': 'Alex', 'b': 'Ronald', 'c': 'Ronn'}

We can display all elements

my_dict={'a':'Alex','b':'Ronald','c':'Ronn'}
my_keys=my_dict.keys()
for i in my_keys:
    print(i)

Output is here

a
b
c

Get maximum or minimum element based on Keys or Values of a dictionary.

Merging of two Dictionary

Using update

my_dict1={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict2={'d':'Rabi','b':'Kami','c':'Loren'}
my_dict1.update(my_dict2)
print(my_dict1)# {'a': 'Alex', 'b': 'Kami', 'c': 'Loren', 'd': 'Rabi'}

my_dict1={'a':'Alex','b':'Ronald','c':'Ronn'}
my_dict2={'d':'Rabi','b':'Kami','c':'Loren'}
my_out={**my_dict1,**my_dict2}
print(my_out) # {'a': 'Alex', 'b': 'Kami', 'c': 'Loren', 'd': 'Rabi'}

Dictionary containing List

We can store multiple elements by using List and store them in a dictionary. Check this code , the outputs are also written within comments.

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
print(my_dict['a'][0]) # Output : Alex
print(my_dict['c'][1]) # Output : 50
print(my_dict['a'][:1])# Output : ['Alex']
print(my_dict['a'][:2])# Output : ['Alex', 30]

We can add more elements like one more list to this dictionary

my_dict={'a':['Alex',30],'b':['Ronald',40],'c':['Ronn',50]}
print(my_dict['a'][0]) # Output : Alex
print(my_dict['c'][1]) # Output : 50

my_dict.update({'d':['Ravi',60]})
print(my_dict)

Dictionary from database record set

More about MySQL database and connection here

from sqlalchemy import create_engine
my_conn = create_engine("mysql+mysqldb://id:pw@localhost/my_tutorial")
query = "SELECT *  FROM student LIMIT 0,5"
my_data = list(my_conn.execute(query))  # SQLAlchem engine result set
my_dict = {} # Create an empty dictionary
my_list = [] # Create an empty list
for row in my_data:
    my_dict[[row][0][0]] = row  # id as key
    my_list.append(row[1]) # name as list
print(my_dict)
print(my_list)
# Print the other values for matching Name
for i, j in my_dict.items():
    if j[1] == "Arnold":
        print(i, j[0], j[1], j[2])

Question

Find out the frequency of occurrence of chars in a string?

my_str='Welcome to Python'
my_dict={}
for i in my_str:
  if i in my_dict:
    my_dict[i] = my_dict[i] + 1
  else:
    my_dict[i]  = 1
print(my_dict)

Output

{'W': 1, 'e': 2, 'l': 1, 'c': 1, 'o': 3, 'm': 1, ' ': 2,
 't': 2, 'P': 1, 'y': 1, 'h': 1, 'n': 1}

Getting unique elements and counting the occurrence by using get().

Getting unique elements and counting the occurrence by using Counter.

Exercise on Dictionary
Ask for Name and mark of three students
Create a dictionary with this and print them
Following details of the student are to be stored.

Name ( this will be key of the dictionary )
Mark in three subjects i.e Physics , Chemistry , Math
Attendance in number of days

Create a dictionary and store three student details as mentions above.

Print the dictionary
Display all students name and attendance
Create one csv( comma separated value ) file with id ( integer) , name(string), mark1(integer ), mark2(integer),mark3(integer), attendance(integer)
Read the file and using the data create one dictionary. The first column id of the csv file should be the key and rest of the data should be value of the dictionary.
One sample csv file is kept here with some data. Same can be used.
```
1,Ravi,20,30,40,120
2,Raju,30,40,50,130
3,Alex,40,50,60,140
4,Ronn,50,60,70,150
```

Solution

Python

set list tuple

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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