Python Dictionary
In
this python lecture you will master all about Python Dictionary with examples;
how to create a dictionary, how to modify a python dictionary, how to remove
and standard operations on dictionaries. Besides these, we will learn about the
methods to manipulate dictionaries.
A
dictionary is a collection of items that are unordered and mutable. Dictionary
items are usually represented as Key: Value pairs which are enclosed in curly braces
{}. Each Key: Value pair is separated using commas. In a nutshell, a dictionary
is a key-value mapping as visualized below.
Key
Features of Python Dictionary
· Dictionary is mutable.
· Dictionaries are dynamic in
nature.
· Dictionary can be nest like a
list
· Keys are unique and immutable
while values are mutable
How
to create a dictionary in Python?
A
dictionary in python can be defined by enclosing comma-separated key-value
pairs inside curly braces {}. Another way to create a dictionary is through the
built-in dict() function.
Syntax of Dictionary
<dict_name> = {
<key>:<value>,
<key>:<value>,
.
.
<key>:<value>
}
Empty
Dictionary: Python
dictionary with zero items.
Example: How to define empty Dictionary
emp_dict ={}
print(type(emp_dict))
Output:
<class
'dict'>
Homogeneous
Dictionary: A Dictionary with keys of the same immutable datatype.
Example: How to create a Homogeneous dictionary
int_dict={1:'Odd',2:'Even'}
print(int_dict)
Output:
{1:
'Odd', 2: 'Even'}
Mixed
Dictionary: A
Dictionary with keys of the different immutable datatypes.
Example: Mixed dictionary
mix_dict={1:'Student','name':'Tom','Marks':[45,46,47,48,50
]}
print(mix_dict)
Output:
{1:
'Student', 'name': 'Tom', 'Marks': [45, 46, 47, 48, 50]}
So
far we have discussed various types of dictionaries and their creation. Another
possible way to create a dictionary is using a function called dict().
Example: dict() function
fn_dict =
dict({'Name':'John','Age':30})
print(fn_dict)
Output:
{'Name':
'John', 'Age': 30}
Properties
of Key and Values in Dictionary
Two
important factors in a python dictionary are Keys and Values. Their properties
are listed below.
1.
The key must be unique means there should be no duplication.
If a dictionary contains duplicate keys then the last assignment will be
considered.
Example:Illustrates keys doesn't hold duplicates
fn_dict =
dict({'Name':'John','Age':30 ,'Name':'Charles'})
print(fn_dict)
Output:
{'Name':
'Charles', 'Age': 30}
2.
Keys are immutable means they are unchangeable over the
lifetime after creation. Keys can be integer, string, tuples but never be a
list.
Example:shows
error when key is a list
D = {['odd']:(1,3,5,7,9)}
print(D)
Output:
D
= {['odd']:[1,3,5,7,9]}
TypeError:
unhashable type: 'list'
3.
Values are free of restrictions and they can be any data
type and can be of any length.
Example: shows values can be of any type and any length
D = {'odd':[1,3,5,7,9]}
print(D)
Output:
{'odd':
[1, 3, 5, 7, 9]}
What
is a Nested Dictionary in Python?
In
Python, a dictionary containing another dictionary is referred to as Nested
Dictionary. Nesting can be done to any level without any restriction. This can
be visualized for better understanding.
Nested Dictionary
University= {
'UN_Name':'Oxford',
'Stud':{
'St1_Name':'Tom','St2_Name':'Becky' }
}
print(University)
Output:
{'UN_Name':
'Oxford', 'Stud': {'St1_Name': 'Tom', 'St2_Name': 'Becky'}}
How
to access dictionary values in Python?
We
know that a dictionary is an unordered collection of key: value pairs. Even though
they appear as how they were defined, when it comes to accessing, numerical
indexing does not work. This is because python interprets the numerical value
as the key instead of index. So then how can we access values from a dictionary?
Dictionary
uses keys to retrieve values from a dictionary. To access values we use the
same square brackets with Keys inside rather than indices. We can also use the
get() method to accomplish the same.
To
understand let’s examine the below example.
Accessing Values from Dictionary
mix_dict={1:'Student','name':'Tom','Marks':[45,46,47,48,50
]}
print('mix_dict[1] = ',mix_dict[1])
print("mix_dict['name'] =
",mix_dict['name'])
print("mix_dict['Marks'] =
", mix_dict.get('Marks')
Output:
mix_dict[1]
= Student
mix_dict['name']
= Tom
mix_dict['Marks']
= [45, 46, 47, 48, 50]
Accessing
undefined Values
Now
suppose you try to access a value with a key that is not defined in the
dictionary. What will happen?
Obviously,
an exception will be the result. If we refer to a key that is not present in
the dictionary, the interpreter raises a KeyError.
This occurs only when we use square [] brackets to access value. The get() method returns None as the output.
KeyError while accessing an undefined key
D= { 'A':'Apple','B':'Banana'}
print(D.get('C'))
print(D['C'])
Output:
None
print(D['C'])
KeyError:
'C'
How
to access values from a Nested Dictionary
Examples : Accessing values from a nested dictionary
D={
'D1':{2:4,3:9},
'D2':{4:64,5:125}
}
print(D['D1'])
print(D['D2'])
print(D['D1'][3])
print(D['D2'][5])
Output:
{2:
4, 3: 9}
{4:
64, 5: 125}
9
125
How
to modify Python Set
The
mutable behavior of a dictionary makes it possible to modify the dictionary
either by adding individual or group of entries or by replacing the existing
value. Three ways of adding entries to a dictionary is listed below.
1.
adding single entries to an empty dictionary
Example: How to add an element to an empty set
Student ={}
Student[' First_Name']='Charles'
Student['Last_Name'] = 'Smith'
print("\n Dictionary after
adding two entries one at a time :",Student)
Output:
Dictionary
after adding two entries one at a time :
{'
First_Name': 'Charles', 'Last_Name': 'Smith'}
2.
update() function : to add multiple entries to dictionary in
one shot
Example: How to add multiples element in one shot
Student = {' First_Name': 'Charles',
'Last_Name': 'Smith'}
Student.update({'ID':10001,
'Department':'Science'})
print("\n Dictionary after
adding multiple entries in one shot :", Student)
Output:
Dictionary
after adding multiple entries in one shot :
{'
First_Name': 'Charles', 'Last_Name': 'Smith', 'ID': 10001, 'Department':
'Science'}
3.
Nesting a dictionary by adding another dictionary
Example: addition of dictionary to an existing dictionary
Student = {' First_Name': 'Charles',
'Last_Name': 'Smith', 'ID': 10001, 'Department': 'Science'}
Student['Marks'] ={'Physics':100
,'Chemistry':98}
print("\n Dictionary after
nesting : \n ",Student)
Output:
Dictionary
after nesting :
{'
First_Name': 'Charles', 'Last_Name': 'Smith', 'ID': 10001, 'Department':
'Science', 'Marks': {'Physics': 100, 'Chemistry': 98}}
We
can also modify the dictionary by changing or replacing the existing values.
This can be done by referring to its key. Below example, change the existing
value, Smith to David, by referring to its key Last_Name.
We
can also modify the dictionary by changing or replacing the existing values.
This can be done by referring to its key. Below example, change the existing
value, Smith to David, by referring to its key Last_Name.
Example: Changing values in the dictionary
Student = {' First_Name': 'Charles',
'Last_Name': 'Smith'}
Student['Last_Name']='David'
print(Student)
Output:
{'
First_Name': 'Charles', 'Last_Name': 'David'}
How
to remove elements from a Dictionary in Python
Another
way to modify the dictionary is by removing the elements from it. There are
various ways to remove or delete elements in the dictionary. They are listed
below.
1.
pop() Method: is a built-in function in a dictionary. It is
used to remove a distinct item from a dictionary by referring to its key and
this method returns the removed value.
Example: Removing an element from a dictionary using pop()
method
Student = {' First_Name': 'Charles',
'Last_Name': 'Smith'}
print(Student.pop('Last_Name'))
print("Dictionary after removal
is:",Student)
Output:
Smith
Dictionary
after removal is: {' First_Name': 'Charles'}
2.
popitem() Method: is a built-in function used to remove the last
inserted key-value pair from the dictionary and will return the key-value
pairs.
Example: Removing element from a dictionary using popitem()
method
Student = {' First_Name': 'Charles',
'Last_Name': 'Smith','Age':27}
print(Student.popitem())
print("Dictionary after removal
is:",Student)
Output:
('Age',
27)
3.
clear() Method: is a built-in function used to remove or clear
all the elements in a dictionary.
Example: Removing element using clear() method
Student = {' First_Name': 'Charles',
'Last_Name': 'Smith','Age':27}
print(Student.clear())
print("Dictionary after removal
is:",Student)
Output:
None
4.
del keyword: can be used to remove either individual elements or
the entire dictionary itself.
Example: Removing element using del keyword
Student = {' First_Name': 'Charles',
'Last_Name': 'Smith','Age':27}
del Student['Age']
print("Dictionary after removal
is:",Student)
del Student
print("Dictionary after removal
is:",Student)
Output:
Dictionary
after removal is: {' First_Name': 'Charles', 'Last_Name': 'Smith'}
print("Dictionary after removal
is:",Student)
NameError:
name 'Student' is not defined
How
to perform Membership Validation in a Python Dictionary
As
in other data types, we can validate the presence of a key in a dictionary
using the membership operators. The two membership operators are :
· in: returns true only when the key is present in the
dictionary.
· not in: returns false when the key is absent in the dictionary.
Example: Membership Validation
D ={
0:'Red',
2:'Green',
4:'Blue'
}
print(0 in D)
print(3 not in D)
Output:
True
True
Python
Dictionary built-in functions
To
accomplish some specific tasks python has got some built-in functions for
dictionaries. They are well tabulated below.
|
Function |
Description |
|
all() |
Returns True if all keys in the
dictionary are True |
|
any() |
Returns True if any of the keys in
the dictionary is True |
|
len() |
Returns the number of key-value
pairs in the dictionary |
|
sorted() |
Returns a list of keys which are
sorted |
Python
Dictionary Methods
Like
functions, the Python dictionary does have methods to perform special tasks and
return specific values. They are listed below in the table below.
|
METHODS |
DESCRIPTION |
|
Clears all items in the dictionary |
|
|
copy() |
Returns a shallow copy of the
dictionary |
|
fromkeys(seq[,v]) |
Returns a new dictionary with keys
from the seq and associated values v. |
|
get(key[,d]) |
Returns the associated values of
the key. |
|
items() |
Returns a list of key-value pair
format |
|
keys() |
Returns the list of keys in the
dictionary |
|
pop(key[,d]) |
Removes elements in a dictionary
referring to its key |
|
popitem() |
Removes and returns the last
inserted entry |
|
setdefault(key[,d]) |
To set the key to default if the
key is not available |
|
update() |
Returns the updated dictionary
with new entries or replacing the existing one. |
|
values() |
Returns the list of values in a
dictionary |