Python, being a user-friendly language, allows us to use many English words. One such keyword is ‘not’. It is a very crucial keyword, having many use-cases as various operators. For example, it can be used as a logical operator to combine conditional statements. Not only as a logical operator but also not in Python, it can be used as ‘is not an identity and as ‘not in’ in the membership operator.
Identity operators are generally used to compare the objects, whether they are equal and in the same memory location or not. A membership operator is used to test whether a particular sequence is present in the object.
Let us understand every use case of the ‘not’ keyword in Python.
3 Unique Forms of Not in Python
- As a Logical Operator
- As a Membership Operator
- Identity Operator
1. Not in Python as a Logical Operator
Logical operators are used for combining conditional statements. There are three types of logical operators – ‘and,’ ‘or’ and ‘not.’
‘not’ is a case–sensitive keyword and only returns Boolean values (True or False). It returns the reverse of the original result, i.e., if the first output was coming to be True, ‘not’ will convert it into False and vice versa.
Let’s understand it with an example, and then we will discuss it in detail.
Without ‘not’’-
x=5
y=2
if (x>y):
print(x)
else:
print(y)
Output-
5 
Now, let us use the ‘not’ keyword
x=5
y=2
#Here, x>y and it would have return True but due to ‘not’ it returned as False.
if not(x>y):
print(x)
else:
print(y)
Output-
2 
Let us make a program to make a list empty and see where ‘not’ can be used.
empty=False
list1=[1,2,3,4,5,6,6,7,8,9,10]
print("list before",list1)
# not empty means 'TRUE'
while (not empty):
# while the list1 is not empty
if list1 !=[]:
list1.pop()
else:
empty=True
print("list after",list1)
Output-
list before [1, 2, 3, 4, 5, 6, 6, 7, 8, 9, 10]
list after [] 
2. Not in Python as a Membership operator
A very common use-case of not in any programming language, which you may have used often, is ‘not in.’ We use it to determine whether a particular sequence is in an object or not.
Suppose we want to check whether ‘peas’ is in the list of fruits or not.
fruits=["mango",
"apple",
"banana",
"strawberry",
"pineapple",
"melon"]
if 'peas' not in fruits:
print("Vegetable")
else:
print("Fruit")
Output-
Vegetable 
To make it shorter and sweeter, We can use these operators to be used inside the print().
ages=[] # making a list of all ages allowed to vote for i in range(18,101): ages.append(i) # If the given age is not allowed to vote print True print(17 not in ages) Output- True
Program to List down marks in a list and add ‘Absent’ if name not in the dictionary
dict1={"Ashwini":48,
"khush":41,
"Lamba":49,
"Shera":30,
"Arjun":47}
list1=['Ashwini','Shalu','khush','Shabnam','Lamba','Manish']
list2=[]
for i in list1:
if i not in dict1.keys():
list2.append("Absent")
else:
list2.append(dict1[i])
print(list2)
Output-
[48, 'Absent', 41, 'Absent', 49, 'Absent']
Let’s create a very small login system using ‘not in’ keyword
password_iscorrect=False
login={'ashwini':'mandani'}
while(not password_iscorrect):
username=input("Enter username ")
password=input("Enter password ")
if username in login.keys() and password not in login.values():
password=input("Incorrect!! Enter correct password")
password_iscorrect=False
else:
print("Welcome ",username)
password_iscorrect=True
Output-
Enter username ashwini
Enter password mandani
Welcome ashwini 
3. Not in Python As an Identity Operator
Identity operators are used to compare objects. We have the ‘is not’ operator in Python for this purpose.
Program to divide list1 from list2, but if any element in list2=0 (Denominator cannot be equal to ‘0’).
list1=[27,14,15,60,1,8]
list2=[9,7,5,15,0,16]
list3=[]
for i in range(len(list1)):
#Denominator cannot be 0
if list2[i] is not 0:
list3.append(list1[i]/list2[i])
else:
list3.append("Not Possible")
print(list3)
Output-
[3.0, 2.0, 3.0, 4.0, 'Not Possible', 0.5] 
Another example can be if we want to sort elements on the basis of type, we can use is not.
list1=[210.2,100,122,199.99,88,17.1,111,88,33.1,1011]
# list containing integers
int_list=[]
# list containing float values
float_list=[]
for i in list1:
# if the data type of
if type(i) is not int:
float_list.append(i)
else:
int_list.append(i)
print("float list",float_list)
print("integer list",int_list)
Output-
float list [210.2, 199.99, 17.1, 33.1]
integer list [100, 122, 88, 111, 88, 1011] 
Also Read:
Python Remove Duplicates From List
Python Print Without Newline
Learn Python the Hard Way Review PDF
Python Map
Conclusion-
We have seen many examples of using not in Python in many different ways. In many places, we could have used ‘!=.’ instead of not a keyword. But the primary purpose of including these types of keywords in Python is to make it more user friendly by adding more English words. So, we must use these types of keywords to encourage the primary purpose behind the language.