Python Keywords

Introduction

In programming, a keyword is a "reserved word" by the language which conveys special meaning to the interpreter. It may be a command or a parameter. Keywords cannot be used as a variable name in the program snippet.
Keywords in Python are reserved words that can not be used as a variable name, function name, or any other identifier.

List of all keywords in Python

We can also get all the keyword names using the below code.
Example: Python Keywords List

Python3

# Python code to demonstrate working of iskeyword()

  

# importing "keyword" for keyword operations

import keyword

  

# printing all keywords at once using "kwlist()"

print("The list of keywords is : ")

print(keyword.kwlist)

Output:

The list of keywords is : 

['False', 'None', 'True', 'and', 'as', 'assert', 'async', 'await', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield']

Let's discuss each keyword in detail with the help of good examples.

True, False, None

• True: This keyword is used to represent a boolean true. If a statement is true, "True" is printed.
• False: This keyword is used to represent a boolean false. If a statement is false, "False" is printed.
• None: This is a special constant used to denote a null value or a void. It's important to remember, 0, any empty container(e.g empty list) does not compute to None. 

  It is an object of its datatype - NoneType. It is not possible to create multiple None objects and can assign them to variables.

Example: True, False, and None Keyword

Python3

print(False == 0)

print(True == 1)

  

print(True + True + True)

print(True + False + False)

  

print(None == 0)

print(None == [])

Output

True

True

3

1

False

False

and, or, not, in, is

• and: This a logical operator in python. "and" Return the first false value. If not found return last. The truth table for "and" is depicted below.
  

3 and 0 returns 0
3 and 10 returns 10
10 or 20 or 30 or 10 or 70 returns 10 
The above statements might be a bit confusing to a programmer coming from a language like C where the logical operators always return boolean values(0 or 1). Following lines are straight from the python docs explaining this:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.
The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

Note that neither and nor or restrict the value and type they return to False and True, but rather return the last evaluated argument. This is sometimes useful, e.g., if s is a string that should be replaced by a default value if it is empty, the expression s or 'foo' yields the desired value. Because not has to create a new value, it returns a boolean value regardless of the type of its argument (for example, not 'foo' produces False rather than ".)

• or: This a logical operator in python. "or" Return the first True value.if not found return last. The truth table for "or" is depicted below.
  

3 or 0 returns 3
3 or 10 returns 3
0 or 0 or 3 or 10 or 0 returns 3  

• not: This logical operator inverts the truth value. The truth table for "not" is depicted below.
• in: This keyword is used to check if a container contains a value. This keyword is also used to loop through the container.
• is: This keyword is used to test object identity, i.e to check if both the objects take the same memory location or not.

Example: and, or, not, is and in keyword

Python

# showing logical operation

# or (returns True)

print(True or False)

  

# showing logical operation

# and (returns False)

print(False and True)

  

# showing logical operation

# not (returns False)

print(not True)

  

# using "in" to check

if 's' in 'geeksforgeeks':

    print("s is part of geeksforgeeks")

else:

    print("s is not part of geeksforgeeks")

  

# using "in" to loop through

for i in 'geeksforgeeks':

    print(i, end=" ")

  

print("\r")

  

# using is to check object identity

# string is immutable( cannot be changed once allocated)

# hence occupy same memory location

print(' ' is ' ')

  

# using is to check object identity

# dictionary is mutable( can be changed once allocated)

# hence occupy different memory location

print({} is {})

Output: 

True

False

False

s is part of geeksforgeeks

g e e k s f o r g e e k s 

True

False

Iteration Keywords - for, while, break, continue

• for: This keyword is used to control flow and for looping.
• while: Has a similar working like "for", used to control flow and for looping.
• break: "break" is used to control the flow of the loop. The statement is used to break out of the loop and passes the control to the statement following immediately after loop.
• continue: "continue" is also used to control the flow of code. The keyword skips the current iteration of the loop but does not end the loop.

Example: For, while, break, continue keyword

Python3

# Using for loop

for i in range(10):

  

    print(i, end = " ")

      

    # break the loop as soon it sees 6

    if i == 6:

        break

      

print()

      

# loop from 1 to 10

i = 0

while i <10:

      

    # If i is equals to 6,

    # continue to next iteration

    # without printing

    if i == 6:

        i+= 1

        continue

    else:

        # otherwise print the value

        # of i

        print(i, end = " ")

          

    i += 1

Output

0 1 2 3 4 5 6 

0 1 2 3 4 5 7 8 9 

Conditional keywords - if, else, elif

• if : It is a control statement for decision making. Truth expression forces control to go in "if" statement block.
• else : It is a control statement for decision making. False expression forces control to go in "else" statement block.
• elif : It is a control statement for decision making. It is short for "else if"

Example: if, else, and elif keyword

Python3

# Python program to illustrate if-elif-else ladder

#!/usr/bin/python

  

i = 20

if (i == 10):

    print ("i is 10")

elif (i == 20):

    print ("i is 20")

else:

    print ("i is not present")

Output

i is 20

def

def keyword is used to declare user defined functions.
Example: def keyword

Python3

# def keyword

def fun():

    print("Inside Function")

      

fun()

Output

Inside Function

Return Keywords - Return, Yield

• return : This keyword is used to return from the function.
• yield : This keyword is used like return statement but is used to return a generator.

Example: Return and Yield Keyword

Python3

# Return keyword

def fun():

    S = 0

      

    for i in range(10):

        S += i

    return S

  

print(fun())

  

# Yield Keyword

def fun():

    S = 0

      

    for i in range(10):

        S += i

        yield S

  

for i in fun():

    print(i)

Output

45

0

1

3

6

10

15

21

28

36

45

class

class keyword is used to declare user defined classes.
Example: Class Keyword

Python3

# Python3 program to

# demonstrate instantiating

# a class

  

  

class Dog:

      

    # A simple class

    # attribute

    attr1 = "mammal"

    attr2 = "dog"

  

    # A sample method

    def fun(self):

        print("I'm a", self.attr1)

        print("I'm a", self.attr2)

  

# Driver code

# Object instantiation

Rodger = Dog()

  

# Accessing class attributes

# and method through objects

print(Rodger.attr1)

Rodger.fun()

Output

mammal

I'm a mammal

I'm a dog

With

with keyword is used to wrap the execution of block of code within methods defined by context manager. This keyword is not used much in day to day programming.
Example: With Keyword

Python3

# using with statement

with open('file_path', 'w') as file:

    file.write('hello world !')

as

as keyword is used to create the alias for the module imported. i.e giving a new name to the imported module. E.g import math as mymath.
Example: as Keyword

Python3

import math as gfg

  

print(gfg.factorial(5))

Output

120

pass

pass is the null statement in python. Nothing happens when this is encountered. This is used to prevent indentation errors and used as a placeholder.
Example: Pass Keyword

Python3

n = 10

for i in range(n):

      

# pass can be used as placeholder

# when code is to added later

pass

Lambda

Lambda keyword is used to make inline returning functions with no statements allowed internally.
Example: Lambda Keyword 

Python3

# Lambda keyword

g = lambda x: x*x*x

  

print(g(7))

Output

343

Import, From

• import : This statement is used to include a particular module into current program.
• from : Generally used with import, from is used to import particular functionality from the module imported.

Example: Import, From Keyword

Python3

# import keyword

import math

print(math.factorial(10))

  

# from keyword

from math import factorial

print(factorial(10))

Output

3628800

3628800

Exception Handling Keywords - try, except, raise, finally, and assert

• try : This keyword is used for exception handling, used to catch the errors in the code using the keyword except. Code in "try" block is checked, if there is any type of error, except block is executed.
• except : As explained above, this works together with "try" to catch exceptions.
• finally : No matter what is result of the "try" block, block termed "finally" is always executed.
• raise: We can raise an exception explicitly with the raise keyword
• assert: This function is used for debugging purposes. Usually used to check the correctness of code. If a statement is evaluated to be true, nothing happens, but when it is false, "Assertion Error" is raised. One can also print a message with the error, separated by a comma.

Example: try, except, raise, finally, and assert Keywords

Python3

# initializing number

a = 4

b = 0

  

# No exception Exception raised in try block

try:

    k = a//b # raises divide by zero exception.

    print(k)

  

# handles zerodivision exception

except ZeroDivisionError:

    print("Can't divide by zero")

  

finally:

    # this block is always executed

    # regardless of exception generation.

    print('This is always executed')

  

# assert Keyword  

# using assert to check for 0

print ("The value of a / b is : ")

assert b != 0, "Divide by 0 error"

print (a / b)

Output

Can't divide by zero

This is always executed

The value of a / b is :

Assertion Error: Divide by 0 error

del

del is used to delete a reference to an object. Any variable or list value can be deleted using del.
Example: del Keyword

Python3

my_variable1 = 20

my_variable2 = "GeeksForGeeks"

  

# check if my_variable1 and my_variable2 exists

print(my_variable1)

print(my_variable2)

  

# delete both the variables

del my_variable1

del my_variable2

  

# check if my_variable1 and my_variable2 exists

print(my_variable1)

print(my_variable2)

Output

20

GeeksForGeeks

NameError: name 'my_variable1' is not defined

Global, Nonlocal

• global: This keyword is used to define a variable inside the function to be of a global scope.
• non-local : This keyword works similar to the global, but rather than global, this keyword declares a variable to point to variable of outside enclosing function, in case of nested functions.

Example: Global and nonlocal keywords

Python3

# global variable

a = 15

b = 10

  

# function to perform addition

def add():

    c = a + b

    print(c)

  

# calling a function

add()

  

# nonlocal keyword

def fun():

    var1 = 10

  

    def gun():

        # tell python explicitly that it

        # has to access var1 initialized

        # in fun on line 2

        # using the keyword nonlocal

        nonlocal var1

          

        var1 = var1 + 10

        print(var1)

  

    gun()

fun()

Output

25

20