Python Keywords

October 26, 2021October 26, 2021 User HandleUser Handle

Python Keywords

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

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

We can also get all the keyword names using the below code.

Example: Python Keywords List

# 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

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.

and keyword python

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.

or

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

# 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

# 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

# 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

# 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

# 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 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

# 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

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

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

# 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

# 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, “AssertionError” is raised. One can also print a message with the error, separated by a comma.

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

# 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 :
AssertionError: 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

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

# 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

 

Last Updated on October 26, 2021 by admin

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