This document discusses functions in Python. It defines functions as sub-programs that carry out well-defined tasks. Functions are categorized as built-in functions and user-defined functions. User-defined functions are defined by the user as needed. The document explains function definition, calling functions, arguments, return values, scope of variables as local and global, mutability and immutability, and provides examples of functions to add numbers, check even/odd, check palindrome, count odd/even in a list, search a list, and more.

1. LECTURE – V
BY: ABHISHEK BHARDWAJ
PGT- COMPUTER SCIENCE

2. Functions
 Functions are sub – programs (part of the program), which
carries out some well defined tasks.
 Functions are categories into two parts:-
(i) Built - in Functions / Library Functions
(ii) User Defined Functions
 Built – in Functions are already defined functions and we just
need to use that.
Examples :- print(), input() etc.
 User defined functions are defined by the user as per
requirements.

3. User Defined Functions
 Any function definition is not executed until it is called.
 In Python function definition starts with “def” keyword.
Syntax :-
def function_name():
Statement…
Statement…
funtion_name()
Keyword
Function Call
Function Definition

4. “Program to Add Two Numbers” by Function
Step : 1
Step : 2
Step : 3

5. “Program to Check Even/ Odd Number” by Function
Step : 1
Step : 2
Step : 3

6. Argument/ Return from Function
 Arguments are those values which are passed to the
function.
 Arguments are provided during the function call.
 Arguments are the values (Inputs) on which action is
to be performed. (Don’t write this point in exam, just to
understand)
 Return is usually a value which a function returns to its
calling place.
 Return is final calculated value. (Don’t write this point
in exam, just to understand)

7. “Program to Add Two Numbers” by Argument
Step : 1
Step : 2
Step : 3
Step : 1
Step : 2
Step : 3

8. “Program to Add Two Numbers” by Argument

9. “Add Two Numbers” by Argument & Return
In place of a,b as
parameters you may
take x, y also.
Then c = x + y.
z = add (a, b) will
remain same as a, b
are arguments.
1
2
3
4

10. Types of Function Arguments
 Positional Argument/ Required Argument
Default Argument
Keyword Argument/ Named Argument
Positional Argument/ Required Argument:- If a function has
arguments then we need to pass the values to the function.
These values are passed in a sequence and this sequence need
to be followed strictly. Sequence means while
passing arguments
remember argument 4 will
be passed to parameter ‘a’
and argument 5 will be
passed to parameter ‘b’.
That’s why add (4, 5) will
be termed as positional
argument.
Positional Argument

11. Types of Function Arguments
Default Argument:- When we want to provide a default value to
the argument. In case no values are passed during the function
call, it will be initialized by the default value otherwise with the
past value. This concept is called default argument.
 Rule:- Once you assign an argument as default then rest of
the arguments after this must also be default arguments.
Example:- (a, b=5, c=10) Correct
(a, b=5, c) Incorrect
Default Argument
Output

12. Types of Function Arguments
Keyword Argument/ Named Argument:- Sequence of the
arguments may be changed using the concept of Keyword/
Named Argument.
Keyword Argument
Output

13. Scope of Variable (Local & Global Variables)
Global Variable:- A global variable is one which is not the part of
any function. It can be used in any function being a global
variable.
Local Variable:- A local variable is one which is the part of a
function and that’s why it can be used only within that function.
NOTE:
 Program Execution starts from main function.
_ _main_ _ (Understood in Python don’t need to write)
 Don’t need to write main function, it is understood.

14. Scope of Variable (Local & Global Variables)
 We may define global variable at two places.
(i) In the starting of the Program where variable is not
a part of any function.
(ii) Inside main function.
At add () call, value of m & n will be passed to a and b.
At sub () call, value of m & n will be passed to x and y.

15. Scope of Variable (Local & Global Variables)
What does happen if, a variable is available as “Local” as well as
“Global”?
 If same name variable has been declared globally as well as
locally, then first of all program control will be at main function
i.e at global variable. In this case value of global variable will be
printed. When control will be at local section then local value of
variable will be printed.
Output
1
2
3
1
2
3

16. Scope of Variable (Local & Global Variables)
If, we want a global variable need to be used in local section of a function!
 From below Example it is clear that first of all the program execution will
start from main and the value of global variable is printed as a = 10 then
demo() will be called and variable a will be set as global which means no
new variable will be created, so existing global variable will be used and its
value will be changed to 5 from 10. This value will not change during whole
program execution now. That’s why at step 2 and 3 the print function
produce value of a = 5.
Output
1
2
3
1
2
3
a = 10 5 5
At Step 1 Variable a is set to 10
At Step 2 Variable a is set to 5 (Declared Global)

17. Mutability & Immutability of Function Parameter
All the data types in Python have been divided into two types:-
Immutable Objects:- These are of in-built types like int, float,
bool, string, unicode, tuple. In simple words, an immutable
object can’t be changed after it is created.
Mutable Objects:- These are of type list, dictionary, set. Custom
classes are generally mutable. If the value of an object can
change, the object is called mutable.
NOTE:
When we pass arguments in a function, we actually pass any data
type (eg. int, float, list, bool, string etc). If we pass any mutable
object as an argument in function, we may change its value but if we
pass immutable object as an argument in function we may not
change its value. Anyhow, if we are able to change immutable
objects, we cannot replicate that change in main().

18. Mutability & Immutability of Function Parameter
1
2
3
Global ‘a’
Local ‘a’
Global ‘a’
1
2
3
Global ‘a’
Local ‘a’
Global ‘a’

19. Mutability & Immutability of Function Parameter
1
2
3
20 20 30 40
mylist
list1
10 20 30 40
list1
1
2
3
List is Mutable Data Type in Python
0 1 2 3 0 1 2 3

20. Program to Check a Number is Prime or Not using
Function?
Function
Definition
_ _main_ _

21. Program to Check a Number is Prime or Not using
Function & Return?
Function
Definition
_ _main_ _

22. Program to Check a Number is Palindrome or Not
using Function?

23. Program to Check a Number is Palindrome or Not
using Function and Return?

24. Program to Count Total Odd/ Even in a List Using
Function?

25. Program to Count Total Odd/ Even in a List Using
Function and Return?

26. Program to Search a Number in the List Using
Function?
OUTPUT
If break statement is not used
in this program then position in
output section will be shown as
“Num Found at 2 Position”
because 1 Position will be
overwritten by 2 Position. In
Searching List the Loop must
terminate as soon as the
element is found. So, less
search time. Best Run Time.

27. Program to Search a Number in the List Using
Function and Return? (Linear/ Sequential Search)
If Value is not found, for this case
position has been initialized as -1.
As we know indexing in list starts
from 0.
A linear search or sequential
search is a method for finding an
element within a list.