The document provides an overview of functions in Python programming, detailing how to define, call, and utilize both value-returning and non-value-returning functions. It covers key concepts such as parameters, default values, and the return statement, while providing multiple examples to illustrate function usage. The document emphasizes the importance of function definitions and highlights the distinction between returning values and side effects in function calls.

1. Course Code – Course Name Prepared by:
Python Programming
Unit III
• Functions
• Program Routines
• Defining Functions
• Calling Value-Returning Functions- Calling Non-
Value-Returning Functions
• Parameter Passing - Keyword Arguments in
Python
• Default Arguments in Python-Variable Scope

2. Course Code – Course Name Prepared by:
Python Programming
Program Routine
• A program routine is a named group of
instructions that accomplishes some task.
• A routine may be invoked (called) as many
times as needed in a given program. A
function is Python’s version of a program
routine.

3. Course Code – Course Name Prepared by:
Python Programming
Program Routine
• When a routine terminates, execution automatically returns to the
point from which it was called. Such routines may be predefined in the
programming language, or designed and implemented by the
programmer.

4. Course Code – Course Name Prepared by:
Python Programming
Function
• A function is a block of code which only runs
when it is called.
• We can pass data, known as parameters, into
a function.
• A function can return data as a result.

5. Course Code – Course Name Prepared by:
Python Programming
How function works in Python

6. Course Code – Course Name Prepared by:
Python Programming
Defining Functions
• The first line of a function definition is the function header.
• A function header starts with the keyword def, followed by an identifier
(avg), which is the function’s name.
• The function name is followed by a comma-separated list of identifiers
(n1,n2,n3) called formal parameters , or simply “parameters.”
• Following the parameter list is a colon ( : ).
• Following the function header is the body of the function, a suite
(program block) containing the function’s instructions.
• As with all suites, the statements must be indented at the same level,
relative to the function header.

7. Course Code – Course Name Prepared by:
Python Programming
Defining Functions
• Functions are generally defined at the top of a
program.
• Every function must be defined before it is called.
• Actual arguments are the values passed to
functions to be operated on.
• Formal parameters are the “placeholder” names
for the arguments passed.

8. Course Code – Course Name Prepared by:
Python Programming
Example 1
def my_function():
print("Hello from a function")
my_function()
Output
Hello from a function

9. Course Code – Course Name Prepared by:
Python Programming
Example 2 - Arguments
def my_function(fname):
print(“Welcome” + fname )
my_function(“SAM")
my_function(“John")
my_function(“William")
Output
Welcome SAM
Welcome John
Welcome William

10. Course Code – Course Name Prepared by:
Python Programming
Example 2.1 - Number of Arguments
By default, a function must be called with the correct number of
arguments. That is, if your function expects 2 arguments, you
have to call the function with 2 arguments, not more, and not
less.
def my_function(fname, lname):
print(fname + " " + lname)
my_function(“Narendra", “Modi")
Output: Narendar Modi

11. Course Code – Course Name Prepared by:
Python Programming
Example 2.2 - Number of Arguments
def my_function(fname, lname):
print(fname + " " + lname)
my_function("Emil")
Output:

12. Course Code – Course Name Prepared by:
Python Programming
Example 2.3 - Arguments
def my_function(child3, child2, child1):
print("The youngest child is " + child3)
my_function(child1 = "Emil", child2 = "Tobias",
child3 = "Linus")
Output
The youngest child is Linus

13. Course Code – Course Name Prepared by:
Python Programming
Example 3 - Default Parameter Value
def my_function(country = "Norway"):
print("I am from " + country)
my_function("Sweden")
my_function("India")
my_function()
my_function("Brazil")
Output
I am from Sweden
I am from India
I am from Norway
I am from Brazil
Function arguments can have default values in
Python. We can provide a default value to an
argument by using the assignment operator (=).
A default argument is an argument that
assumes a default value if a value is not
provided in the function call for that
argument.

14. Course Code – Course Name Prepared by:
Python Programming
Example 3.1 - Default Parameter Value

15. Course Code – Course Name Prepared by:
Python Programming
Example 4 - Passing a List as an Argument
def my_function(food):
for x in food:
print(x)
fruits = ["apple", "banana", "cherry"]
my_function(fruits)
Output
apple
banana
cherry
We can send any data types of argument to a
function (string, number, list, dictionary etc.),
and it will be treated as the same data type
inside the function.

16. Course Code – Course Name Prepared by:
Python Programming
Example 5 – Return Values
def my_function(x):
return 5 * x
print(my_function(3))
print(my_function(5))
print(my_function(9))
Output
15
25
45
The statement return [expression] exits a function,
optionally passing back an expression to the caller.
A return statement with no arguments is the same as
return None.

17. Course Code – Course Name Prepared by:
Python Programming
Example 5.1 – Return Values

18. Course Code – Course Name Prepared by:
Python Programming
Example – Pass statement
Function definitions cannot be empty, but if you for
some reason have a function definition with no
content, put in the pass statement to avoid getting
an error.
def myfunction():
pass

19. Course Code – Course Name Prepared by:
Python Programming
Categories of Functions
• Value-Returning Functions
• Non-Value-Returning Functions

20. Course Code – Course Name Prepared by:
Python Programming
Value-Returning Functions
• A value-returning function is a program
routine called for its return value, and is
therefore similar to a mathematical function.

21. Course Code – Course Name Prepared by:
Python Programming
• Function avg takes three arguments (n1, n2, and n3) and
returns the average of the three.
• The function call avg(10, 25, 16), therefore, is an
expression that evaluates to the returned function value.
• This is indicated in the function’s return statement of the
form return expr , where expr may be any expression.

22. Course Code – Course Name Prepared by:
Python Programming
Non-Value-Returning Functions
• A non-value-returning function is called not for a returned
value, but for its side effects .
• A side effect is an action other than returning a function
value, such as displaying output on the screen.
• There is a fundamental difference in the way that value-
returning and non-value-returning functions are called.
• A call to a value-returning function is an expression, as for the
call to function avg: result = avg(10, 25, 16) * factor.
• When non-value-returning functions are called, the function
call is a statement.
• Since such functions do not have a return value, it is incorrect
to use a call to a non-value-returning function as an
expression

23. Course Code – Course Name Prepared by:
Python Programming
Non-Value-Returning Functions
• In this example, function displayWelcome is called only for the side-effect
of the screen output produced.
• Finally, every function in Python is technically a value-returning function since
any function that does not explicitly return a function value (via a return
statement) automatically returns the special value None. Such functions as non-
value-returning functions.
A non-value-returning function is a function called for its side effects, and not
for a returned function value.