Functions

CS 149 Chapter 4 Base Slides

A couple of the slides in this set are adapted from the POGIL activities or from Prof Chao.

Basic Patterns of Program Flow

• Sequence
• Instructions are executed one after another
• Selection (Conditionals)
• A group of instructions may or may not be executed, depending on some condition
• Function calls
• Jump to the function at the function call
• Return to the next step after the function call
• Iteration (Loops)
• A group of instructions is repeated multiple times

Functions

• Function definition/implementation:

def functionName(arg_x, arg_y):

# everything indented is part of the function

x = arg_x + 2

y = arg_y * 2

# a return statement is optional

return (x + y)

​

• Function call/invocation

functionName(17, 4)

Functions

Function

doSomething

Data in

Data out

Functions

Function

doSomething

Data in

Data out

Arguments (parameters)

Return value

Functions - compare to input/output

Function

doSomething

Data in

Data out

Arguments (parameters)

Return value

Program

input()

print()

Program

Program

Functions

• Reuse the same code over and over
• Write code for others to use
• Function parameters
• Allow the function to take the same action on different input data
• Function return values
• Use the output of a function in further computations
• If there are no return statements in a function
• The function returns special type None at its end

Function style and documentation

• Two blank lines before & after each function
• Function docstring

​

def divide(number, divisor):

"""

Calculate the quotient and remainder.

​

Args:

number(int): the number to divide

divisor(int): the divisor

​

Returns:

int: the quotient

int: the remainder

"""

Program flow

• Default behavior is sequential
• Functions change the flow

​

Python Programs

• A Python program consists of one or more functions
• Each function consists of one or more statements
• Functions help organize programs into reusable blocks of code
• Also more manageable and easier to understand

​

• A statement may call (invoke) a function (or multiple functions)
• When called, the function’s code begins to execute
• When the function reaches a return statement (or it reaches its end), execution continues just after the point where the function was called
• If a value(s) is returned, it is used in the expression where the function was called

Python Programs

• A Python program may consist of multiple modules (.py files) of code
• Modules may import other modules
• Multiple files embedded into one another
• Forming one large program

​

Where does a Python program start?

Where does a Python program start?

• In most languages, a program starts with a function called main
• If you execute a Python file, it will start at the first executable statement
• The first unindented executable statement
• Function definitions are not executable
• Many Python programmers set up a main function like this

def main():

print("Hello World!")

​

if __name__ == "__main__":

main()

• The special __name__ variable’s value distinguishes between whether the module is executed directly, or imported

https://realpython.com/python-main-function/

Program Flow

14

15

10

11

5

2

6

2

7

2

12

​

Order of execution

(by line #)

Scope

• Local variables
• Variables defined within a function
• Function parameters are local to a function
• Global variables
• Variables defined outside of a function
• Can be accessed from inside functions
• To change a global variable inside a function requires a special global statement

Global variables

student_id = 1000

​

def define_student(fname, lname):

global student_id # required to change the global

eid = lname[0:6] + fname[0:2]

email = eid + "@dukes.jmu.edu"

student_id += 1;

return student_id, eid, email

​

print(define_student("mona", "rizvi"))

print(define_student("james", "madison"))

Function parameters and arguments

• If a function call argument is immutable,
• If the local variable (parameter) is modified, a new object is created
• Simple data types (int, float, bool, etc.) are immutable
• If a function call argument is mutable,
• If the local variable is modified, the original object is changed
• More on this later …

Multiple returns

student_id = 1000

​

def define_student(fname, lname):

global student_id

eid = lname[0:6] + fname[0:2]

email = eid + "@dukes.jmu.edu"

student_id += 1;

return student_id, eid, email

​

x = define_student("james", "madison")

print(x[0])

print(x[2])

​

Multiple returns

• It appears that you are returning many values separated by commas
• A series of values separated by commas automatically creates a tuple