Object: Function

A function object refers to a named block of Python code, designed to perform one specific task and only runs when it’s called, not defined. In this way, code can be reused easily. This is called modular programming.

You’ve already used builtin functions that are provided by the Python language.

x = input("Enter your name: ")
print(x)
y = len("seneca")

Functions: Visualization

Consider the len() function: we give it a string or list and it returns an integer equal to the length of the input. We don’t need to see the code to understand how to use it. And so we can save time by using functions.

Functions: Visualization

Consider the len() function: we give it a string or list and it returns an integer equal to the length of the input. We don’t need to see the code to understand how to use it. And so we can save time by using functions.

y = len("seneca")

Defining Your Own Functions

Functions are composed of two parts:

• Function definition (function name and parentheses)
• Body (defined by an indent level)

The body must contain something in order not to cause a Syntax Error. If you need to, you can simply use pass as a placeholder.

For example:

def hello():  # definition
    print("Hello!")  # body

def goodbye():  # TODO: finish tomorrow
    pass

Calling Functions

Once you define a function, it won’t run by default. It will only run when you call it. Write the name of the function, and include the parentheses.

hello()

By calling this function, the Python interpreter will print “Hello!” on screen.

Function Types

Functions can be broken into four major categories.

• Does not accept arguments or return a value (no in, no out)
• Does not accept arguments but returns a value ( no in, but out)
• Accepts arguments but doesn’t return a value (in, but no out)
• Accepts arguments and returns a value (both in and out)

Functions: Arguments

A function that accepts arguments is one that allows you to give it input when it’s run. For example:

def birthday(year):
    print('You were born in the year ' + year)

birthday('1970')
birthday('2005')

Functions: Arguments

You can have many arguments, but you must supply the same number as defined when you run it.

def birthday(year, month, day):
    print('You were born on ' + month + day + year)

# Main Program - Run Function
birthday('1970', 'January', '1')

Functions: Default Arguments

There is one exception to the rule about arguments: you can give an argument a default value. Then you can choose to pass it a value, or leave it.

def school_func(name, school="Seneca"):
    print(name + " attends " + school)

>>>> school("Eric")  # 1 arg, default for 2nd arg is used
Eric attends Seneca
>>>> school("Hong", "Centennial")  # 2 args, default is overridden
Hong attends Centennial

Functions: Parameters

Parameters are what we set up in the function definition. Each argument is passed into the body of the function and assigned a parameter name. Once the function stops, those parameter values are lost and the memory is freed up.

def get_age(year):
    now = 2024
    age = now - year
    print('You are ' + str(age) + ' years old.')

>>>> get_age(1991)
You are 33 years old.
>>>> print(now)
Error: Varible 'now' is not defined!

Functions: Return Values

One thing to keep in mind is that any parameters inside a function aren’t available to the rest of the code (not a global variable). If you do work inside the function and want to use it elsewhere, you have to return the value.

Note: as soon as your code encounters a return, it will stop running the function immediately!

# Function Definition
def calculate_age():
    age = 2024 - 1970
    return age  # stops the function
    print("We will never reach this line")

# Main Program - Run Function
print(calculate_age()) # Prints "54"

Functions: Arguments And Return Values

You can combine both to create an incredibly useful function. This example returns a True or False.

def can_drive(year):
    age = 2024 - year
    if age >= 16:  # legal age to get G1 license in Ontario
        return True
    else:
        return False

# Main Program - Run Function
x = input("Enter your date of birth: ")
if can_drive(x):
    print("You are old enough to drive.")

Function Annotations

• Generally Python functions will accept any datatype, even when this is going to cause an issue.
• You can choose to add annotations to your function definition, which will suggest datatypes for parameters and return values.
• Note: Depending on your configuration, breaking guidelines won’t stop your code. Consider these like comments.

def is_even(number: int) -> bool:
    "accepts an integer, and returns True or False"
    ...

Function Annotations

• parameter annotation comes after variable:
• return value annotation comes after ->

def student_grade(name: str, gpa: float) -> dict:
    "Puts name and gpa into a dictionary"
    ...

• int, float, list, dict, set, str, etc.

Some Guidelines For Creating Functions

• Shorter functions are always better than longer functions.
• Function names should be short: one verb only! 2-3 words!

Good:

• circle_area()
• is_odd()
• get_age()

Bad:

• do_stuff()
• process_form_and_find_all_names_and_put_names_into_a_file()
• calculation()

Some Guidelines For Creating Functions

• In programming, we try to avoid redundancy. This means if you have two blocks of code that are identical or very similar, you can probably replace them with one function.
• Functions can only return one thing. If your function is returning more than one thing, it probably needs to be two functions.

Scope

When we call a function, we pass in our arguments and associate them with parameters. Parameters are considered local, that is, they only exist inside of the function. Variables outside of functions are called global variables.

Don’t access global variables inside functions!! Pass them in as arguments if you need them! This will make your life easier.

Scope: Don’t Do This If You Want To Enjoy Your Life

x = 34

def a_func(x, y):  # local and global vars can have the same name
    global x  # don't do this
    z = x + y  # what is this going to do?
    return z

a_func(12, 45)

Main Block: Why?

This is important for when your code is going to be imported. Importing a python file will cause all the code to run, which is not what we want!

# this is a file called func.py

def a_useful_func():  # this is what I want to use
   ...

print("This is bad stuff")

>>>> import func
This is bad stuff

Main Block

A main block is useful here. It will run if you execute func.py by itself, but will be ignored during imports.

# this is a file called func.py

def a_useful_func():  # this is what I want to use
   ...

if __name__ == "__main__":
    print("This is best practice")

Concept: List Object

A list object is a special datatype. It refers to a series of items in a particular order, and each item in the list can be accessed using an index.

mylist = ['seneca', 'ict', 'cns', 'cty'] 

mylist is the name of a list object which contains four string objects: seneca, ict, cns, and cty. Your list can contain any combination of datatype you like: integers, floats, strings, other lists, etc.

Accessing Items Inside a List

We can navigate through this list using its index number to call it, a range, or whole list. Note: the last number in the slide tells us the first element not included in the slice.

mylist = ['seneca', 'ict', 'cns', 'cty']
print(mylist[0])  # This will print: seneca
print(mylist[1:3]) # This will print: ['ict', 'cns']
print(mylist)  # This will print ['seneca', 'ict', 'cns', 'cty']

Manipulating Lists

We can also manipulate the items in a list.

mylist = ['seneca', 'ict', 'cns', 'cty']
mylist.append('2019')
mylist.insert(1, 'S@Y')
mylist.remove('cty')
mylist.sort()

List Iteration

If you’d like to go through all the items in a list (to use them in a function, for example) you can do that as well.

mylist = ['seneca', 'ict', 'cns', 'cty']
for item in mylist:
    print('This item is: ' + item)
# This will print:
# This item is seneca
# This item is ict
# This item is cns
# This item is cty

For Loops

The For Loop is our second method of Iteration. The first was the while loop.

There are two variables: the first is a special iterating variable, which is singular. It contains each individual item, one at a time.

The second is our iterable. This is the name of the variable that we will iterate through.

Naming your variables correctly (singular and plural) will reduce confusion when you are writing code.

menu = ['falafel', 'pizza', 'burger', 'coffee']
healthy_choices = []  # empty list

for food in menu:
    if is_healthy(food):  # this calls a function
        healthy_choices.append(food)
    else:
        print("We should probably avoid too much " + food)