Python Tutorial - 7 - Functions

Functions

/*

• File: functions.md
• Project: 7_functions
• File Created: Wednesday, 15th March 2023 5:09:08 pm
• Author: Hanlin Gu ([email protected])

---

• Last Modified: Wednesday, 15th March 2023 8:44:59 pm
• Modified By: Hanlin Gu ([email protected]>)
  */

1. Define a function

In the def fun_name():, parenthesis are where the parameters are going into when the function is called.

If one would like to leave a function for later, one can use pass to avoid error. To execute the function, one need to add the parenthesis, otherwise it equals to the function itself.

def hello_func():
    pass

print(hello_func)

Output:

<function hello_func at 0x000001A6CB023E20>

def hello_func():
    pass


print(hello_func())

Output:

None

def hello_func():
    print('Hello Function!')


hello_func()

Output:

Hello Function!

Function allows one to reuse code without repeating oneself. This is called keeping your code DRY.

def hello_func():
    print('Hello Function.')


hello_func()
hello_func()
hello_func()

Output:

Hello Function.
Hello Function.
Hello Function.

1.1. Function return

def hello_func():
    return 'Hello Function.'


print(hello_func())

Output:

Hello Function.

For the first time to look at a function, focus on the input and what’s returned instead of understanding every details of what each function does.

As we know the function hello_func() returns a string. Then all the methods, functions that work for string can be applied to this function as well.

def hello_func():
    return 'Hello Function.'


print(hello_func().upper())

Output:

HELLO FUNCTION.

2. Pass arguments to a function

If one defined an argument in the function, it’s required to input an argument when calling that function otherwise there’ll be a TypeError.

def hello_func(greeting):
    return '{} Function.'.format(greeting)

print(hello_func())

Output:

Traceback (most recent call last):
  File "d:\Documents\Code\python\tutorial\8_functions.py", line 35, in <module>
    print(hello_func())
TypeError: hello_func() missing 1 required positional argument: 'greeting'

def hello_func(greeting):
    return '{} Function.'.format(greeting)


print(hello_func('Hi'))

Output:

Hi Function.

Notice that this greeting variable doesn’t affect any variable outside the function. It’s only local to the function, so that one doesn’t need to worry about anything that one doesn’t want to affect.

Right now the greeting is a required positional argument, because it doesn’t have a default value.

By setting default value for an argument is called a keyword argument, if the function doesn’t have that argument given then it will use the default value instead.

def hello_func(greeting, name='You'):
    return '{}, {}!'.format(greeting, name)


print(hello_func('Hi'))
print(hello_func('Hi', 'Corey'))
print(hello_func('Hi', name='Corey'))
print(hello_func(name='Corey', greeting='Hi'))

Output:

Hi, You!
Hi, Corey!
Hi, Corey!
Hi, Corey!

Note, the required positional arguments have to come before your keyword arguments. If you try to create a function with those out of order, then it will give you an error.

*args and **kwargs allow us to accept an arbitrary number of positional or keyword arguments. The name does not have to be args and kwargs but it’s good to stay with convention so that it’s more readable for others.

def student_info(*args, **kwargs):
    print(args)
    print(kwargs)


student_info('Math', 'Art', name='John', age=22)

Output:

('Math', 'Art')
{'name': 'John', 'age': 22}

args is a tuple of all the positional arguments, and kwargs is a dictionary of all the keyword values.

Some times one might see function call with arguments using * or **. When it’s used in that context, it will actually unpack a sequency or dictionary and pass those values into the function individually.

def student_info(*args, **kwargs):
    print(args)
    print(kwargs)


courses = ['Math', 'Art']
info = {'name': 'John', 'age': 22}

student_info(courses, info)

Output:

(['Math', 'Art'], {'name': 'John', 'age': 22})
{}

def student_info(*args, **kwargs):
    print(args)
    print(kwargs)


courses = ['Math', 'Art']
info = {'name': 'John', 'age': 22}

student_info(*courses, **info)

Output:

('Math', 'Art')
{'name': 'John', 'age': 22}

3. Snippets - number of days per month

The three quotes """Documents""" in the function is called doc string, and it helps document what a function or a class supposes to do.

# Number of days per month. First value placeholder for indexing purpose.
month_days = [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]


def is_leap(year):
    """Return True for leap years, False for non-leap years."""
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)


def days_in_month(year, month):
    """Return number of days in that month in that year."""

    if not 1 <= month <= 12:
        return 'Invalid Month'

    if month == 2 and is_leap(year):
        return 29

    return month_days[month]


print(is_leap(2023))
print(is_leap(2024))
print(days_in_month(2023, 2))
print(days_in_month(2024, 2))

Output:

False
True
28
29