Functions

Ha Khanh Nguyen (hknguyen)

• 1. What is a Function?
  • 1.1 Defining a function vs. A function call?
• 2. Function Properties
  • 2.1 Namespaces, scope, and local functions
  • 2.2 Returning multiple values
  • 2.3 Anonymous (labmda) functions
• 3. Exercise

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1. What is a Function?

• Function here means user-defined function.
• If you anticipate needing to repeat the same or very similar code more than once, it may be worth writing a reusable function.

# define a function
def my_function(x, y, z=1.5):
    if z > 1:
        return z * (x + y)
    else:
        return z / (x + y)

# execute the function
my_function(1, 2, 3)

9

my_function(1, 2)

4.5

• Use def keyword to define a function.
• A function might have argument(s). Argument(s) is not required for a function.
  • In my_function() above, x, y, and z are the arguments.
  • z=1.5 means that if no value is provided for z, the default value of z is 1.5.
• Here is an example of a function with no argument:

# define a function
def function_without_argument():
    print('I\'m still a function!')

# execute the function
function_without_argument()

I'm still a function!

1.1 Defining a function vs. A function call?

# define the function
def my_function(x, y, z=1.5):
    if z > 1:
        return z * (x + y)
    else:
        return z / (x + y)

# execute the function
my_function(1, 2, 3)

9

# execute the function
my_function(1, 2, 0.5)

0.16666666666666666

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2. Function Properties

2.1 Namespaces, scope, and local functions

• Functions can access variables in two different scopes: global and local.
• A variable scope in Python is also called a namespace.
• Any variables that are assigned within a function by default are assigned to the local namespace.
• The local namespace is created when the function is called and immediately populated by the function’s arguments.
• After the function is finished, the local namespace is destroyed (with a few exceptions).

# example 1
def func():
    a = 5
    return a

func()

5

print(a)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-11-bca0e2660b9f> in <module>
----> 1 print(a)

NameError: name 'a' is not defined

# example 2
a = 0
def another_func():
    b = a + 1
    return b

another_func()

1

a

0

2.2 Returning multiple values

def f():
    a = 5
    b = 6
    c = 7
    return a, b, c

a, b, c = f()
print(a)
print(b)
print(c)

5
6
7

• The function is actually just returning one object, namely a tuple, which is then being unpacked into the result variables.
• We can also return a dictionary instead!

2.3 Anonymous (labmda) functions

• Anonymous or lambda functions are ways of writing functions consisting of a single statement, the result of which is the return value.

def short_function(x):
    return x * 2

equiv_anon = lambda x: x * 2

short_function(2)

4

equiv_anon(2)

4

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3. Exercise

Write a function named largest_mult_of_three() that takes in 4 input: x, y, z and w (which represents a positive number). Find the largest number that is divisible by 3 among those 4 numbers and return it as the function output.

If none of the numbers are divisible by 3, return -1.

def largest_mult_of_three(x, y, z, w):
    # define the function below

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This lecture notes referenced materials from Chapter 3 of Wes McKinney's Python for Data Analysis 2nd Ed.