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Introduction to Python - Lesson 2 - Control Flow, Functions, Scopes

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By Tomislav Dukez 9 min read

2.3. Functions

Function in Python Documentation

  • function is a reusable block of code that performs a specific task.
  • function can be defined in two ways:
    • function definition
    • function declaration

2.3.1. Function definition in Python

  • syntax: def function_name(parameters):
  • parameters are optional
  • return is optional
  • : is required
  • indentation is required
  • pass is optional
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# declaration of the function to print the title
def print_title():
    print('---------------------------------------------------------------')
    print('{:^63}'.format('Lesson 2: Functions'))
    print('---------------------------------------------------------------')
  • Why nothing happened when we run the code? Because we only defined the function, but we didn’t call it. Functions are like a recepie for our code. We have the instructions, but if we don’t say - ok make it so, nothing happens.
  • Therefore, we need make a function call. We do that by writing the name of the function and parentheses.
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# function call
print_title()

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---------------------------------------------------------------
                      Lesson 2: Functions                      
---------------------------------------------------------------

2.3.2. Function definition with return value

  • if a function doesn’t have a return value, the default value is None. We call that function a void function.
  • usually, we expect a function to return a value, after some data manipulation
  • we use the return keyword to return a value from a function
  • we also use the return keyword to exit the function

Because of this, the value of the function needs to be assigned to a variable, stored in a file or passed to another function.

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# weather forecast functin

def weather_forecast():
    return 'The weather will be sunny with a high of 75 degrees.'

print(weather_forecast())

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The weather will be sunny with a high of 75 degrees.

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# news
def news():
    return 'The news today is that the stock market is up 100 points.'

headline = news()
print(headline)

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The news today is that the stock market is up 100 points.

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# using Pass in a function
def check_triangle():
    pass

check_triangle()

We use pass to create an empty function. It is useful when we want to create a function that we will fill in later.

2.3.3 Functions with parameters

  • functions can take parameters
  • parameters are variables that are passed to the function when it is called
  • we use the : operator to specify the type of a parameter (optional)
  • we user the -> operator to specify the type of the return value (optional)
  • we use the = operator to assign a default value to a parameter if it is not specified in a function call
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# function with parameters
def add_two_numbers(num1, num2):
    return num1 + num2

# function with specified types of parameters
def add_three_numbers(num1: int, num2: int, num3: int) -> int:
    return num1 + num2 + num3

# function with default parameters
def multiply_by_two(num1, num2=2):
    return num1 * num2

sum_of_two = add_two_numbers(1, 2)
sum_of_three = add_three_numbers(1, 2, 3)
product_of_two_one_parameter = multiply_by_two(2)
product_of_two = multiply_by_two(2, 3)

print(sum_of_two)
print(sum_of_three)
print(product_of_two_one_parameter)
print(product_of_two)

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2.3.4. Functions with default parameters

  • as we saw in the previous example, we use the = operator to assign a default value to a parameter if it is not specified in a function call
  • there is a rule that limits the use of default value of the parameters. That is, if we specify a default value of the parameter, the parameters that follow also need to have a default value, whilst the parameters that come before the one with the default value do not need to have a default value.
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# function with default values
def multiply_by_two(num1, num2=2):
    return num1 * num2

print(multiply_by_two(2))
print(multiply_by_two(2, 3))

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# function with default values folowed by undefined
def area_of_triangle(side_a=3, side_b=4, side_c):  # side_c is undefined and will cause an error
    return side_a * side_b / 2

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  Cell In[3], line 2
    def area_of_triangle(side_a=3, side_b=4, side_c):  # side_c is undefined and will cause an error
                                             ^
SyntaxError: parameter without a default follows parameter with a default

2.3.5. Docstrings

  • docstrings are used to document functions
  • docstrings are used to provide information about the function
  • they are written in triple quotes
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# example of docstring
def print_title():
    """Prints the title of the lesson"""
    print('---------------------------------------------------------------')
    print('{:^63}'.format('Lesson 2: Functions'))
    print('---------------------------------------------------------------')

print_title.__doc__


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'Prints the title of the lesson'
  • We can also give the explanation of the function input and output
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# docstring with explanation of input and output
def check_triangle(side1, side2, side3):
    """Checks if a triangle is valid
    Parameters
    ----------
    side1 : int
        The length of the first side
    side2 : int
        The length of the second side
    side3 : int
        The length of the third side
    Returns
    -------
    bool
        True if the triangle is valid, False otherwise
    """
    return side1 + side2 > side3 and side1 + side3 > side2 and side2 + side3 > side1

print(check_triangle.__doc__)

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Checks if a triangle is valid
    Parameters
    ----------
    side1 : int
        The length of the first side
    side2 : int
        The length of the second side
    side3 : int
        The length of the third side
    Returns
    -------
    bool
        True if the triangle is valid, False otherwise

2.3.6. Scope

  • scope is the visibility of a variable. It determines where a variable can be accessed.

  • variables can be accessed from anywhere in the program, depending on their scope

  • there are 2 important types of scopes in Python:
    • local scope
    • global scope

2.3.6.1. Local scope

  • local scope is the scope of a variable in a function or class
  • local scope is defined by indentation
  • local scope is only accessible within the function or class
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# function with variable definition
def sum_numbers(num1, num2):
    total = num1 + num2
    return total

print(sum_numbers(1,3))

print(total)

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4



---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

Cell In[16], line 8
      4     return total
      6 print(sum_numbers(1,3))
----> 8 print(total)


NameError: name 'total' is not defined

2.3.6.2. Global scope

  • global scope is the scope of a variable in the program
  • global scope is defined by the global keyword
  • global scope is accessible from anywhere in the program
  • global scope is not limited to a function or class
  • global scope can be overwritten by a local scope
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name = 'John'
def print_name():
    # we can use name in our function, but that will not change the global variable value
    name = 'Jane'
    print(name)

print_name()
print(name)

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Jane
John

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# we can access a global variable in a function
name = 'John'
def print_name():
    global name
    name = 'Jane' # this will change the global variable value
    print(name)

print_name()
print(name)

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Jane
Jane

We have to be careful when we overwrite a global variable with a local variable. It can lead to unexpected behavior of our program.

2.4. Type Conversion Functions

  • type conversion is the process of converting a value from one type to another
  • we use the type() function to get the type of a value
  • we use the int() function to convert a value to an integer
  • we use the str() function to convert a value to a string
  • we use the float() function to convert a value to a float
  • we use the bool() function to convert a value to a boolean

We have already seen that we can use the type() function to get the type of a value. And when we were talking about input, we have also used the int() function to convert a value to an integer. Let us combine this knowledge with functions.

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# make a function that requires an input of a number from the user
def get_number():
    return int(input('Enter a number: '))

print(get_number())

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34

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# let as reverse a number backwards
num = 123456789

#first we need to make it a string
str_num = str(num)

#now we can reverse it with a slice
str_num = str_num[::-1]

# now we can convert it back to an int
num2 = int(str_num)

print(num2)




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987654321

We will often convert from floats to integers and vice versa in order to do some calculations.

For example, if we want to take the half of a string, we need to take the length of the string, divide it by two and then slice the string, but the index can be only an integer.

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# slice a string in half
str = 'Let us slice this string in half'

half = len(str)/2
print(half)  # this is not an int
print(int(half))

half_index = int(half)
print(str[:half_index])
print(str[half_index:])

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16.0
16
Let us slice thi
s string in half

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# Different approach
str = 'Let us slice this string in half'

half = len(str)//2  # this is an int, result of floor division
print(half)  # this is an int

print(str[:half])
print(str[half:])

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16
Let us slice thi
s string in half

Sometimes we need from the user the input of a decimal number. We can use the float() function.

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# let us calculate the total price based on a purchase
purchase = float(input('Enter the amount of the purchase: '))
tax = 0.08
total = purchase * (1 + tax)
print(total)


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108.0
python, lesson
python programming code
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