Guide to Python (University at Buffalo Version)

Chapter 5: Working With Functions

Python Functions

Python functions provide a way to create a group of statements that perform a certain task and then "call" that function from your Python code.

def factorial(n):
    """
    Calculate the factorial of a given number n.
    
    Parameters:
    n (int): The number to calculate the factorial for.
    
    Returns:
    int: The factorial of the number n.
    """
    if n < 0:
        return "Factorial is not defined for negative numbers."
    elif n == 0 or n == 1:
        return 1
    else:
        result = 1
        for i in range(2, n + 1):
            result *= i
        return result

# Example usage:
number = 5
print(f"The factorial of {number} is {factorial(number)}")

Figure 5-1: A Python Function

The factorial of 5 is 120

Figure 5-2: A Python Function Output

The Python `import` Statement

Python allows you to have a function in a separate file (called a module), such as functions.py and, using the Python import statement access that function in any Python program.

from functions import printName

Figure 5-3: Using import (functions.py)

The factorial of 5 is 120

Figure 5-4: Python import Output

Function Parameters

Python allows you to pass arguments to a function by declaring them as the parameters of the function.

# module functions2.py
def welcome(name):
	print(f"Hello {name}!")
	

# welcome.py
from functions2 import welcome

welcome("Jim")
welcome("D'arcy")

Figure 5-5: Passing Arguments to a Function

Hello Jim!
Hello D'arcy!

Figure 5-6: Passing Arguments to a Function Output

Using Default Function Parameters

Python also allows you to have default values parameters if that parameter is not in the arguments passed to a function by assigning a default value to them as the parameters of the function.

# module functions2.py
def welcome(name  "Default Name"):
	print(f"Hello {name}!")
	

# welcome.py
from functions2 import welcome

welcome("Jim")
welcome("D'arcy")
welcome()

Figure 5-7: Using Default Arguments with a Function

Hello Jim!
Hello D'arcy!
Hello Default Name!

Figure 5-8: Using Default Arguments with a Function Output

Using Optional Function Parameters

Python also allows you to have optional parameters if that parameter is not in the arguments passed to a function.

# module functions2.py
def welcome(*name):
	print(f"Hello {name}!")
	

# welcome.py

from functions2 import welcome

welcome(Jim)

Figure 5-9: Using Optional Arguments with a Function

Hello Jim!

Figure 5-10: Using Optional Arguments with a Function Output

Using the `math` and `cmath` Modules

The Python math and cmath modules for performing calculations. The cmath module is used when working with complex numbers. These modules provide many methods.

#math_module.py

import math

# find the square root of a number
num = float(input('Enter a number to find its square root: '))
num_sr = math.sqrt(num)
print(f'The square root of {num} is {num_sr:.2f}.\n')

# Use the math.pi constant to calculate the circumference of a circle
rad = float(input('Enter the radius of the circle: '))
circ = float(2 * math.pi * rad)
print(f'The circumference of a circle with a radius of {rad} is {circ:.2f}.\n')

# use the math.pow to raise a number to a power
num = float(input('Enter a number: '))
pow = float(input(f'Enter a power for {num:.2f}: '))
num_pow = float(math.pow(num, pow))
print(f'The number {num:.2f} raised to the power of {pow:.2f} is: {num_pow:,.2f}.')

Figure 5-11: Using the math Module

Enter a number to find its square root: 65
The square root of 65.0 is 8.06.

Enter the radius of the circle: 89
The circumference of a circle with a radius of 89.0 is 559.20.

Enter a number: 5
Enter a power for 5.00: 9
The number 5.00 raised to the 9.00 power is: 1,953,125.00.

Figure 5-12: Using the math Module Output