Functions, Modules and Libraries

AESB2122 - Signals and Systems with Python

Geet George

Functions

A function is a reusable block of code that performs a specific task.
Syntax:

def function_name(arguments):
    # code block
    return result

def: keyword used to define a function. After def, Python understands that a function is being defined
arguments: a list of values that the function can accept as input. These could be any in number, any data type, but position matters (unless they are keyword arguments)
return: keyword used to exit a function and send back a value. Anything after return is the value that gets sent back. Also, the function is considered finished when it hits a return statement.

Convert from Celsius to Kelvin

Let’s say we want to convert temperatures to Kelvin

temperature_celsius = 25
temperature_kelvin = temperature_celsius + 273.15
print(temperature_kelvin)

298.15

How to make this a function?

Identify inputs (argument) and outputs (return)
Place the code that converts input to outputs between the def statement and return statement
Ensure that the argument keyword is used correctly in the code

def convert_celsius_to_kelvin(temperature_C):
    temperature_K = temperature_C + 273.15
    return temperature_K

Calling the Function

temperature_kelvin = convert_celsius_to_kelvin(25)
print(temperature_kelvin)

298.15

Practice exercise

Create a function that identifies whether a given input in degree Celsius is a possible surface temperature in the Netherlands and if so, convert to Kelvin. Otherwise return the input value.

def convert_celsius_to_kelvin(temperature_C):
    if -70 <= temperature_C <= 70:  # Surface temperature range in the Netherlands
        temperature_K = temperature_C + 273.15
        return temperature_K
    return temperature_C

Use functions in a different file

Create a new Python file named sine_wave.py. In it:

import numpy as np

def create_sine_wave(frequency, duration):
    t = np.linspace(0, duration, int(44100 * duration))
    return np.sin(2 * np.pi * frequency * t)

Check the output:

print(create_sine_wave(440, 2))

[ 0.00000000e+00  6.26490336e-02  1.25051934e-01 ... -1.25051934e-01
 -6.26490336e-02  1.25522677e-13]

Import our function to a different file

Create a new file named script.py. In it:

from sine_wave import create_sine_wave

sine_wave = create_sine_wave(440, 2)
print(sine_wave)

In using a function from a different file, what we did right now is import a module!

Modules

A module is simply a file containing any Python code (functions, classes, variables).

You can simply import such modules and use them in other Python scripts. This is called modular programming and it helps in organizing code, reusability, and maintaining the codebase.
A module can have other modules imported within it, allowing for a hierarchical organization of code.
So, this is essentially what you do when you, for instance, do import numpy as np. You are importing the NumPy library which gives access to all the functions and classes defined in that library.
Libraries are essentially collections of modules, usually thought of as a package.

Exercise

A. Create a Python script sine.py that:

1. Defines a function `create_sine_wave` that generates a sine wave given a frequency and duration.

2. Plot the output of the function and save it as `sine_wave.png`.

B. Run the script from the terminal and check the output.

C. Create another script script.py that imports the create_sine_wave function from sine.py and uses it to plot 3 sine waves with different frequencies in the same plot.

D. Run script.py from the terminal and check the output.