Lecture VI - Using Modules and Packages

Programming with Python

Dr. Tobias Vlćek

Kühne Logistics University Hamburg - Fall 2024

Quick Recap of the last Lecture

Exceptions and Error Handling

  • Exceptions are discovered errors during program execution
  • Common built-in exceptions: ValueError, TypeError, etc.
x = int("Hello, World!")

>ValueError: invalid literal for int() with base 10: ‘Hello, World!’

Try-Except Blocks

  • try-except blocks are used to handle exceptions
  • try block contains code that might raise an exception
  • except block contains code executed if an exception occurs
try:
    # Code that might raise an exception
    # ...
except ExceptionType as e:
    # Code to handle the exception
    # ...
except Exception as e:
    # Code to handle any other exceptions
    # ...

Raising Exceptions

  • We can raise exceptions using the raise statement
  • Allows for more controlled error handling
  • Can include custom error messages
raise ValueError("This is a custom error message")

Note

The type if raised exception has to exist or you have to create a custom error type before.

Assertions

  • Assertions check if a condition is true
  • If the condition is false, an AssertionError is raised
  • Useful for checking calculations or variable types
x = -1
assert x > 0, "x must be positive"

Question: Will this raise an AssertionError?

Debugging

  • Debugging is the process of finding and fixing errors in code
  • Using print and assert statements
  • Using logging
  • Using built-in debugging tools in IDEs

Tip

That’s why IDEs are so helpful in coding.

Modules

Why Modules?

  • Modular programming breaks large tasks into smaller subtasks
  • Modules are like building blocks for larger applications
  • Individual modules can be combined to create a complete program
  • This approach enhances code organization and reusability

Creating Modules

  • Modules are simply .py files containing Python code
  • They can define functions, classes, and variables
  • They can be imported into other Python scripts
# The script new_module.py is in the same directory as this script
import new_module # Here we import the module
new_module.my_function() # Here we call the function from the module
Hello from my_function!

Importing functions from modules

  • We can also import specific functions from a module
  • This is useful if we only need a few functions from a module
  • Analogously, we can import classes or variables from a module
# Multiple imports from a module are possible as well!
from new_module import another_function, yet_another_function
another_function()
yet_another_function()
Hello from another_function!
Hello from yet_another_function!

Tip

This is a good way to avoid importing too much from a module. In addition, we don’t need to use the module name before the function name when we use the functions from the module.

Built-in Modules

Python comes with many built-in modules. Common ones include:

Module Description
math Different mathematical functions
random Random number generation
datetime Date and time manipulation
os Operating system interaction
csv Reading and writing CSV files
re Regular expression operations

Importing from the Standard Library

Task: Use Python’s math module to calculate the area of a circle.

# Import the `math` module.
# Define a function named `calculate_area` that takes the radius `r` as an argument.
# Inside the function, use the `math.pi` constant to get the value of π.
# Calculate the area in the function and return it.

# Your code here

assert calculate_area(5) == 78.53981633974483

Tip

Note, how assertations can be used to check if a function works correctly.

Standard Libraries

Random Numbers

The random module provides functions for random numbers

  • random.random(): random float between 0 and 1
  • random.uniform(a, b): random float between a and b
  • random.randint(a, b): random integer between a and b
  • random.choice(list): random element from a list
  • random.shuffle(list): shuffle a list

Tip

There are many more functions in the random module. Use the help() function to get more information about a module or function.

Random Numbers in Action

Task: Time for a task! Import the random module and create a small number guessing game with the following requirements:

# Generate a random integer between 1 and 10 using randint().
# Ask the user to guess the number with input().
# Print whether the guess was correct.
# Give a hint if the guess was too high or too low.
# Repeat the game until the user guesses the number.

# Your code here

Tip

Remember, that the input function always returns a string!

OS Module

  • The os module provides functions to interact with the OS
  • os.listdir(path): list all files and directories in a directory
  • os.path.isfile(path): check if a path is a file
  • os.path.exists(path): check if a path exists
  • os.makedirs(path): create a directory

Tip

These can be quite useful for file handling. The os module contains many more functions, e.g. for changing the current working directory, for renaming and moving files, etc.

CSV Module

  • Comma-Separated Values files are used to store tabular data
  • Write: csv.writer(file)
  • Read: csv.reader(file)
import csv # Import the csv module

with open('secret_message.csv', 'w') as file: # Open the file in write mode
    writer = csv.writer(file) # Create a writer object
    writer.writerow(['Entry', 'Message']) # Write the header
    writer.writerow(['1', 'Do not open the file']) # Write the first row
    writer.writerow(['2', 'This is a secret message']) # Write the second row

Task: Copy the code and run it. Do you have a new file?

OS and CSV Module in Action

Task: Time for another task! Do the following:

# First, check if a directory called `module_directory` exists.
# If it does not, create it.
# Then, list all files in the current directory and save them in a CSV file called `current_files.csv` in the new `module_directory`.

import os
if not os.path.exists('module_directory'):
    pass
# Your code here

Regular Expressions

What are Regular Expressions?

  • Regular expressions are a way to search for patterns in text
  • They are a useful tool for string manipulation
  • We can use the re module to work with regular expressions
import re
pattern = r'World' # This is the pattern we are searching for
string = 'Hello, World!' # This is the string we are searching in
print(re.search(pattern, string)) # This will search for the pattern in the string
<re.Match object; span=(7, 12), match='World'>

Note

So far, we could also have achieved this with the find method of a string.

Why Regular Expressions?

import re
pattern = 'World' # This is the pattern we are searching for
string = 'Hello, World!' # This is the string we are searching in
print(string.find(pattern)) # No regular expressions here!
7
  • But regular expressions are more powerful and flexible
  • They have special characters that allow for complex patterns
  • They are widely used in text processing and web scraping

Using Regular Expressions

  • re.search(pat, str): search for a pattern in a string
  • re.findall(pat, str): find all occurrences of a pattern
  • re.fullmatch(pat, str): check if entire string matches pattern
  • re.sub(pat, repl, str): replace a pattern in a string
  • re.split(pat, str): split a string by a pattern

Note

As always, there is more. But these are a good foundation to build upon.

Regular Expression in Action

Task: Replace all occurences of Python by “SECRET”.

import re
string = """
Python is a programming language. 
Python is also a snake. 
Monty Python was a theater group.
"""
# Your code here

Note

Regular expressions are even more powerful when combined with special characters.

Special Characters I

  • . matches any character
  • * matches zero or more of the preceding element
  • + matches one or more of the preceding element
  • ? matches zero or one of the preceding element
  • [] matches any character in the brackets
  • | matches either the left or the right side
  • \d matches any digit
  • \w matches any word character (alphanumeric and underscore)
  • \s matches any whitespace character

Special Characters II

  • There are many more special characters in regular expressions
  • In order to keep things simple, we will not cover them here
import re
print(re.findall(r'\d{3}-\d{2}-\d{4}', 'Here is a phone number: 123-45-6789.'))
['123-45-6789']

Tip

It can be quite complicated to get the hang of these special characters, especially at the beginning. Gladly, there are tools like regexr.com that can help with building the right pattern. Apart from that, help(re) in the terminal can also be very helpful.

Advanced Regular Expressions in Action

Task: Use regular expressions to extract all dates from the text.

dates = """
On 07-04-1776, the United States declared its independence. Many years later, 
on 11-09-1989, the Berlin Wall fell. In more recent history, the COVID-19 
pandemic was declared a global emergency on 04-11-2020.
"""
# Try to find all dates in the above text with findall()
# Your code here

Packages

What are Packages?

  • Packages are esentially collections of modules
  • They can contain multiple modules, subpackages, and data files
  • Many packages are available in the Python Package Index (PyPI)
  • You don’t have to invent the wheel yourself
  • A lot of functionality is already implemented by others!

Installing Packages

  • Packages are installed in the shell
  • Use pip install <package_name> to install a specific package
  • Afterward you can import from the package in your Python scripts

Tip

With Thonny you can install packages directly in the IDE. Simply click on Tools -> Manage packages and search for the package you want to install.

Packages in Action

Task: Install the pandas and numpy packages, which are commonly used for data analysis. We will use them together next week!

pip install pandas numpy

Tip

If you install packages like this, you can use the shell to do so! Alternatively, you can use pip install <package_name> in the Python terminal or Thonny.

Virtual Environments

  • Virtual environments are used to manage dependencies
  • They allow you to have different environments for projects
  • They can be created using the venv module
  • This becomes important if you work on several projects at once

Note

And that’s it for todays lecture!
We now have completed the first step into data science in Python. Next week, we can use this new knowledge to start to work with some tabular data and matrices.

Literature

Interesting Books

  • Downey, A. B. (2024). Think Python: How to think like a computer scientist (Third edition). O’Reilly. Link to free online version
  • Elter, S. (2021). Schrödinger programmiert Python: Das etwas andere Fachbuch (1. Auflage). Rheinwerk Verlag.

Tip

Nothing new here, but these are still great books!

For more interesting literature to learn more about Python, take a look at the literature list of this course.