👩‍💻 Programming

🪟 Editors

Let's get familiar with the two main parts of our code editor, Thonny.

1. The code editor: This is where we will write our Python code. We use this section when we write multiple lines of code that we want to execute all at once.

Running programs

Press the green run button to execute the code in the code editor

1. The REPL: This stands for Read-Evaluate-Print-Loop. This bit is interactive. When we enter a valid Python instruction (that is, type the instruction after the >>> then press the enter/return key), we will immediately see the result. We usually write short lines of code here when we want to test something quickly.

Command History

When using the REPL, you can press the up arrow key to load a previous command

Thonny is an IDE: An Integrated Development Environment. An IDE is any kind of editor that has multiple sections that are for different things, like editing code, running coding interactively (like a REPL), and managing files.

Code IDEs vs Word Processors

MS Word and Google Docs are examples of word processors. These apps deal with rich text, which is text that is embellished with lots of extra notation called markup so that the text can have fancy styles like bolding, colours, highlights, and different sizes.

However, all the markup makes it difficult to write Python code in MS Word. Please don't write code using Microsoft Word

Instead we write Python code in a plain text file with the file extension .py. Plain text is the kind you would usually write in Notepad (Windows) or TextEdit (Mac). You can't change the styling of the text, but it's much simpler; it's plain.

We use Thonny, instead of Notepad, because it has features that make it more convenient to edit Python code. These include (but are not limited to):

• Syntax highlighting: some words get a different colour, without extra markup
• Debugging: This feature lets you run code one instruction at a time
• Shell: This lets you run Python interactively

To see your files, select "View" then check "Files" in the menu.

💡 Coding is Writing is Thinking

Writing Python code is more than typing some words into Thonny. It is writing, and writing is thinking on paper. People who write well know how to sort out their thoughts, critique them, refine them, and express them in an accessible way to other people. The same can be said for Python. You can write convoluted, broken code, or write simple, elegant, and correct code that says nothing beyond what it needs to say, to do the things it needs to do.

Just like writing, learning to code well takes time. Google developer and leading AI-researcher Peter Norvig wrote about this best in his blog post Teach Yourself Programming in Ten Years. Do not be tempted to skim! Read it over in its entirety to set your expectations for learning to code.

Read it!

Even if you don't understand most of it, please read Teach Yourself Programming in Ten Years. The blog post is aimed at people interested in software development, but even if that is not you, all I want you to get out of it is that learning to code will take time and effort, so don't worry if you feel it's taking a long time for you get the hang of Python. We're all in the same boat!

🔨 Solving Problems with Code

It's important to have a good grasp of the problem you're trying to solve with Python (or knowing if you even need Python at all!). For some problems, it makes sense to write a program, i.e. a recipe of computations. Allen Downey, author of the wonderful (free!) book Think Python, says that there are really only 5 kinds of instructions that compose a program:

Instruction	Description
Input	Get data from the keyboard, a file, the internet, or some other device
Output	Display data on the screen, save it in a file, send it over the internet, etc
Math	Perform some number-crunching (add a number here, multiply some numbers there)
Conditional Execution	Check for certain conditions and run the appropriate code
Repetition	Perform some action repeatedly, usually with some variation each time

Basically, if you can express your problem as some combination of these instructions, you can write a program in Python to get your computer to solve your problem for you.

🛞 Process & Design

It's very easy to write a terrible essay; you simply need to write, and then stop writing. Writing a great essay takes research, planning, writing, and multiple rounds of revisions and gathering feed-back. A similar thing happens (or at least, should happen) when writing software (i.e. code). It's easy to write code that doesn't actually solve your problems correctly or robustly.

In his book, Code Complete, Steve McConnell laid out a few key components of the software development process¹:

Step of Process	Description
Problem definition	What problem are we trying to solve? (Don't offer solutions yet!)
Requirements development	What features does our solution have to have? Does Python offer these features?
Construction planning	Think about how we could implement the solution, step by step
Coding & debugging	Implement the solution in Python. Fix bugs.
Testing	Test! Test! Test! Write lots of tests for your Python code. Retest when you make changes

📦 Scripts vs Apps vs Libraries

A script is a short piece of code, usually in a single file, that does one main thing. For example, this script fetches a random useless fact from the internet:

uselessfact.py

Try it yourself

You don't need to how this code works at this point in the guide, but try typing this code into the code editor section of Thonny (the top portion, not the Shell). Then press the green run button, and look at the Shell section to see the result.

Try re-running the script a few times to see different useless facts.

Scripts are different from applications, or apps, which are bundles of code that you use to accomplish one or more tasks. Apps are typically more complicated than scripts. Compare the script above to something like your calendar app, which can send notifications, set reminders, store data related to events, dates, and times, has a user interface, and can synchronize data across multiple devices.

There is another kind of software called a library. A library is a bundle of code that is meant to be used in other people's code. Libraries tend to be pretty hard to write because you have to think about how the code will be used on many different operating systems, and write lots of tests, publish documentation on how to use the library, and maintain it when people find bugs or are looking for more features. Python developers typically use the word "packages" instead of library. PyPI is where you can find Python packages.

Specific to Python, we have modules. These are files with reusable bits of code. Python comes pre-installed with many modules. For example, we can use the random module which lets us write code that can generate random numbers

Python random module

import random
print("Here's a random number from 1 to 10:", random.randint(1, 10))

We need to write import random to tell the Python interpreter to load the code from the random module. It's a good thing the interpreter doesn't automatically load every single built-in module, otherwise it would take a while to start up. Better for us to explicitly ask for just the specific modules we need.

Note

In this guide, we will stick to writing modules and scripts

🏋️‍♂️ Exercises

Q1. What's an IDE?

Answer

An Integrated Development Environment. It's a piece of software that makes it easier to write and edit code.

Q2. What is a program? What are the different instruction types for a program?

Answer

A program is a sequence of instructions of how to perform a computation. In other words, a recipe of computations.

Instruction	Description
Input	Get data from the keyboard, a file, the internet, or some other device
Output	Display data on the screen, save it in a file, send it over the internet, etc
Math	Perform some number-crunching (add a number here, multiply some numbers there)
Conditional Execution	Check for certain conditions and run the appropriate code
Repetition	Perform some action repeatedly, usually with some variation each time

Q3. What is the process for writing code?

Answer

Part of Process	Description
Problem definition	What problem are we trying to solve? (Don't offer solutions yet!)
Requirements development	What features does our solution have to have? Does Python offer these features?
Construction planning	Think about how we could implement the solution, step by step
Coding & debugging	Implement the solution in Python. Fix bugs.
Testing	Test! Test! Test! Write lots of tests for your Python code. Retest when you make changes

Q4. What is the difference between a script, an app, and a library? Provide an example for each

Answer

Type of Software	Description	Example
Script	Typically a single file of code that accomplishes a few simple/short tasks	A script to download files from the internet
App	A bundle of software that people use to accomplish a few complicated tasks	Thonny is an app for editing Python code
Library	A bundle of software that people can use in their own code	pandas is data analysis library for Python

Q5. Consider this Python code that verifies if someone is old enough to drink

age = int(input("Please enter your age: "))

drinking_age = 19

if age == drinking_age:
    print("You may enter the bar")
else:
    print("Sorry, you are not old enough to drink")

Type it into Thonny, then run it. You can enter an age by typing it into the Shell, then pressing the enter/return key.

i. Does the program do what it's supposed to? Why or why not? Rerun the code and try a few different ages

Answer

No, the code produces an incorrect result for ages greater than 19. It should respond "You may enter the bar", but doesn't.

This is because the code checks if age equals drinking age. Instead it should check if age is greater than or equal to drinking_age.

Don't worry if you weren't sure how to read the Python script, yet (after all, we haven't properly gone over the basic words and phrases you can use in Python).

If you were able to figure it out, great! Can you try and fix the code?

ii. Can you think of any inputs you can give that will crash the program (i.e. cause it to throw an error)?

Answer

The program will crash if you provide inputs that don't make any sense, like letters or punctuation.

If you haven't already, try running the code again and input abc or something similar.

iii. Why is testing your code so important?

Possible Answer

Testing brings your attention to errors in your code. There are many reasons for errors:

• It's easy to accidentally mistype something without realizing it
• Sometimes your solution to the problem isn't actually correct, or doesn't work the way you expect
• When you change one part of your code, you could inadvertently break some other place in your code
• Users of your code will find a way to break your code by accidentally giving inputs you could have never expected
• Malicious actors can purposely give unexpected inputs that exploit security vulnerabilities in your code

Testing helps ensure your code is more robust!

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1. The full software development process is much lengthier and much more involved, but this list is good enough for most of the software you write as beginner Python programmer ↩