Python Programming I
Senior 4 · ICT
Python Programming I
Lesson 1 of 5
22 min
In Unit 3 you wrote algorithms that a person could follow: Euclid's method, the canteen till, the leap-year rule. A computer cannot read pseudocode. It needs the algorithm in a programming language, written so exactly that there is no room for guessing. In this unit that language is Python, one of the most widely used languages in the world, in schools, in science, in web servers and in artificial intelligence. Python is free, and its programs read almost like the pseudocode you already know.
Definition
Programming language: a formal language with exact rules (its syntax) for writing instructions a computer can carry out. Source code: the text of a program.
High-level language: a language close to human language and independent of the processor, such as Python; it must be translated into the machine's own instructions.
Interpreter: a program that translates and runs source code one statement at a time. Compiler: a program that translates the whole source code into machine code before it runs. Python is normally interpreted.
IDE (integrated development environment): an application for writing, running and debugging code. Python comes with a simple one called IDLE.
Guido van Rossum began Python at the end of the 1980s at the CWI research centre in Amsterdam and posted the first public version to the internet in February 1991. He named it after the BBC comedy series Monty Python's Flying Circus, which he was reading at the time: he wanted a name that was short, unique and slightly mysterious. Python 3, the version used today and in this unit, appeared in 2008, and a new feature release now comes out every year. Python is open source: anyone may download, use and share it free of charge.
Because the interpreter works statement by statement, a program can run correctly up to line 9 and then stop with an error on line 10. The error message tells you where to look.
Getting Python. Download Python 3 free from python.org/downloads and run the installer; many macOS and Linux computers already have it. The code boxes in these lessons have a Run button that runs Python for you, so you can practise on a phone or a school computer without installing anything.
>>> prompt. Type one statement, press Enter, and Python runs it at once and shows the result. It is a calculator and a place to try ideas, but nothing is saved..py (for example hello.py), and choose Run → Run Module (F5). The output appears in the shell window. Programs you want to keep are always written in script mode.
print() is a built-in function: a named piece of code that does a job. Whatever you put between its brackets is shown on the screen. Text must be in quotation marks (single or double); numbers and calculations are not. A line starting with # is a comment: Python ignores it, but people reading the code do not.
Press Run. Each print() starts a new line. When print is given several items separated by commas, it shows them with a space between. 7 * 6 has no quotation marks, so Python calculates it and prints 42. An empty print() prints a blank line.
Worked example 1: predict, then run
What does this print?
print("2 + 3")
print(2 + 3)
print("Total:", 2 + 3, "Frw")Line 1: the quotes make it text, so it prints 2 + 3 exactly. Line 2: no quotes, so Python calculates and prints 5. Line 3: three items, text, a calculation and text, printed with spaces: Total: 5 Frw. Type the three lines into the code box above (replacing its lines) and run them to check.
Programmers call a mistake a bug and finding it debugging. Python reports two kinds you will meet first:
A third kind, a logic error, gives no message at all: the program runs but gives the wrong answer, which is why you test with data whose answer you know.
Press Run. The first line prints, then Python stops with NameError: name 'Print' is not defined and reports line 2. Python is case-sensitive: print and Print are different names. Change the capital P to p and run again.
Worked example 2: find and fix three bugs
print("Welcome to S4 ICT)
print("Lessons per week: " 3)
prnt("Have a good term")print("Welcome to S4 ICT").print("Lessons per week:", 3).prnt is misspelled: a NameError when the line runs. Fix: print("Have a good term").Because of the syntax errors, Python would not run even the first line; it reports the first error it finds, and you fix them one at a time.
Corey Schafer: Python Tutorial for Beginners 1, Install and Setup for Mac and Windows. Watch how Python is installed, how the interactive prompt works and how a .py file is run. The version numbers in the video are older; always download the current Python 3.
Warning
Copy the punctuation exactly. Quotation marks come in pairs, every opening bracket needs a closing one, and word processors replace straight quotes " with curly ones “ ”, which Python rejects. Type code in IDLE or a code editor, not in Google Docs or Word.
Read the last line of an error first. It names the error (SyntaxError, NameError…) and the line above it shows where.
Tip
Use the shell as a calculator and a test bench. Not sure what 7 / 2 gives? Type it at the >>> prompt and see. Trying a small idea in the shell is faster than guessing.
Practice
print("5 * 4 =", 5 * 4).print("Hello) (b) Print("Hello") (c) a program that should add two marks but subtracts them.print("Uwase Keza"), print("G.S. Rubavu"), print("S4 B").5 * 4 = 20.Key points
>>>) runs one statement at a time; the editor saves a .py program that is run as a whole (Run Module, F5).print() displays text in quotes and the values of calculations; # starts a comment.Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
# My first Python program
print("Muraho, Rwanda!")
print("Python can calculate:", 7 * 6)
print("Days in a leap year:", 366)
print() # an empty line
print("Goodbye")print("This line runs first")
Print("Python is case-sensitive")
print("This line is never reached")Quick check
What does print("3 + 4") display?