PCC·01 · Chapter record

Getting Started

Chapter 1 — how Python actually runs your code

Done

The book's own Chapter 1 spends most of its time on installing Python and a text editor per operating system — not needed here, since every demo on this site already runs in-browser. These three sessions keep the ideas that carry over: what the interpreter actually does, the two ways to run code, and how to read an error when something breaks.

1.1

The Python Interpreter

Concept

Every Python program starts as a plain text file saved with a .py extension — a script.
A program called the interpreter reads your file from the very first line to the very last and carries out each instruction immediately.
There's no separate "build" step the way some other languages use — whatever line the interpreter reaches is the line that runs next.
When it sees print followed by parentheses, it displays whatever is inside them on the screen.
A good editor helps with syntax highlighting — coloring function names, strings, and keywords differently — so mistakes are easier to spot before you even run the file.

Key Terms

interpreter: the program that reads your .py file and runs it line by line
script: a saved .py file containing a full program
syntax highlighting: an editor's way of coloring code by what each part means

Exercise

Activity 1

Change the message inside print() to introduce yourself, then run it again.

1.2

Script vs. Interactive Prompt

Concept

There are two ways to run Python code.
Saving instructions in a .py file and running the whole thing at once is running a script — that's how real programs are built.
Python also has an interactive prompt, shown as >>> in examples, which runs one line at a time and immediately shows you the result.
In a script, 2 + 3 sitting on its own line does nothing visible — you have to print() it to see the result.
At the interactive prompt, Python echoes back the value of anything you type — that's why examples show >>> 2 + 3 followed directly by 5, with no print() at all.

Key Terms

interactive prompt: Python's line-by-line mode, shown as >>> in examples — also called a REPL
>>>: the prompt symbol shown before code you type at the interactive prompt
expression: something Python can reduce to a single value, like 2 + 3

Exercise

Activity 2

Which of these would only show a visible result if typed at the interactive prompt, not inside a saved script: 2 + 3, or print(2 + 3)?

1.3

Reading a Traceback

Concept

When Python hits code it can't run, it doesn't fail silently — it prints a traceback: a report showing which line caused the problem and what kind of error occurred.
A SyntaxError means Python couldn't even understand the structure of what you wrote — often a missing colon, a missing quote, or a mismatched parenthesis.
Read a traceback from the bottom up — the last line names the error type, and the line just above it points to roughly where it happened.
Not every error is a SyntaxError: a misspelled function or variable name raises a NameError instead, because Python understood the structure fine but couldn't find anything by that name.

Key Terms

traceback: the report Python prints when it hits an error it can't recover from
SyntaxError: raised when Python can't understand the structure of your code
NameError: raised when Python can't find anything matching the name you used

Exercise

Activity 3

Line 4 above is prin("Hello!") — what type of error would this raise, and what's the one-character fix?