Python basics: your first line of code

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1Summary 重點摘要

Python is a typed language whose lines run top to bottom. print() displays things; a variable is a named box that stores data.

2Vocabulary cards Vocabulary

3Step by step Steps

1. How a program runsPython runs a program from the first line down to the last, one line at a time, never skipping forward or back. This is exactly how Scratch blocks run from top to bottom — a sequence structure. The idea matters because if line three changes a variable, line four reads the new value, not the old; the result depends on the order of the lines, and swapping two lines can change everything. When reading someone else's program, trace it line by line with a finger, imitating the computer — the most effective way for a beginner to understand what happens.
2. The first line of codeBy tradition the first program in any language prints a greeting, and in Python it is print('Hello, world!'). The word print is the command to show output on the screen; what goes in the brackets is what will be shown. Rule one: text (a string) must be wrapped in quotes, single or double — print('Hello') and print("Hello") behave identically. Rule two: numbers need no quotes; print(7) shows 7. Without quotes, print(Hello) raises an error, because the computer takes Hello as a variable name. Everything inside the quotes displays exactly as typed, spaces included; outside the quotes is where code lives.
3. A variable is a labelled boxA variable is like a labelled box storing a piece of data. Writing age = 12 means: put the value 12 into the box named age; later print(age) opens the box and shows 12. The contents can change at any time: after age = 12, writing age = 13 leaves the box holding 13, with the old 12 overwritten. The crucial idea: in programs, the equals sign means 'put the right-hand value into the left-hand box', not mathematical equality. That is why x = x + 1 is absurd in mathematics yet perfectly normal in code — 'take x's value, add 1, and store it back in x'.
4. Getting input from the userThe input() command lets the program talk to the user. Writing name = input('What is your name?') shows the question in the brackets, and whatever the user types is stored in the box name; type Ming and print(name) shows Ming. One essential caveat: input() always returns a string, even when the user types digits. After age = input('Age?'), the box age holds the text '12', which cannot be used in arithmetic; convert it with int(): age = int(input('Age?')), after which age + 1 computes correctly. Forgetting the conversion raises an error — one of the commonest beginner mistakes.
5. Putting it togetherThree instructions combine into one complete mini-program:
   
   name = input('What is your name?')
   age = int(input('How old are you?'))
   print('Next year you will be', age + 1)
   
   The order of running: line one asks the name and stores it; line two asks the age and converts the text to a whole number; line three joins text with the computed result, so entering 12 displays 'Next year you will be 13'. This tiny program shows the skeleton of every program: input first, then processing (computing age + 1), then output (print) — matching the hardware jobs of input, processing, output and storage one to one.
6. Running and commentingA finished program must be run to show results. Method one: save the file as hello.py (the extension must be .py), open a command line and type python hello.py. Method two: press the Run button in an online editor — beginners should start there, with no installation needed. A line beginning with # is a comment, such as '# work out next year's age'. Comments are notes for humans that the computer skips entirely; they label what each part of the code does so that you or a classmate can read it later. A good habit is to comment as you write, especially on the harder calculation lines, so the code stays clear three months on.
7. Common errors and debuggingThree errors dominate a beginner's first week. First, unbalanced quotes or brackets: print('Hello) with no closing quote shows SyntaxError; the fix is to type the closing partner the moment you open one. Second, mixing strings and numbers: after age = input(), writing age + 1 raises TypeError because text cannot be added to a number; write age = int(input()) instead. Third, mistyped names: having defined name, printing nam raises NameError; Python names are case-sensitive, and Name and name are two different things. When an error appears, do not panic: read the error type first, then the line number, and the cause is usually obvious at once.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Python runs top to bottom, line by line
• print() displays; quote text, not numbers
• A variable is a named box: x = 5 means 'put in', not 'equals'
• input() gives strings; convert with int() before maths
• Input → processing → output: programs follow the same idea as hardware