Python's Vocabulary: Reserved Words and Your First Sentence

From Instructions to Output

Python is a programming language: a formal system for giving instructions to a computer. When you write Python, you express instructions in a format that both people and computers can understand. The computer reads those instructions through a Python interpreter, executes them, and displays results such as text, numbers, or error messages on your screen.

The editor is where you write. The interpreter is the program that reads what you wrote and performs the requested actions. The output area is where you see the result. These are connected, but they are not the same thing: text sitting in the editor has not yet been executed.

Your First Python Statement

The simplest way to make Python display something is to use print. The print function is a built-in tool for displaying text, numbers, and other values. It requires parentheses, and the value you want to display goes inside those parentheses as an argument.

print("Hello, World!")

Hello, World!

Python's Vocabulary

Beginning programmers need to separate two kinds of words in code. Some words belong to Python's vocabulary and rules. The source raises tokens such as for and in as examples of words to examine when deciding which parts of code are defined by Python. Other names are choices made by the programmer, such as names connected with words, pizza, or slices. Python does not have a human understanding of pizza or slices; it follows the language rules represented by the code.

The first statement also shows this distinction. The word print is not a name you invented for this line; it is a built-in Python tool. The text inside quotation marks is data supplied to that tool. The quotation marks tell Python that the characters are a string rather than a command or a variable name.

Text, Numbers, and Values

Quotation marks change how Python interprets what you write. A string is text surrounded by quotation marks. A number is written without quotation marks. Python treats these values differently internally, even when their printed appearance looks similar.

print(42)
print(3.14)
print(10 + 5)

42
3.14
15

The first statement prints the number 42, and the second prints the decimal number 3.14. In the third statement, Python evaluates 10 + 5 and then print displays the result, 15. By contrast, "42" would be a string because the quotation marks identify it as text. The string "42" and the number 42 may look alike in the output, but Python treats them as different kinds of values.

Writing Versus Running

Writing code and running code are separate actions. When you type print("Hello") into the editor, you have written an instruction, but the instruction is only waiting in the editor. When you press the control that runs the code, or the course environment triggers execution, the interpreter reads and performs it.

Top-to-Bottom Execution

A program contains one or more statements. Python executes multiple statements sequentially, from top to bottom. Each line is a separate instruction in the example below, so the output appears in the same order as the statements.

print("First") print("Second") print("Third")

The Course Workspace

You do not need to install Python on your computer for this course. The course provides a built-in environment in your browser. When a lesson includes a code example or practice exercise, you can type Python statements into the editor and run them through the connected Python interpreter. The result appears in an output area below or beside the code.

This arrangement lets you experiment immediately. You can change a statement, run it again, compare the output, and observe what happens when the interpreter finds a mistake.

Mistakes Beginners Make

• Assuming that typing code automatically runs it

  Writing and running are separate actions. The interpreter does not execute the statement until execution is triggered.

  Fix: Write the statement in the editor, then run it and inspect the output area.

• Leaving quotation marks out of a text value

  Quotation marks tell Python that the characters are a string rather than a command or variable name.

  Fix: Write the text as a quoted string, as in print("Hello").

• Putting quotation marks around a number when you intend a numeric value

  Quotation marks make 42 a string. The string "42" and the number 42 are different to Python.

  Fix: Use print(42) when the value should be written as a number.

• Changing syntax that Python requires

  The print function requires parentheses around the value it receives.

  Fix: Use print("Hello") with parentheses and quotation marks in the appropriate places.

• Treating every word in an example as a fixed Python word

  Some parts of code come from Python's vocabulary, while other names are choices made by the programmer. Python does not fundamentally understand pizza or slices.

  Fix: Separate Python-defined vocabulary and syntax from names selected by the programmer.

Try the Interpreter