Python Syntax Basics

Python's syntax was deliberately designed to read almost like plain English while still being precise enough for a computer to execute. Unlike languages such as Java or C, Python does not use curly braces {} to mark blocks of code, and it does not require a semicolon at the end of every statement. Instead, Python uses indentation itself as part of the language's grammar.

This design choice — sometimes called the "off-side rule" — forces every Python program to be visually organized, which is one reason Python is popular for teaching, scripting, and data work. Learning to read and write correct indentation is the very first skill every Python programmer needs, because a misplaced space can change what your program does or stop it from running at all.

Why it matters

In most languages indentation is a matter of style. In Python it is part of the grammar: it decides which statements belong to a loop, a function or an if. A line indented one level too far, or not far enough, changes what the program does or stops it from running, so this is the first rule to get right.

Indentation Defines Blocks

In Python, a colon (:) at the end of a line — after an if, for, while, def, class, or similar statement — signals that an indented block follows. All statements in that block must be indented by the same amount, conventionally four spaces. There are no braces to open or close a block; the indentation level itself marks where the block starts and ends.

• A block begins right after a line ending in a colon (:).
• Every line in the block must use consistent indentation (spaces are recommended; mixing tabs and spaces raises a TabError).
• Reducing the indentation level ends the block and returns control to the enclosing level.

No Semicolons or Braces Required

Python statements normally end at the end of the physical line — no semicolon is needed. A semicolon can technically be used to place multiple statements on one line, but this is discouraged by Python's style guide (PEP 8) because it hurts readability. Similarly, there are no braces {} anywhere in Python's control-flow syntax; parentheses () are used only for function calls, tuples, and grouping expressions.

The Python Interpreter

Python is an interpreted language: source code in a .py file is read and executed line by line by the Python interpreter (CPython is the standard, most widely used implementation), rather than being compiled ahead of time into a standalone native executable. You can run a script with "python filename.py" from a terminal, or run Python interactively using the REPL (Read-Eval-Print Loop) by simply typing "python" or "python3", which lets you test expressions one at a time and see results immediately.

Comments

A single-line comment starts with a hash symbol (#) and continues to the end of the line; the interpreter ignores everything after it. Python has no dedicated multi-line comment syntax, but a triple-quoted string (''' ... ''' or """ ... """) that is not assigned to anything is commonly used as a block comment, and the same triple-quoted syntax is also used for docstrings that document functions, classes, and modules.

Examples

age = 20

# Comment: check whether age qualifies as an adult
if age >= 18:
    print("You are an adult")
    print("You can vote")
else:
    print("You are a minor")

print("This line always runs, regardless of the if/else")

You are an adult
You can vote
This line always runs, regardless of the if/else

if True:
    print("First line indented with 4 spaces")
   print("Second line indented with only 3 spaces")

  File "<stdin>", line 3
    print("Second line indented with only 3 spaces")
                                                     ^
IndentationError: unindent does not match any outer indentation level

Exercise

The program should add up the numbers and print the total once, but it stops with an IndentationError. Fix the indentation.

Expected output: 6

total = 0
for number in [1, 2, 3]:
total += number
print(total)

total = 0
for number in [1, 2, 3]:
    total += number
print(total)

Quick quiz

Interview questions