Python Syntax Basics: Indentation, Case Sensitivity, Statements and Keywords

Syntax is the set of grammar rules a language follows. Python's syntax is famously clean: there are no semicolons at the end of lines and no curly braces around blocks. Instead, Python relies on indentation and a few simple rules, which this lesson covers.

One statement per line#

A statement is one instruction. In Python, each statement usually sits on its own line, with no semicolon:

name = "Asha"
age = 21
print(name, age)

You can put several statements on one line with semicolons (x = 5; y = 10; print(x + y)), but it is considered poor style.

Indentation defines blocks#

Many languages group lines with { }. Python uses indentation: the spaces at the start of a line. A line ending in a colon : starts a block, and every line of that block is indented by the same amount:

age = 20
if age >= 18:
    print("Adult")
    print("Can vote")
print("Done")

Adult
Can vote
Done

The two indented lines belong to the if. print("Done") is back at the left edge, so it runs regardless of the condition.

Forgetting to indent is an error:

if True:
print("no indent")

  File "example.py", line 2
    print("no indent")
    ^^^^^
IndentationError: expected an indented block after 'if' statement on line 1

Blocks appear after if, else, for, while, def, class, with and try. You will meet each of them in this course.

Python is case-sensitive#

name, Name and NAME are three different names:

name = "Asha"
Name = "Ravi"
print(name, Name)

Asha Ravi

Keywords and built-in functions are lowercase (print, if, for), except True, False and None, which start with a capital letter.

Long lines#

Lines longer than about 80 to 100 characters are hard to read. Inside parentheses, brackets or braces, you can break a line freely:

total = (10 +
         20 +
         30)
print(total)

60

Python knows the statement is not finished until the ( is closed. A backslash \ at the end of a line also continues it, but parentheses are clearer and preferred.

Keywords#

Python reserves some words for its own syntax. You cannot use them as variable names:

import keyword
print(keyword.kwlist[:10])
print(len(keyword.kwlist))

['False', 'None', 'True', 'and', 'as', 'assert', 'async', 'await', 'break', 'class']
35

The full list includes def, for, if, import, in, return, while, and more. Writing class = "A" causes a SyntaxError.

Naming conventions (PEP 8)#

Python's official style guide, PEP 8, recommends:

These are conventions, not rules, but following them makes your code look familiar to every Python developer.

Whitespace inside lines#

Spaces inside a line are mostly up to you: x=1+2 and x = 1 + 2 work the same. PEP 8 recommends spaces around = and operators, and after commas, for readability.

Common mistakes#

Practice#

number = 7
If number > 0
print("Positive")
print("Finished")

number = 7
if number > 0:
    print("Positive")
print("Finished")

Positive
Finished

Key takeaway#