Python Syntax

Every language has grammar rules. Python’s are few and simple, but strict: one wrong space or a missing colon stops the program. This lesson covers the rules you need for everything that follows.

Statements#

A Python program is a list of statements, run from top to bottom. Normally, one statement per line:

price = 250
quantity = 3
total = price * quantity
print(total)

No semicolons are needed at the end of lines. (You *can* put two statements on one line with ;, but nobody does.)

Blocks and indentation#

Statements that end with a colon : start a block: if, else, for, while, def, class, with, try. The block is everything indented beneath them:

temperature = 31

if temperature > 30:
    print("It's hot.")
    print("Stay hydrated.")
else:
    print("Nice weather.")

print("Done.")

Rules:

• The line before a block ends with :.
• Every line in the block is indented by the same amount, by convention 4 spaces.
• The block ends when the indentation goes back.

Blocks can be nested:

score = 85
submitted = True

if submitted:
    if score >= 80:
        print("Excellent!")
    else:
        print("Submitted.")
else:
    print("Please submit your work.")

Indentation errors#

if score > 50:
print("Pass")

This gives IndentationError: expected an indented block. Mixing tabs and spaces causes similar errors. Let your editor insert 4 spaces when you press Tab (VS Code does this by default for Python).

An empty block: pass#

A block can’t be empty. Use pass as a placeholder:

def coming_soon():
    pass  # TODO: write this later

coming_soon()
print("No error.")

Comments#

# A full-line comment

total = 499 * 3  # a comment after code

# Python has no special multi-line comment.
# Just use several # lines.

Write comments that explain why, not what. # add 18% GST is useful; # multiply by 1.18 repeats the code.

Docstrings#

A string right at the start of a function, class or file is a docstring, the documentation for it:

def area(width, height):
    """Return the area of a rectangle."""
    return width * height

print(area(4, 5))
print(area.__doc__)

Editors show docstrings when you hover over the function. Triple quotes """ allow several lines.

Long lines#

Keep lines under about 80–100 characters for readability. Inside brackets (), [] or {}, you can break lines freely:

total = (
    120 * 3
    + 250 * 1
    + 299 * 2
)

languages = [
    "HTML",
    "CSS",
    "JavaScript",
    "Python",
]

print(total, len(languages))

Names (identifiers)#

Variable and function names:

• Can contain letters, digits and underscores, but can’t start with a digit.
• Are case-sensitive: score, Score and SCORE are different.
• Can’t be keywords like if, for, def, class, True, None, import.

Python’s naming style (from the official style guide, PEP 8):

Kind	Style	Example
Variables and functions	snake_case	total_price, get_user()
Constants	UPPER_CASE	MAX_RETRIES = 3
Classes	PascalCase	ShoppingCart

Quotes#

Strings can use single or double quotes; pick one style and be consistent:

a = 'single'
b = "double"
c = "It's easy"            # double quotes when the text has an apostrophe
d = """Several
lines of text"""
print(a, b, c)
print(d)

Reading error messages#

Errors are normal. Python tells you exactly what happened in a traceback:

Traceback (most recent call last):
  File "shop.py", line 3, in <module>
    total = price * quantty
NameError: name 'quantty' is not defined. Did you mean: 'quantity'?

Read it from the bottom:

1. The last line says the type of error and a message: a misspelled variable.
2. The lines above show where: file shop.py, line 3, and the code on that line.

Modern Python even suggests fixes (“Did you mean…?”). Common error types:

Error	Usually means
SyntaxError	Grammar problem: missing colon, bracket or quote
IndentationError	Wrong or inconsistent indentation
NameError	Using a name that doesn’t exist (typo?)
TypeError	Wrong type, e.g. adding a number to text
ValueError	Right type, bad value, e.g. int("abc")

The Zen of Python#

Type this in the REPL:

import this

It prints 19 short principles behind Python’s design, such as “Readability counts” and “Simple is better than complex”. They’re good advice for any code you write.

Common mistakes#

• Missing colon after if, for, def.
• Mixing tabs and spaces.
• Unclosed brackets or quotes: the error may point to the next line, so check the line above too.
• Using a keyword as a name, like class = "A".

Exercise#

1. Write an if/else that checks whether a number is even, with two print lines inside the if block.
2. Deliberately remove a colon and read the error. Then remove an indentation and read that error.
3. Write a function with a docstring and print the docstring.
4. Rename these to Python style: TotalPrice (a variable), getuser (a function), maxsize (a constant).

Summary#

• One statement per line; no semicolons.
• A line ending in : starts a block; indent the block by 4 spaces.
• # starts a comment; """docstrings""" document functions.
• Use snake_case for names, and read tracebacks from the bottom.

Next, you’ll store values in variables.