Naming and readability

Programs are read far more often than they are written — by colleagues, by reviewers and by you, three months later. Compare these two lines:

c = a * b * 0.85

sale_price = unit_price * quantity * DISCOUNT_FACTOR

Python treats them identically. A person does not. The second line tells you what is being calculated without a single comment. This lesson covers the rules Python enforces for names and the conventions Python programmers follow so their code reads like the second line.

The rules (enforced by Python)

A variable name:

• may contain letters, digits and underscores _;
• must not start with a digit (2nd_price is invalid, price_2 is fine);
• may not contain spaces, hyphens or other symbols (total-cost reads as "total minus cost");
• may not be a keyword — a word with a fixed meaning in Python, such as if, for, class, True, None, import, def, and, or, not;
• is case-sensitive: price, Price and PRICE are three different names.

Break a rule and the program does not start at all: Python reports a SyntaxError before running any line.

price = 10
Price = 20
PRICE = 30
print(price, Price, PRICE)

The conventions (followed by Python programmers)

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

Python does not check these — UnitPrice = 5 runs fine — but every Python developer expects them, so following them makes your code instantly familiar.

A constant is a value that should not change while the program runs: a tax rate, a delivery fee, the number of seats in a room. Python has no way to lock a variable, so the UPPER_CASE name is a promise to the reader: "don't reassign this". Put constants near the top of the program so they are quick to find and update.

VAT_RATE = 0.15
SERVICE_FEE = 5

ticket_price = 80
vat_amount = ticket_price * VAT_RATE
amount_due = ticket_price + vat_amount + SERVICE_FEE

print("VAT:", vat_amount)
print("Amount due:", amount_due)

An unexplained number in the middle of a calculation, such as 0.15 or 5, is often called a magic number: it works, but nobody can tell what it means or where else it is used. Naming it fixes both problems.

Choosing meaningful names

• Say what the value is: hours_worked, not h or num.
• Include the unit when it matters: weight_kg, duration_minutes.
• Avoid names of Python's built-in functions, such as print, input, sum, max, list, str. They are not keywords, so Python lets you reuse them — and then the original function stops working for the rest of the program.
• Very short names are fine only where the meaning is obvious from context — x and y for coordinates, for instance.

print_count = 3
total = 5
print("Copies:", print_count * total)

VAT: 15.0
Total: 140.0

Gross pay: 2100.0
Tax: 210.0
Net pay: 1890.0

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