4.1.1.2, 4.1.1.3, 4.1.1.6 - Variables, constants and arithmetic (Python)

Assignment and sequence

An assignment evaluates the expression on its right and binds the resulting value to the name on its left. score = score + 5 reads the old score before replacing its binding; it does not assert an algebraic equality.

score = 10
bonus = 5
score = score + bonus
bonus = 2
print(score, bonus)

The output is 15 2. Changing bonus does not recompute an earlier expression. Statements run in sequence, and using a name before assigning it raises NameError. Python does not require a separate declaration before ordinary assignment. An optional annotation such as score: int = 10 describes the intended type but does not enforce a fixed runtime type.

Meaningful identifiers explain a value's role: distance_metres is more useful than x. They reduce misinterpretation and help someone inspect, debug or maintain a program. Names are case-sensitive, cannot contain spaces or start with a digit, and must not be reserved words such as if.

input() reads one line as a string. To calculate with a guaranteed whole-number input, use count = int(input()); for a fractional input use measurement = float(input()). print() displays a value. Conversion and error handling are developed in later lessons; these basic input/output operations let you supply changing values to an arithmetic program.

Sequence, selection and iteration are the three basic ways of combining imperative statements. Selection chooses a branch; iteration repeats statements. A subroutine groups statements behind a name so they can be called where needed. The next lessons develop branches and loops; the subroutines lesson develops parameters and results. Here the assessed demand is state changes in sequence.

Named constants

A variable is intended to take different values during execution, while a constant is intended to retain its value. Python normally represents a named constant by an uppercase name and a convention to leave it unchanged:

METRES_PER_KILOMETRE = 1000
distance_km = 2.5
distance_metres = distance_km * METRES_PER_KILOMETRE
print(distance_metres)

Python still permits reassignment of METRES_PER_KILOMETRE; uppercase spelling is not runtime protection. Nevertheless, the name makes the conversion's meaning clear. If a configurable rate changes, updating one definition is safer than searching through several unrelated occurrences of the same literal. One shared definition also prevents inconsistent rates in different calculations. A constant need not be a number: DEFAULT_CATEGORY = "standard" is also useful.