6.3.1–6.3.2 – Values, variables and constants

Choosing primitive data types

A data type describes the kind of value and the operations that make sense for it. Choose a type from the meaning of the data, not just its appearance on screen. Pearson names four primitive types: each represents one simple value rather than a collection of items.

Python has no separate char type. It represents a character using a string of length one; longer text also uses str. The strings "7" and "False" are text, whereas 7 is an integer and False is a Boolean. Boolean literals start with a capital letter and have no quotation marks.

attempts = 4
lap_time = 12.6
door_open = True
player_initial = "K"

Each statement associates a name with a value. A quantity such as an attempt count is an integer; a measurement such as a lap time may need a real. A flag such as whether a door is open is Boolean. A one-letter initial is a character represented by a string. A reference with leading zeros, such as "0042", should remain text if those zeros are part of its identity.

Initialising and updating variables

A variable is an identifier associated with a value that can change. Its first assignment is its initialisation. Later assignments replace the current value; they do not retain a history in the variable.

score = 3
saved_score = score
score = 5
print(score)
print(saved_score)

Python evaluates the right-hand side before assigning to the left. Here saved_score receives the integer 3. Later changing score does not change that separate stored integer.

5
3

print() displays its argument and starts a new output line. This lets us inspect stored values. The assignment sign = means set/change a value, not test equality.

PLS identifiers start with a letter and then use letters, digits or underscores. Names are case-sensitive: score and Score differ. Choose names that explain a role, such as questions_remaining, rather than an unexplained x. Do not use a Python keyword as an identifier. Python can assign a different type to an existing name, but repeatedly changing the type of one stable role often makes the program harder to understand.

Converting values

Type conversion returns a value of another allowable type. Assign the returned value if you need to retain it. Conversion does not automatically rewrite the source variable.