Use of Data Types

A real value can have a fractional part, such as 12.47 seconds or a temperature of -2.5 degrees Celsius. Python represents real-number values using its float type. A float can also represent a value such as 5.0: the value need not have a non-zero fractional part.

A Boolean has one of two logical values, True or False. It is useful for recording whether something is the case, such as whether a door is locked. In Python, these values have capital initial letters and no quotation marks. "True", with quotation marks, is text rather than a Boolean value.

A character is one symbol, such as "A", "7" or "?". A string is a sequence of characters, such as "Aisha" or "AB12 3CD". Python has no separate character type: it represents a character using a string containing one character.

These assignments show how the types are used in Python:

goals = 3                 # integer
finishing_time = 12.47     # float: represents a real value
door_locked = True        # Boolean
grade = "A"               # character data: one-character string
name = "Aisha"            # string

The quotation marks identify text in the program; they are not part of the stored name or grade.

Choosing a type from the meaning of the data

Choose a data type by considering the values that must be represented and what the program will do with them.

Digits do not automatically mean that data should be numeric. A postcode is a string because it is an identifier, not a quantity used in arithmetic. Similarly, a telephone number is usually stored as text: its digits identify a contact, and any leading zero must be preserved.

An unsuitable type can lose information or prevent an operation. An integer cannot represent a finishing time of 12.47 seconds accurately because it cannot retain the fractional part. A real value is more suitable.

Casting makes a value usable in another form

Casting converts a value from one data type to another. This is particularly useful with keyboard input: Python's input() returns a string, even when the user types digits.

Common Python casts include:

Converting a real value to an integer discards its fractional part; it does not round to the nearest whole number. Choose the conversion according to the data you need to preserve.

Casting can provide a temporary converted value without changing the original variable:

age_text = "19"
old_enough = int(age_text) >= 18

Here, int(age_text) provides the integer 19 for the comparison with 18. The comparison produces the Boolean value True, stored in old_enough. Afterwards, age_text still contains the string "19".

To retain a converted value, assign it to a variable:

age = int(age_text)

Now age contains an integer, while age_text still contains a string. Assigning the converted result back to the original name, as in age_text = int(age_text), would replace its stored text value with an integer.

Using casting in a working program

A shop needs a price that may include pence and a quantity that must be a whole number. The following Python program converts each input before calculating the order total:

price = float(input("Enter price in pounds: "))
quantity = int(input("Enter quantity: "))
total = price * quantity
print(total)

If the user enters 4.50 and 3, the program displays 13.5, representing £13.50. float() preserves the fractional price, while int() supplies a whole-number quantity. The multiplication therefore uses numeric values rather than the text returned by input().

Casting is not needed just because data comes from the keyboard. A name or postcode should remain a string. Conversion is useful when the current type differs from the type needed for the intended operation.