Values, types and input/output

Why does the till say 66 coffees?

• A café till asks "Coffees?" and the cashier types 6. The program doubles it for a two-for-one offer and prints 66, not 12.
• Nothing is broken. The program got the text "6", and doubling text repeats it. This lesson is about knowing what kind of thing each value is.

coffees = input("Coffees? ")   # the user types 6
print(coffees * 2)             # prints 66

Every value has a type

• A value 值 is a piece of data. Its type 类型 says what kind it is: int (whole number), float (decimal number), str (text, a string 字符串) or bool (True or False).
• type() tells you the type: type(42) is int, type(4.0) is float, type("42") is str. The quotes make "42" text.

The type decides what an operator does

• + adds numbers but joins strings: 3 + 4 is 7, but "3" + "4" is "34". Mixing them, "3" + 4, raises a TypeError: Python refuses to guess.
• Division has three forms: 7 / 2 is 3.5 (always a float), 7 // 2 is 3 (whole part) and 7 % 2 is 1 (the remainder).

print(3 + 4, "3" + "4", 7 / 2, 7 // 2, 7 % 2)
# 7 34 3.5 3 1

A variable is a name tag on a value

• A variable 变量 is a name bound to a value by =. price = 3.50 makes the name price point at the value 3.50.
• Assigning again moves the tag to a new value. After a = 5, b = a, a = 7, the name b still points at 5.

input always gives a string

• input() returns whatever the user typed, as a str, even if it looks like a number. Convert it on purpose: int(text) or float(text).
• A conversion that cannot work raises a ValueError: int("4.5") fails because 4.5 is not a whole number. Use float("4.5") for decimals.

coffees = int(input("Coffees? "))   # 6 -> the int 6
print(coffees * 3.50)               # 21.0

Floats are close, not exact

• Computers store float values in binary, so many decimals are rounded a tiny amount. 0.1 + 0.2 prints 0.30000000000000004, and 0.1 + 0.2 == 0.3 is False.
• Compare floats with a tolerance instead: abs(x - y) < 1e-9, or math.isclose(x, y). For money, many programs count whole pence as int.

Format at the edge, not in the middle

• An f-string 格式化字符串 puts values into text: f"Total: £{total:.2f}" shows two decimal places.
• Formatting changes only what is printed. The stored value is unchanged, so keep calculating with the number and format once, when you display it.

Marks that slip away

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