Names

Typing the same value over and over gets old. A name refers to a value so you can use it again. You create one with =.

price = 40
print(price)
print(price * 3)

price = 40 means "from now on, price refers to 40". It does not mean "price is equal to 40" in the maths sense. The value is on the right, the name on the left, always.

Names can change

Assigning again replaces the old value.

stock = 10
print(stock)
stock = 7
print(stock)

The second print shows 7. The old value is gone.

A very common pattern is to update a name using its own current value:

stock = 10
stock = stock - 1
print(stock)

Read stock = stock - 1 right to left: compute stock - 1, which is 9, then make stock refer to 9. Python has a shortcut for this: stock -= 1. The same works for +=, *= and /=.

Names for strings

Any value can be named, including text.

item = "bread"
print(item)
print("You bought", item)

Choosing names

Names are made of letters, digits and underscores, and cannot start with a digit. Python is case sensitive: Price and price are different names. Use names that say what the value is. unit_price beats x.

Using a name before it exists is an error. Run this and read the message:

print(total)

NameError: name 'total' is not defined means exactly what it says: there is no name total yet. You will see this error often. It almost always means a typo or a forgotten line.

Try it

Name and print

Create a name item that refers to the text bread, then print the line You bought bread using the name rather than typing the word again; that is the skill being practised.

You bought bread

item = ...
print("You bought", ...)

Update stock

The shop starts the day with 10 loaves of bread. It sells 3. Starting from the code below, reduce stock by 3 and then print it. Let Python do the subtraction rather than typing 7 yourself; that is the skill being practised.

7

stock = 10
# sell three loaves
print(stock)

Swap two names

first refers to "apple" and second refers to "bread". Make them swap, so that first refers to "bread" and second to "apple", then print both on one line, first then second.

bread apple

Do this using only the names, without writing "bread" or "apple" again; moving values between names is the skill being practised.

first = "apple"
second = "bread"
# swap them

print(first, second)

Next: How a program runs.