Variables

Giving a value a name

Read this short program and decide what it prints. Take a real guess before you reveal the answer.

price = 250
print(price)
price = 300
print(price)

250
300

price = 250 gives the value 250 a name, price. From then on, wherever you write price without quotes, Python uses the value it refers to. A name like this is a variablevariableA name that refers to a value. You give it a value with =, and you can point it at a different value later..

Read = as "gets": "price gets 250". It is not the equals sign from math. It's an instruction: work out the value on the right, then make the name on the left refer to it. We'll say "price is 250" or "price refers to 250".

Step through the same program one line at a time. Watch the table: after each line it shows what every name refers to.

250
300

Now run it yourself. Then change 300 to 275 and run it again.

price = 250
print(price)
price = 300
print(price)

A name refers to one value at a time

Think of a variable as a name tag, not a box. The tag hangs on one value. When you write price = 300, Python takes the tag off 250 and hangs it on 300. Nothing piles up. What does this print?

name = "Abel"
name = "Hana"
print(name)

Hana

If you picked AbelHana or Abel Hana, you were imagining a box that collects things. That picture is the most common one beginners have, and it's wrong for Python. A new value replaces the old one; the name now refers only to the new value.

Using a name to make a new value

The right side of = can use names, including the name on the left. In math, count = count + 1 is impossible. In Python it's normal. Read it as "count gets count plus one": Python works out the right side first, using the value count has now, then moves the name to the result.

6

One more trap. total = price * 2 works out a value once, at that moment. It doesn't set up a rule that keeps total up to date. Predict this one.

price = 250
total = price * 2
price = 300
print(total)

500

Change the program so it prints 600: after price = 300, work out the total again.

price = 250
total = price * 2
price = 300
print(total)

Swapping two values

Here's a classic puzzle. a is "tea" and b is "coffee", and you want to swap them. This looks right, but step through it and watch what happens to "tea".

coffee coffee

After line 3, nothing refers to "tea" any more, so line 4 can't bring it back. The fix is a third name that keeps hold of "tea" first:

a = "tea"
b = "coffee"
temp = a
a = b
b = temp
print(a, b)

Python also has a shortcut, a, b = b, a, which you'll understand properly in Module 4.

Naming rules

A name can use letters, digits, and underscores, but it can't start with a digit, and it can't be one of Python's own words, like if or from. Case matters: price and Price are two different names. Python style is lowercase words joined with underscores, like taxi_fare or price_per_cup. Choose names that say what the value means: fare beats x, even though Python accepts both.

The name always goes on the left of =, and the value on the right:

A typo in a name is the most common error of all. Python can only find a name you've actually given a value to: