Where a variable lives

What you will be able to do

After this lesson you can

• say where a variable lives and when it disappears,
• explain a NameError for a variable inside a function,
• predict what happens when a function and the main program use the same name, and
• write functions that work alone.

A variable has a home

When a function runs, Python makes a small private room for it. The variables that you make inside are in that room. When the function ends, the room is closed, and the variables disappear. This is called local scope. Variables made outside any function are in the open space of the program, the global scope.

What happens if you try to use a local variable outside?

The same is true of parameters. name in make_greeting(name) is a local variable too.

The same name in two places

What if the outside program and a function use the same name?

Now the second part. A function can read a variable from outside. The function add_tax uses rate without making one, so Python looks outside and finds it. But this is a risk. If someone changes rate later, the function gives a different answer for the same argument, and the reason is not in the function. A beginner is surprised.

To get a value out of a function, return it, and store the answer: x = change().

A good habit

Make a function depend on its parameters only, and give its result with return. Then

• you can understand it by reading it alone,
• you can test it alone,
• you can use it in any program.

Pass the tax rate as a parameter: def add_tax(price, rate):. The call is then a little longer, but the function has no hidden inputs.

The word global

Python has a word, global, to let a function change an outside variable. You will meet it in other people's programs. It is better to avoid it, for the same reason: a function that changes things outside is hard to follow. Return the new value, and let the caller store it.

A picture to remember

A function is a room with a door. What goes in comes through the parameters (the door). What comes out goes through return (the door again). Nothing passes through the walls.

Common mistakes

• Using a local variable outside the function. A NameError.
• Expecting an assignment inside a function to change the outside variable. It makes a new local one.
• Relying on an outside variable that may change.
• Using global for convenience.
• Forgetting that parameters are local.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Return, do not reach out. A starter function uses an outside variable, and you make it use parameters.
2. Build. Open the problem Where does a variable live? and the problem Same name, two variables.
3. Stretch. Write a function with a local variable of the same name as a global one, and explain to a friend which is printed where.

Remember these

• A variable made inside a function is local. It disappears when the function ends.
• Outside the function, its name is unknown: a NameError.
• An assignment inside a function makes a new local variable, even if the name exists outside.
• Use parameters in and return out.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Where does message live in the first program of this lesson?

2. After x = 10 and a function that sets x = 99 inside, what is the outside x?

3. How do you get a value out of a function?