Copies, and lists inside lists

What you will be able to do

After this lesson you can

• explain why b = a does not copy a list,
• make a real copy,
• predict when a function changes your list, and
• make and read a table, a list that holds lists.

Two names for one list

Here is a puzzle.

A variable does not hold the list itself. It holds a pointer, like a label on a box. When we write b = a, Python does not make a second list. It sticks a second label on the same box. A change through b is a change to the one box, and we see it through a too. The word is asks: "are these the same box?" a is b is True.

To make a real copy, with its own box, use a.copy(). Other ways: a[:] (a slice of everything) or list(a). Now c.append(5) changes only c, and a is c is False.

This is not a fault of Python. It is useful: a list can be big, and Python does not copy it unless you ask. But it is a famous trap.

For a string or a number, the problem does not exist, because they cannot be changed. Only a changeable thing needs this care.

A function gets the same list

The same happens when you give a list to a function. The function receives a label on your box.

A list inside a list: a table

A list can hold any values, and a list is a value too. So a list can hold lists. This is how we make a table, with rows and columns.

One more warning. Making a table with [[0] * 3] * 3 looks neat, but it makes three labels on one row. A change to one row shows in all three rows. Build the rows with a loop instead:

table = []
for i in range(3):
    table.append([0, 0, 0])

Read t[1][2] from the left: first t[1], the row [6, 2, 9], then item 2 of that row, which is 9.

When to use a table

A table is good for data that has two directions: students and subjects, days and cities, rows and columns of a grid for a game. For one direction only, a plain list is enough.

Common mistakes

• Believing that b = a makes a copy. It makes a second name.
• A function that changes a list by surprise. Say in a comment if it does.
• Mixing the two positions of a table. The first is the row, the second is the column.
• **[[0] * 3] * 3** for a table. All the rows are one row.
• A copy that is only one level deep. a.copy() of a table copies the outer list, but the inner rows are still shared. For that, make a new row for each row.

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 Make a real copy. The checker tests that the original does not change.
2. Build. Open the problem Two names, one list and the problem Is the copy safe?
3. Stretch. Open the problem Look inside the table and find the total of the third column of a table.

Remember these

• b = a gives the list a second name. a.copy() makes a real copy.
• is asks whether two names are the same list.
• A list given to a function is not copied.
• A list inside a list makes a table: table[row][column].

Check yourself

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

1. After a = [1, 2], b = a, b.append(3), what is a?

2. How do you make a separate copy of a?

3. What does t[2][0] mean for a table t?