LESSON 3 / 5 · 7 MIN

Variables

LET'S MAKE IT CLICK

Name your values

A variable is a name that refers to a value. Names let you reuse values and explain what your program is working with.

WATCH THE IDEA

Value10

A number to work with.

Namescore → 10

Give that value a meaningful label.

Readprint(score) → 10

Use the name to retrieve the current value.

A variable is a name that refers to a value.

Move through one idea at a time. Watch, experiment, and check your understanding along the way.

Give a value a name

WATCH THE IDEA

Evaluate10

Read the right-hand side.

Bindscore → 10

Attach the name to the value.

print(score) displays that value.

Assignment evaluates the value before binding the name.

In score = 10, Python evaluates the value on the right, then binds the name score to it. Reading score later gives you the value it currently refers to.

See the Python code

score = 10
print(score)

Keep in mind

A useful beginner picture is a label attached to a value. Python variables are names referring to objects, rather than fixed typed boxes.

Change what the name refers to

WATCH THE IDEA

Beforescore → 10

Read the current score.

Calculate10 + 5 → 15

Evaluate the right-hand side.

Afterscore → 15

Rebind score to the new value.

The old value is used in the calculation before the binding changes.

Assigning again changes the binding. In score = score + 5, the right-hand side uses the current score, calculates a new number, then assigns that result back to score.

Assignment is not a mathematical equality test. A single = assigns; == compares two values.

YOUR TURN TO EXPLORE

Watch a score change, line by line.

Choose a starting score and an increase. Step through the assignments and watch the current binding update.

Starting scoreIncrease

example.pyPython

CURRENT VALUESReady

No assignments yet.

OUTPUT

No output yet.

Predict the result, then take the first step.

Enable JavaScript to change inputs and step through this guided example. Here is the initial program and its result:score = 10 score = score + 5 print(score)Expected output:15

Choose names that explain

WATCH THE IDEA

Unclearx = 5

What does x represent?

Descriptivelesson_count = 5

The name tells you what the value means.

Validletters + underscores

Digits are allowed after the first character.

Names should explain their purpose. total_score is valid; 2scores is not.

Use names such as total_price or user_name instead of mysterious letters when they clarify the purpose. Python names can contain letters, digits, and underscores, but cannot start with a digit or be a keyword such as for.

Names are case-sensitive: score and Score are separate names. Use lowercase words separated by underscores for ordinary variables.

See the Python code

user_name = "Asha"
lesson_count = 5

Names are not copies

WATCH THE IDEA

First namea → [1, 2]

a refers to a list object.

Second nameb = a

b refers to that same list, without copying it.

Shared objecta, b → [1, 2]

There are two names and one list.

Giving an object another name is different from creating a copy.

For immutable numbers, reassignment gives a name a different value. With mutable objects such as lists, several names can refer to the same object; changing that object is different from rebinding a name.

We will explore shared references later. For now, distinguish creating a value, assigning a name, and reading that name.

TAKE IT WITH YOU

What you have learned

Assignment evaluates the right-hand side before updating the name.
Reassignment changes the value a name refers to.
Descriptive, case-sensitive names make code easier to read.

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