Variables and Constants

Store, change, name, and swap values with variables and recognize constants by convention.

10 min read 10 quiz questions

Store values in variables

A variable is a name that points to a value. Use the equals sign (=) to assign a value to a name.

Think of a variable as a labeled box: the label is the name and the value is what the box currently holds.

Example 1: Assignment

city = "Dhaka"
print(city)

This assignment gives the name city the text value "Dhaka". Printing the variable shows its value.

Reassigning variables

Giving a variable a new value is called rebinding. The name now points to the new value instead of the old one.

Example 2: Rebinding

level = 1
level = 2
print(level)

The first value is replaced when level = 2 runs. Python therefore prints 2.

Names and multiple assignment

Variable names can use letters, digits, and underscores, but they cannot start with a digit. Names are case-sensitive, so score and Score are different names. Use descriptive lowercase names with underscores, such as player_score.

Example 3: Multiple assignment and swapping

first_name, age = "Asha", 12
left, right = "L", "R"
left, right = right, left
print(first_name, age)
print(left, right)

Multiple assignment gives several names values at once. The swap works because Python evaluates the values on the right before assigning them to the names on the left.

Example 4: Constants and literals

MAX_LIVES = 3
pi_approximation = 3.14
print(MAX_LIVES)
print(pi_approximation)

Python does not enforce true constants. By convention, an ALL_CAPS name such as MAX_LIVES tells readers that the value should not change. The values 3 and 3.14 written directly in code are called literals.

Key points

Assignment uses = to give a variable a value.
Rebinding gives an existing name a new value.
Names cannot start with a digit.
Multiple assignment can swap values.
ALL_CAPS names signal constants by convention.

Worked example

Track a mobile wallet balance: start with 1000 taka, pay 250 for a train ticket, and see what remains.

Example

balance = 1000
balance = balance - 250
print(balance)

How it works, step by step

balance starts as 1000.
The right side (balance - 250) is worked out first: 750.
Then the result is stored back into balance — assignment always goes right to left.

References & Further Reading

Related entries from PyLabs's own reference library.

id() (Return an object's identity number during this run.)
type() (Find out what type of value an object is.)

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Practice exercise

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Create variables first and second with values "sun" and "moon". Swap them, then print first followed by second.

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