Python Variables

A variable in Python is a name that refers to a value stored in memory. Unlike statically typed languages, Python does not require you to declare a variable's type in advance — you simply assign a value to a name, and Python figures out the type automatically at runtime. This is called dynamic typing.

Because variables in Python are really just labels pointing to objects, the same name can be reassigned to a value of a completely different type later in the program. This flexibility is powerful, but it also means naming variables clearly and consistently matters more in Python than in languages where the type is fixed by a declaration.

Why it matters

A variable in Python is a name attached to an object, not a box that holds a value. That one idea explains why no types are declared, why two names can refer to the same list, and why swapping two values takes a single line. It is the basis for understanding everything that follows about mutable and immutable data.

Dynamic Typing

When you write "x = 5", Python creates an integer object with value 5 and makes the name x refer to it. If you later write "x = \"hello\"", the name x now refers to a string object instead — no error occurs, and no type declaration was ever needed. The type is a property of the object, not of the variable name itself, which is why Python is described as "dynamically typed."

Variable Naming Rules

Variable names in Python must start with a letter (a-z, A-Z) or an underscore (_), followed by any combination of letters, digits, and underscores. Names are case-sensitive (age and Age are different variables), and they cannot be a reserved keyword such as if, for, class, or return. By convention (PEP 8), variable names use lowercase words separated by underscores (snake_case), such as total_price or user_name.

• Valid: age, _count, user_name, total2
• Invalid: 2total (starts with a digit), user-name (hyphen not allowed), class (reserved keyword)
• Convention: use snake_case for variables and functions; UPPER_CASE for constants.

Multiple Assignment

Python allows several convenient assignment patterns in a single line. You can assign the same value to multiple names at once (a = b = c = 0), or assign different values to multiple names in one statement by unpacking a sequence (x, y, z = 1, 2, 3). This unpacking form also enables the classic one-line variable swap, "a, b = b, a", without needing a temporary variable.

Example

x = 5
print(type(x), x)

x = "hello"
print(type(x), x)

a, b, c = 1, 2, 3
print(a, b, c)

a, b = b, a
print(a, b)

<class 'int'> 5
<class 'str'> hello
1 2 3
2 1

Exercise

Swap the values of the two variables without using a third variable, then print them.

Expected output: 10 5

a = 5
b = 10

# TODO: swap a and b in one line

print(a, b)

a = 5
b = 10

a, b = b, a

print(a, b)

Quick quiz

Interview questions