Python Basics — Variables

Variables

A variable is a name that refers to a value stored in your computer’s memory. Think of it as a labeled box — you give the box a name, and you put something inside it.

In Python, you don’t need to declare the type of a variable. Just assign a value with the = operator, and Python handles the rest.

Creating Variables

name = "Alice"
age = 25
temperature = 36.6
is_student = True

No int, string, or var keyword needed — Python automatically determines the type based on the value you assign.

Assignment Operator (=)

The = sign in Python is the assignment operator. It means “store this value in this variable.” It does NOT mean “equals” in the mathematical sense.

x = 10       # Store 10 in variable x
x = x + 5    # Take current x (10), add 5, store result (15) back in x
print(x)     # 15

Multiple Assignment

Python lets you assign values to multiple variables in one line:

# Assign different values to multiple variables
a, b, c = 1, 2, 3
print(a)  # 1
print(b)  # 2
print(c)  # 3

You can also assign the same value to multiple variables:

# Chained assignment
x = y = z = 0
print(x, y, z)  # 0 0 0

Swapping Variables

Python makes variable swapping elegant:

a = 10
b = 20

# Swap in one line (no temp variable needed!)
a, b = b, a

print(a)  # 20
print(b)  # 10

Dynamic Typing

Python is dynamically typed, meaning a variable can hold different types of data at different times:

x = 42          # x is an integer
print(type(x))  # <class 'int'>

x = "hello"     # Now x is a string
print(type(x))  # <class 'str'>

x = [1, 2, 3]   # Now x is a list
print(type(x))  # <class 'list'>

This is different from statically typed languages (like Java or C++) where a variable’s type is fixed at declaration.

Feature	Python (Dynamic)	Java (Static)
Declaration	x = 10	int x = 10;
Type change	Allowed	Not allowed
Type checking	At runtime	At compile time

The type() Function

Use type() to check what type a variable holds:

name = "Bob"
age = 30
height = 5.9
is_active = True

print(type(name))      # <class 'str'>
print(type(age))        # <class 'int'>
print(type(height))     # <class 'float'>
print(type(is_active))  # <class 'bool'>

The id() Function

Every object in Python has a unique identity — a number that represents its memory location. Use id() to see it:

x = 10
print(id(x))  # Something like 140234866423056

y = 10
print(id(y))  # Same number! Python reuses small integers

z = 10000
print(id(z))  # Different number

Why this matters: Python caches small integers (-5 to 256) for performance. Two variables holding 10 point to the same object in memory.

Variable Naming Best Practices

Rule	Example	Notes
Use snake_case	user_name, total_price	PEP 8 standard
Start with letter or _	_count, name	Cannot start with digit
Be descriptive	student_age not sa	Self-documenting code
Avoid single letters	Exception: i, j, k in loops	Conventions are OK
Avoid reserved words	Don’t use class, for, if	Python keywords
Use meaningful names	is_valid not flag	Booleans start with is_, has_, can_

# ✅ Good variable names
first_name = "Alice"
total_items = 42
is_logged_in = True
max_retries = 3

# ❌ Bad variable names
fn = "Alice"       # Too short, not descriptive
x = 42             # What does x represent?
flag = True        # Flag for what?
data = 3           # Too vague

Deleting Variables

Use the del keyword to delete a variable:

x = 10
print(x)  # 10

del x
# print(x)  # NameError: name 'x' is not defined

You’ll rarely need to delete variables in practice. Python’s garbage collector handles memory cleanup automatically.

Variables Are References

In Python, variables don’t “contain” values — they reference (point to) objects in memory.

a = [1, 2, 3]
b = a           # b now references the SAME list as a

b.append(4)     # Modifying through b...
print(a)        # [1, 2, 3, 4] — a is also changed!

This is because a and b point to the same list object. We’ll explore this concept deeper when we cover lists and mutable vs immutable types.

Common Mistakes

Mistake	Example	Fix
Using before assignment	print(x) (x not defined)	Assign before using
Starting with digit	2name = "Bob"	Use name2 = "Bob"
Using keywords	class = "Math"	Use class_name = "Math"
Case sensitivity	Name ≠ name	Be consistent with casing
Spaces in names	my var = 10	Use my_var = 10

Summary

• Variables are names that reference values in memory
• Use = to assign values — no type declaration needed
• Python uses dynamic typing — variable types can change
• Use type() to check a variable’s type and id() for its identity
• Follow snake_case naming and PEP 8 conventions
• Multiple assignment and swapping work in a single line
• Variables are references, not containers

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Next up: Numbers →