Variables and Data Types

What Is a Variable in Python?

A variable is a named location in memory used to store a value your program can use and change. Python doesn't require you to declare a variable's type upfront — it figures it out automatically based on the value assigned. This is called dynamic typing.

name = "Aman"
age = 21
price = 499.99
is_active = True

Rules for Naming Variables

• Must start with a letter or an underscore (_) — never a number
• Can contain letters, numbers, and underscores only
• Case-sensitive — age and Age are different variables
• Cannot be a reserved keyword like class, for, or import

# Valid
student_name = "Riya"
_marks = 90

# Invalid
2ndYear = "error"   # cannot start with a number
class = "error"     # 'class' is a reserved keyword

Python's Core Data Types

Every value in Python belongs to a data type. Here are the ones you'll use constantly:

1. Numeric Types

x = 10        # int
y = 10.5      # float
z = 2 + 3j    # complex

2. String (str)

message = "Learning Python at Coding Now"
print(message.upper())   # LEARNING PYTHON AT CODING NOW

3. Boolean (bool)

is_enrolled = True
has_paid = False

4. Sequence Types — List & Tuple

skills = ["Python", "SQL", "Pandas"]   # list — mutable
coordinates = (28.70, 77.13)           # tuple — immutable

5. Mapping Type — Dictionary

student = {"name": "Aman", "course": "Data Science"}

6. Set

unique_ids = {101, 102, 103}

List, Tuple, Set, and Dictionary each get their own dedicated note on this hub — this page focuses on the fundamentals.

Checking a Variable's Type

print(type(10))        # <class 'int'>
print(type(10.5))      # <class 'float'>
print(type("hello"))   # <class 'str'>

Type Conversion (Type Casting)

a = "25"
b = int(a)      # converts string to integer → 25
c = float(b)    # converts integer to float → 25.0
d = str(c)      # converts float to string → "25.0"

Key Takeaways

• Python variables don't need explicit type declarations — the type is decided at runtime
• Core types include int, float, complex, str, bool, list, tuple, set, and dict
• Use type() to inspect a value and functions like int(), str(), float() to convert between types