Python - Variables

In Python, a variable is a name that refers to a value stored in memory. It's like a label you attach to a box so you can identify what's inside it later.

Key Points

Variables are containers for storing data values.
Python is dynamically typed, so you don’t need to declare the type.
Type is inferred from the value assigned.

sh
name = "Alice"       # string
age = 25             # integer
height = 5.7         # float
is_student = True    # boolean

Why are Variables Important?

They make your code readable and manageable.
Allow you to store, reuse, and modify values.

Example

sh
radius = 5
pi = 3.14159
area = pi * radius ** 2
print("Area of the circle is:", area)

Rules for Naming Variables

Must start with a letter or an underscore (_).
Can contain numbers after the first character (e.g., x1, value_2).
Case-sensitive: score and Score are different.
Cannot use Python keywords (e.g., class, for).

Valid vs Invalid Examples

sh
# Valid
x = 10
_name = "Raj"
temperature_celsius = 36.6

# Invalid
# 2cool = "no"       # starts with number
# my-var = "dash"    # hyphens not allowed
# class = "error"    # 'class' is a keyword

1. Declaring Variables

python
# This is how you declare variables in Python
x = 5  # Integer
y = 3.14 # Float
name = "Alice" # String
status = True # Boolean

2. Variable Naming Rules

Must start with a letter or underscore (_)
Cannot start with a number
Can only contain alpha-numeric characters and underscores
Case-sensitive (e.g., age, Age, and AGE are different)

3. Multiple Assignments

python

    a, b, c = 1, 2, 3 # Assigning multiple variables in one line
    x = y = z = 0 # Assigning same value to multiple variables

4. Variable Types and Type Casting

python

    x = int(1) # 1
    y = float("3.5") # 3.5
    z = str(10) # "10"
    b = bool(0) # False

5. Variable Scope

python

    def example():
        local_var = "I'm local"
        print(local_var)

    global_var = "I'm global"
    example()
    print(global_var)

6. Constants

Python doesn't have true constants, but you can indicate a constant by using uppercase letters:

python

    PI = 3.14159
    GRAVITY = 9.8

7. Deleting Variables

python

    x = 10
    del x # x is now deleted

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