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Python Variables

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1. What is a Variable?

• A variable is a named location in memory used to store data.
• It acts as a container for storing values that can be used and manipulated throughout a program.

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2. Variable Naming Rules

• Valid Names:
  • Must start with a letter (a-z, A-Z) or an underscore (_).
  • Can contain letters, numbers (0-9), and underscores.
  • Case-sensitive (age, Age, and AGE are different variables).
• Invalid Names:
  • Cannot start with a number.
  • Cannot contain spaces or special characters (e.g., @, #, $).
  • Cannot be a Python keyword (e.g., if, else, for, while).

valid_variable = 10
_valid_name = "Python"
invalid-variable = 5  # Error: hyphens are not allowed
2invalid = 20         # Error: cannot start with a number

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3. Assigning Values to Variables

• Use the assignment operator = to assign a value to a variable.
• Python is dynamically typed, so you don’t need to declare the type of a variable explicitly.

x = 10          # Integer
name = "Alice"  # String
is_valid = True # Boolean
pi = 3.14       # Float

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4. Multiple Assignments

• You can assign multiple variables in a single line.

a, b, c = 1, 2, 3  # a=1, b=2, c=3
x = y = z = 10     # x=10, y=10, z=10

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5. Variable Types

• Python variables can hold data of different types:
  • Integer: Whole numbers (e.g., 10, -5)
  • Float: Decimal numbers (e.g., 3.14, -0.001)
  • String: Text (e.g., "Hello", 'Python')
  • Boolean: True or False
  • List: Ordered, mutable collection (e.g., [1, 2, 3])
  • Tuple: Ordered, immutable collection (e.g., (1, 2, 3))
  • Dictionary: Key-value pairs (e.g., {"name": "Alice", "age": 25})
  • Set: Unordered, unique elements (e.g., {1, 2, 3})

age = 25            # Integer
price = 19.99       # Float
name = "John"       # String
is_student = True   # Boolean
numbers = [1, 2, 3] # List

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6. Dynamic Typing

• Python is dynamically typed, meaning the type of a variable is determined at runtime and can change.

x = 10      # x is an integer
x = "Hello" # x is now a string

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7. Type Checking

• Use the type() function to check the type of a variable.

x = 10
print(type(x))  # Output: <class 'int'>

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8. Variable Scope

• Local Scope: Variables defined inside a function are local to that function.
• Global Scope: Variables defined outside a function are global and can be accessed anywhere.

x = 10  # Global variable

def my_function():
    y = 5  # Local variable
    print(x)  # Access global variable
    print(y)

my_function()
print(y)  # Error: y is not defined globally

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9. Deleting Variables

• Use the del keyword to delete a variable.

x = 10
del x
print(x)  # Error: x is not defined

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10. Constants

• Python doesn’t have built-in support for constants, but by convention, use uppercase names for constants.

PI = 3.14159  # Convention for constants

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11. Best Practices

• Use descriptive variable names (e.g., user_age instead of ua).
• Follow naming conventions (e.g., snake_case for variable names).
• Avoid using single-letter variable names unless in loops or mathematical contexts.