Python Variables

valid_name = 10 # Valid
_valid_name = 20 # Valid
name123 = "Alice" # Valid
1invalid = 30 # Invalid (cannot start with a number)
for = 40 # Invalid (cannot use reserved word)
name! = "Bob" # Invalid (cannot use special characters)


4. Assigning Values to Variables

Python allows multiple types of value assignments to variables:

• Single Assignment:
  python

  a = 10


  Multiple Assignments: You can assign the same value to multiple variables simultaneously:

  python

  x = y = z = 100


  Multiple Variables, Multiple Values: You can assign different values to multiple variables in one line:

  python

  a, b, c = 1, 2, 3


  5. Data Types of Variables

  Python is dynamically typed, meaning you don’t need to declare the type of a variable explicitly. The type is inferred based on the value assigned.

  Common data types:

  • Integer (int): Whole numbers.
    python

    age = 25


    Float (float): Numbers with a decimal point.

    python

    height = 5.9


    String (str): A sequence of characters enclosed in quotes.

    python

    name = "Alice"


    Boolean (bool): True or False values.

    python

    is_active = True
List (list): An ordered collection of values.

    python

    fruits = ["apple", "banana", "cherry"]


    Tuple (tuple): An ordered, immutable collection of values.

    python

    coordinates = (10, 20)


    Dictionary (dict): A collection of key-value pairs.

    python

    person = {"name": "Alice", "age": 25}


    Set (set): An unordered collection of unique values.

    python

    unique_numbers = {1, 2, 3}


    6. Changing the Value of a Variable

    Variables can be reassigned to new values, even of different types.

    python

    x = 10 # x is initially an integer
x = "Hello" # Now x is a string


    7. Checking the Type of a Variable

    You can check the type of a variable using the type() function.

    python

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

y = "Hello"
print(type(y)) # Output: <class 'str'>

    8. Deleting Variables

    You can delete a variable using the del statement, which removes the variable from memory.

    python

    x = 10
del x
# print(x) # This would raise an error because x no longer exists


    9. Global and Local Variables

    • Global Variables: Defined outside of all functions, accessible throughout the program.
    • Local Variables: Defined inside a function, accessible only within that function.

    python

    x = "global"
def my_function():
x = "local"
print(x) # Outputs "local"
my_function()
print(x) # Outputs "global"


    To modify a global variable inside a function, you can use the global keyword:

    python

    x = "global"
def my_function():
global x
x = "changed"
my_function()
print(x) # Outputs "changed"


    10. Variable Scope

    • Local Scope: Variables defined within a function or a block of code.
    • Global Scope: Variables defined outside all functions, accessible from any part of the program.

    python

    def my_function():
y = 10 # Local variable
print(y)
my_function()
# print(y) # This would raise an error because y is not defined outside the function


  11. Constants

  Python doesn’t have built-in support for constants, but by convention, variables meant to be constants are written in all uppercase letters.

  python

  PI = 3.14159
GRAVITY = 9.8


  While you can technically reassign these values, the uppercase naming convention signals to other programmers that these values should not change.

  12. Dynamic Typing

  Python is dynamically typed, meaning you can change the type of a variable by assigning it a different type of value. For example:

  python

  x = 10 # x is an integer
x = "Python" # Now x is a string


  This flexibility is one of Python’s strengths, but it requires careful management to avoid bugs due to unintended type changes.

  Best Practices for Using Variables

  • Use meaningful names: Choose variable names that describe the data they store. This makes your code more readable and easier to maintain.
    python

    age = 25
total_price = 100.50


    • Follow naming conventions: Use snake_case for variable names (e.g., total_price) and UPPER_CASE for constants (e.g., MAX_LIMIT).
    • Avoid using reserved keywords: Python has a set of reserved keywords that cannot be used as variable names, such as if, else, for, while, True, False, None, etc.
    • Be mindful of scope: Understand the difference between global and local variables and use them appropriately to avoid conflicts and unintended side effects.

    Variables are the foundation of any Python program, enabling you to store, manipulate, and retrieve data dynamically throughout your code. Understanding how to use variables effectively is crucial to writing clear, efficient, and bug-free Python programs.