Python Variables

Python Variables

A variable in Python is a container for storing data values. When you create a variable, you are essentially reserving a portion of memory to hold a value that you can reference, use, or modify later in your program.

In Python:

• Variables do not require an explicit declaration.
• A variable is created the moment you assign a value to it.
• Python is dynamically typed, meaning you don't need to specify the type of the variable; the interpreter infers the type automatically.

Rules for Naming Variables

• Start with a letter or an underscore (_):

  • Valid: name, _age
  • Invalid: 1name, @value

• Followed by letters, numbers, or underscores:

  • Valid: value1, user_name
  • Invalid: user-name, user@name

• Case-sensitive:

  • age, Age, and AGE are treated as three distinct variables.

• Cannot use reserved keywords: Reserved words like class, if, else, and True cannot be used as variable names.

Assigning Values to Variables

In Python, assignment is done using the = operator.

  # Assigning integer value
  x = 10
  print(x)  # Output: 10

  # Assigning string value
  name = "Alice"
  print(name)  # Output: Alice

  # Assigning a float value
  pi = 3.14159
  print(pi)  # Output: 3.14159

Variable Types

Python supports various data types, and variables can hold any of them. Some common types include:

• Integers (int): Whole numbers.

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    age = 25

• Floating-Point Numbers (float): Decimal numbers.

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    price = 19.99
    

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

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    greeting = "Hello, World!"
    

• Booleans (bool): True or False values.

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    is_active = True

• Lists: Ordered, mutable collections.

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   colors = ["red", "blue", "green"]
   

• Dictionaries (dict): Key-value pairs.

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

Multiple Assignments

You can assign values to multiple variables in one line.

  x, y, z = 5, 10, 15
  print(x, y, z)  # Output: 5 10 15

  # Assigning the same value to multiple variables
  a = b = c = 100
  print(a, b, c)  # Output: 100 100 100
  

Changing Variable Values

You can change the value of a variable at any time.

  counter = 1
  print(counter)  # Output: 1

  # Reassigning a new value
  counter = counter + 1
  print(counter)  # Output: 2 

Using type() to Check Variable Type

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

  x = 42
  print(type(x))  # Output: 

  y = "Hello"
  print(type(y))  # Output: 
   

Dynamic Typing

Python allows a variable to change its type during program execution.

  var = 10      # Initially an integer
  print(type(var))  # Output: 

  var = "Python"  # Now a string
  print(type(var))  # Output: 

Best Practices for Variables

• Use meaningful names:

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    temperature = 25  # Good
    t = 25            # Not descriptive
    

• Stick to snake_case for variable names:

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  user_name = "Alice"

• Avoid using single-letter names except in short loops or temporary variables:

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    for i in range(5):
    print(i)

• Use constants (all uppercase) for values that should not change:

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   MAX_USERS = 100
  

Example Program

Here's an example of variables in action:

  # Variables for user information
  name = "Alice"
  age = 30
  is_subscribed = True

  # Display user information
  print("User Information:")
  print("Name:", name)
  print("Age:", age)
  print("Subscribed:", is_subscribed)

  # Changing values
  age = age + 1
  is_subscribed = False

  print("\nUpdated Information:")
  print("Name:", name)
  print("Age:", age)
  print("Subscribed:", is_subscribed)

Output:

• User Information:
• Name: Alice
• Age: 30
• Subscribed: True
• 
  
• Updated Information:
• Name: Alice
• Age: 31
• Subscribed: False