Python Variables

In Python, variables are used to store data values. They act as containers for storing information that can be referenced and manipulated throughout the program. Python has dynamic typing, which means you don’t need to declare the variable type—it is determined automatically based on the value assigned.

 

Key Features of Python Variables

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1. Dynamic Typing: No need to specify data type.

   Copyx = 10 # Integer
   y = "Hello" # String
   z = 3.14 # Float
   

2. Case-Sensitive: Variable names are case-sensitive.

   CopymyVar = 10
   MyVar = 20 # Different from myVar
   

3. Name Rules:
   • Must start with a letter or an underscore (_).
   • Cannot start with a number.
   • Can contain letters, numbers, and underscores.
   • Cannot use reserved keywords like class, def, etc.

     Copy# Valid variable names
     _name = "John"
     age = 25
     number_of_items = 10
     

     Copy# Invalid variable names
     1name = "Invalid" # Starts with a number
     name@ = "Invalid" # Contains a special character
     

4. Reassignment:

   Variables can be reassigned to a different type.

   Copyx = 10
   x = "Hello" # x is now a string
   

 

Variable Assignment

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1. Single Assignment:

   Copyx = 5
   name = "Alice"
   

2. Multiple Assignment:

   Copya, b, c = 1, 2, 3 # Assign multiple variables at once
   x = y = z = 0 # Assign the same value to multiple variables
   

 

Types of Variables

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Python variables can store various data types:

Data Type	Description	Example
int	Integer (whole number)	x = 10
float	Floating-point number (decimal)	y = 10.5
complex	Complex number	z = 3 + 5j
bool	Boolean (True or False)	is_valid = True
str	String (text)	name = “Alice”
list	List (ordered, mutable collection)	fruits = [“apple”, “banana”]
tuple	Tuple (ordered, immutable collection)	coordinates = (10, 20, 30)
dict	Dictionary (key-value pairs)	person = {“name”: “Alice”}
set	Set (unordered, unique items)	unique_numbers = {1, 2, 3}
frozenset	Immutable version of a set	frozen_numbers = frozenset([1, 2])
bytes	Immutable sequence of bytes	byte_data = b”Hello”
bytearray	Mutable sequence of bytes	byte_array_data = bytearray(5)
memoryview	View on binary data	memory_view = memoryview(b”Hello”)
NoneType	Absence of value	value = None

 

Variable Scope

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1. Global Variables:
   • Declared outside any function.
   • Accessible anywhere in the program.

     Copyx = "global"
     def print_global():
         print(x) # Accessing the global variable
     
     print_global() # Output: global
     

2. Local Variables:
   • Declared inside a function.
   • Accessible only within that function.

     Copydef my_function():
         y = "local"
         print(y)
     my_function() # Output: local
     print(y) # Error: y is not defined
     

3. Using global Keyword:
   • Modify a global variable inside a function.

   Copyx = "global"
   def change_global():
       global x
       x = "modified"
   change_global()
   print(x) # Output: modified
   

 

Best Practices for Variables

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• Use descriptive names to make your code self-explanatory.

  Copy# Bad
  x = 10
  # Good
  item_price = 10
  

• Use snake_case for variable names (e.g., total_price, user_name).
• Avoid reserved keywords as variable names (e.g., class, def).
• Initialize variables before using them.
• Use constants for values that should not change. By convention, constants are written in all uppercase.

  CopyPI = 3.14
  MAX_USERS = 100