Variables & Memory Model

Variables in Python — The Reference Model:

Unlike C/C++ where a variable IS a memory box, in Python a variable is a label (reference) that points to an object in memory.

x = 42    # Creates an int object 42 in memory, x points to it  
y = x     # y now points to the SAME object 42  
x = 100   # x now points to a new object 100, y still points to 42  

Variable Naming Rules:

1. Must start with a letter (a-z, A-Z) or underscore (_)
2. Can contain letters, digits (0-9), and underscores
3. Cannot start with a digit
4. Cannot be a Python keyword (if, for, class, etc.)
5. Case-sensitive: age, Age, and AGE are three different variables

Python Naming Conventions (PEP 8):

• snake_case — variables, functions, methods: user_name, calculate_total()
• PascalCase — classes: StudentProfile, BankAccount
• UPPER_SNAKE_CASE — constants: MAX_RETRIES, API_KEY
• _single_leading_underscore — "private" (convention): _internal_value
• __double_leading_underscore — name mangling: __private_attr

The id() Function:
Every object in Python has a unique identity (memory address). Use id() to see it.

Python Keyword List (35 keywords in Python 3.12):
False, None, True, and, as, assert, async, await, break, class, continue, def, del, elif, else, except, finally, for, from, global, if, import, in, is, lambda, nonlocal, not, or, pass, raise, return, try, while, with, yield