python essentials

Objects: bytes vs BytesIO

Feature:             Type
`bytes` Object:      Immutable sequence of bytes
`BytesIO` Object:    In-memory file-like object for binary data

Feature:             Mutability
`bytes` Object:      Immutable (cannot be modified)
`BytesIO` Object:    Mutable (you can write and modify data)

Feature:             Use Case
`bytes` Object:      Fixed, unchangeable binary data
`BytesIO` Object:    Temporary, in-memory file-like operations

Feature:             Memory Storage
`bytes` Object:      Stores data as a sequence of bytes
`BytesIO` Object:    Stores data in memory, can read/write like a file

Feature:             Methods
`bytes` Object:      Supports indexing, slicing, and iteration
`BytesIO` Object:    Supports `.read()`, `.write()`, `.seek()` file-like methods

Feature:             Size
`bytes` Object:      Fixed size once created
`BytesIO` Object:    Size can change as you write/read from it

Note: recv() and Thread Behavior in Python

recv() is a low-level, protocol-independent socket function called by C extensions (e.g., psycopg2) to receive data from a TCP socket, like a DB response. 

Python thread runs a DB call -> C extension releases GIL -> calls recv(), which blocks and thread stays blocked in the OS wait queue during recv().

Network data arrives -> NIC interrupts (per packet or batch) -> OS network stack processes TCP headers, moves payload to socket buffer, updates socket state based on TCP state for instance FIN/RST. OS wakes threads that are waiting on that socket, moving them to the run queue. 

When thread run again -> C extension reacquires GIL -> recv() checks buffer and state, deciding to return or wait again.

recv() Wake and Decision:
-> Checks socket buffer. If data’s there, returns it (e.g., recv(1024) returns up to 1024, but x if that's what' there).
-> If buffer’s empty, checks socket state:
    	-> Open (no FIN): Blocks again if data’s partial and more’s expected, or waits if nothing’s there. Never returns 0—reserved for closure.
    	-> Closed (FIN, buffer empty): Returns 0, “end of stream.”
    	-> Errored (RST): Returns -1 with errno (e.g., ECONNRESET).
	-> Non-blocking mode returns -1/EAGAIN if no data; timeouts error if set.

DB libs loop recv() for protocol completeness.
Key Point: OS wakes thread on data or state change; recv() checks buffer then state, returning data/0/error or waiting based on need.

Existence of variables!

To check the existence of a local variable:

if 'myVar' in locals():
  # myVar exists.

To check the existence of a global variable:

if 'myVar' in globals():
  # myVar exists.

To check if an object has an attribute:

if hasattr(obj, 'attr_name'):
  # obj.attr_name exists.