RAM's default pid is evaluated at import time, so forked workers track the parent process

Problem

RAM.__init__ uses pid: int = psutil.Process().pid as a default argument. Python evaluates default arguments once, when the module is first imported, so the default pid is frozen to the importing process for the life of the interpreter — including across fork(). A child process that constructs RAM() therefore records the parent's pid.

Reproduction

import os, multiprocessing as mp
from codecarbon.external.ram import RAM   # imported in the parent

def worker():
    print(os.getpid(), RAM(tracking_mode="process")._pid)  # these differ

mp.set_start_method("fork")
p = mp.Process(target=worker); p.start(); p.join()

Linux/fork only — with spawn the child re-imports and the default is recomputed.

Root cause

• codecarbon/external/ram.py:37 — pid: int = psutil.Process().pid (evaluated at import).
• codecarbon/external/ram.py:58 — self._pid = pid.
• Neither real construction site passes a pid: codecarbon/core/resource_tracker.py:37 and codecarbon/core/hardware_cache.py:164, so the stale default is always the one in use.

By comparison CPU resolves it correctly inside __init__ (codecarbon/external/hardware.py:216).

Expected vs actual

Expected: with tracking_mode="process", RAM usage is measured for the process that created the tracker.

Actual, in a forked child:

• If the parent is alive, process_memory_GB (ram.py:285-296) measures the parent's RSS plus all its descendants — i.e. every sibling worker — so N workers each report roughly the whole pool.
• If the parent has exited, psutil.Process(pid) raises NoSuchProcess; total_power swallows it (ram.py:337-339) and RAM power silently reads 0 W for the rest of the run.

Machine tracking mode is unaffected (it never reads _pid).

Fix

Default pid to None and resolve os.getpid() in __init__.

Start in codecarbon/external/ram.py at RAM.init and compare its pid handling with CPU in codecarbon/external/hardware.py:216. Check the construction paths in codecarbon/core/resource_tracker.py:37 and codecarbon/core/hardware_cache.py:164, then verify process-mode RAM in ram.py:285-296 under fork. Done means a forked child records and measures its own pid while machine mode remains unaffected.