The underlying API calls made by os.putenv() and os.environ[name] = value syntax are not thread safe on POSIX systems. POSIX _does not have_ any thread safe way to access the process global environment.
In a pure Python program, the GIL prevents this from being an issue. But when embedded in a C/C++ program or using extension modules that launch their own threads from C, those threads could also make the invalid assumption that they can safely _read_ the environment. Which is a race condition when a Python thread is doing a putenv() at the same time.
We should document the danger.
CPython's os module snapshots a copy of the environment into a dict at import time (during CPython startup). But os.environ[] assignment and os.putenv() modify the actual process global environment in addition to updating this dict. (If an embedded interpreter is launched from a process with other threads already running, even that initial environment reading could be unsafe if the larger application has a thread that wrongly assumes it has exclusive environment access)
For people modifying os.environ so that the change is visible to child processes, we can recommend using the env= parameter on subprocess API calls to supply a new environment.
A broader issue of should we be modifying the process global environment state at all from os.putenv() and os.environ[] assignment still exists. I'll track that in another issue (to be opened).