Caveats

Only output during the session is contained

Temporary files created before the session starts (while the initial conftest.py files are imported, in pytest_configure, or in other plugins' pytest_sessionstart hooks) or after it ends (pytest_terminal_summary, pytest_unconfigure) land directly in /dev/shm and stay there until reboot. Create them in fixtures, or remove them yourself.

Caches under the temp root become per-session

A library that caches downloads under tempfile.gettempdir() sees the contained directory, which the plugin frees after the session, so it downloads again every session. If your suite uses such a library, point its cache at a stable directory in your root conftest.py, where tempfile.gettempdir() is still /dev/shm. Which environment variable to set depends on the library. For example, tiktoken reads TIKTOKEN_CACHE_DIR, falling back to DATA_GYM_CACHE_DIR:

# conftest.py at the repository root (tiktoken shown as an example)
import os
import tempfile
from pathlib import Path

if "TIKTOKEN_CACHE_DIR" not in os.environ and "DATA_GYM_CACHE_DIR" not in os.environ:
    os.environ["TIKTOKEN_CACHE_DIR"] = str(Path(tempfile.gettempdir()) / "data-gym-cache")

Temp files live on another filesystem

os.rename or os.replace from a temporary file into your project fails with EXDEV, as it already does wherever /tmp is tmpfs.

Paths get longer

/dev/shm/pytest-of-<user>/pytest-N/popen-gwN/shm-tmp/tmpXXXXXXXX is much longer than /tmp/tmpXXXXXXXX, which matters for the 107-byte limit on AF_UNIX socket paths.

Files use RAM

Everything a session writes counts against memory until the session ends. Base directories of failing sessions stay until the machine reboots or later failing sessions push them out of pytest's retention of three numbered directories; passing sessions reuse the freed number instead of advancing it.