I think you misunderstand; since PyMongo's documentation warns you that a single MongoClient is not fork-safe, you interpret that to mean that PyMongo prohibits your whole program from ever creating subprocesses.
Any single MongoClient is not fork-safe, meaning you must not create it before forking and use the same MongoClient object after forking. Using PyMongo in your program overall, or using one MongoClient before a fork and a different one after, are all safe.
That's why subprocess.Popen is ok: you fork, then exec (to replace your program with a different one in the child process), and therefore you cannot possibly use the same MongoClient in the child afterward.
To quote the PyMongo FAQ:
On Unix systems the multiprocessing module spawns processes using fork(). Care must be taken when using instances of MongoClient with fork(). Specifically, instances of MongoClient must not be copied from a parent process to a child process. Instead, the parent process and each child process must create their own instances of MongoClient. For example:
# Each process creates its own instance of MongoClient. def func(): db = pymongo.MongoClient().mydb # Do something with db. proc = multiprocessing.Process(target=func) proc.start()
Never do this:
client = pymongo.MongoClient() # Each child process attempts to copy a global MongoClient # created in the parent process. Never do this. def func(): db = client.mydb # Do something with db. proc = multiprocessing.Process(target=func) proc.start()
Instances of MongoClient copied from the parent process have a high probability of deadlock in the child process due to inherent incompatibilities between fork(), threads, and locks. PyMongo will attempt to issue a warning if there is a chance of this deadlock occurring.
