The maxPoolSize docs say: "Can be either 0 or None, in which case there is no limit on the number of concurrent connections", and the pool treats both as unlimited (if not self.max_pool_size: self.max_pool_size = float("inf")).

However, _parse_pool_options in pymongo/client_options.py only exempts None from the minPoolSize check:

if max_pool_size is not None and min_pool_size > max_pool_size:
    raise ValueError("minPoolSize must be smaller or equal to maxPoolSize")

so 0 is treated as a real limit.

Reproduction

from pymongo import MongoClient
MongoClient("mongodb://localhost", connect=False, maxPoolSize=None, minPoolSize=5)  # OK
MongoClient("mongodb://localhost", connect=False, maxPoolSize=0, minPoolSize=5)
# ValueError: minPoolSize must be smaller or equal to maxPoolSize

Expected
Both calls are accepted, since 0 and None both mean "no limit".

Proposed fix
Skip the check when maxPoolSize is 0 or None:

if max_pool_size and min_pool_size > max_pool_size:

Environment: PyMongo 4.18.2, Python 3.12, Linux.