PYTHON-6149 Fix condition for minPoolSize validation

doc/changelog.rst

@@ -13,8 +13,13 @@ Bug fixes
   default ``UNSPECIFIED`` UUID representation, and raise
   :class:`~bson.errors.InvalidBSON` when UUID conversion is requested
   (`PYTHON-6172`_).
+- Fixed a bug where ``MongoClient(maxPoolSize=0, minPoolSize>0)`` raised
+  ``ValueError``, even though ``maxPoolSize=0`` means "no limit". The
+  ``minPoolSize <= maxPoolSize`` validation now treats both ``0`` and ``None``
+  as unlimited (`PYTHON-6149`_).
 
 .. _PYTHON-6172: https://jira.mongodb.org/browse/PYTHON-6172
+.. _PYTHON-6149: https://jira.mongodb.org/browse/PYTHON-6149
 
 Changes in Version 4.19.0 (2026/10/14)
 --------------------------------------

pymongo/client_options.py

@@ -160,7 +160,8 @@ def _parse_pool_options(
     max_pool_size = options.get("maxpoolsize", common.MAX_POOL_SIZE)
     min_pool_size = options.get("minpoolsize", common.MIN_POOL_SIZE)
     max_idle_time_seconds = options.get("maxidletimems", common.MAX_IDLE_TIME_SEC)
-    if max_pool_size is not None and min_pool_size > max_pool_size:
+    # A maxPoolSize of 0 or None means no limit.
+    if max_pool_size and min_pool_size > max_pool_size:
         raise ValueError("minPoolSize must be smaller or equal to maxPoolSize")
     connect_timeout = options.get("connecttimeoutms", common.CONNECT_TIMEOUT)
     socket_timeout = options.get("sockettimeoutms")

pymongo/pool_options.py

@@ -498,7 +498,7 @@ def max_pool_size(self) -> float:
         """The maximum allowable number of concurrent connections to each
         connected server. Requests to a server will block if there are
         `maxPoolSize` outstanding connections to the requested server.
-        Defaults to 100. Cannot be 0.
+        Defaults to 100. A value of 0 or ``None`` means no limit.
 
         When a server's pool has reached `max_pool_size`, operations for that
         server block waiting for a socket to be returned to the pool. If

test/asynchronous/test_pooling.py

@@ -200,9 +200,28 @@ async def test_max_pool_size_validation(self):
 
         self.assertRaises(ValueError, AsyncMongoClient, host=host, port=port, maxPoolSize="foo")
 
+        self.assertRaises(
+            ValueError, AsyncMongoClient, host=host, port=port, maxPoolSize=10, minPoolSize=20
+        )
+
         c = AsyncMongoClient(host=host, port=port, maxPoolSize=100, connect=False)
         self.assertEqual(c.options.pool_options.max_pool_size, 100)
 
+    async def test_max_pool_size_no_limit_with_min_pool_size(self):
+        # A maxPoolSize of 0 or None means no limit, so minPoolSize must
+        # still be allowed (PYTHON-6149).
+        host, port = await async_client_context.host, await async_client_context.port
+        for max_pool_size in (0, None):
+            c = AsyncMongoClient(
+                host=host,
+                port=port,
+                maxPoolSize=max_pool_size,
+                minPoolSize=5,
+                connect=False,
+            )
+            self.assertEqual(c.options.pool_options.max_pool_size, max_pool_size)
+            self.assertEqual(c.options.pool_options.min_pool_size, 5)
+
     async def test_no_disconnect(self):
         await run_cases(self.c, [NonUnique, Unique, InsertOneAndFind])
 

test/test_pooling.py

@@ -200,9 +200,28 @@ def test_max_pool_size_validation(self):
 
         self.assertRaises(ValueError, MongoClient, host=host, port=port, maxPoolSize="foo")
 
+        self.assertRaises(
+            ValueError, MongoClient, host=host, port=port, maxPoolSize=10, minPoolSize=20
+        )
+
         c = MongoClient(host=host, port=port, maxPoolSize=100, connect=False)
         self.assertEqual(c.options.pool_options.max_pool_size, 100)
 
+    def test_max_pool_size_no_limit_with_min_pool_size(self):
+        # A maxPoolSize of 0 or None means no limit, so minPoolSize must
+        # still be allowed (PYTHON-6149).
+        host, port = client_context.host, client_context.port
+        for max_pool_size in (0, None):
+            c = MongoClient(
+                host=host,
+                port=port,
+                maxPoolSize=max_pool_size,
+                minPoolSize=5,
+                connect=False,
+            )
+            self.assertEqual(c.options.pool_options.max_pool_size, max_pool_size)
+            self.assertEqual(c.options.pool_options.min_pool_size, 5)
+
     def test_no_disconnect(self):
         run_cases(self.c, [NonUnique, Unique, InsertOneAndFind])