Troubleshooting

Server Reports Wire Version X, PyMongo Requires Y

If you try to connect to MongoDB Server v3.4 or earlier, PyMongo might raise the following error:

pymongo.errors.ConfigurationError: Server at localhost:27017 reports wire version 5, but this version of PyMongo requires at least 6 (MongoDB 3.6).

This occurs when the driver version is too new for the server it's connecting to. To resolve this issue, upgrade your MongoDB deployment to v3.6 or later, or downgrade to PyMongo v3.x, which supports MongoDB Server v2.6 and later.

AutoReconnect

An AutoReconnect exception indicates that a failover has occurred. This means that PyMongo has lost its connection to the original primary member of the replica set, and its last operation might have failed.

When this error occurs, PyMongo automatically tries to find the new primary member for subsequent operations. To handle the error, your application must take one of the following actions:

• Retry the operation that might have failed

• Continue running, with the understanding that the operation might have failed

Timeout When Accessing MongoDB from PyMongo with Tunneling

If you try to connect to a MongoDB replica set over an SSH tunnel, you receive the following error:

File "/Library/Python/2.7/site-packages/pymongo/collection.py", line 1560, in count
  return self._count(cmd, collation, session)
  File "/Library/Python/2.7/site-packages/pymongo/collection.py", line 1504, in _count
  with self._socket_for_reads() as (connection, slave_ok):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/contextlib.py", line 17, in __enter__
  return self.gen.next()
  File "/Library/Python/2.7/site-packages/pymongo/mongo_client.py", line 982, in _socket_for_reads
  server = topology.select_server(read_preference)
  File "/Library/Python/2.7/site-packages/pymongo/topology.py", line 224, in select_server
  address))
  File "/Library/Python/2.7/site-packages/pymongo/topology.py", line 183, in select_servers
  selector, server_timeout, address)
  File "/Library/Python/2.7/site-packages/pymongo/topology.py", line 199, in _select_servers_loop
  self._error_message(selector))
pymongo.errors.ServerSelectionTimeoutError: localhost:27017: timed out

This occurs because PyMongo discovers replica set members by using the response from the isMaster command, which contains the addresses and ports of the other replica set members. However, you can't access these addresses and ports through the SSH tunnel.

Instead, you can connect directly to a single MongoDB node by using the directConnection=True option with SSH tunneling.

AutoReconnect Error

You receive this error if you specify tag-sets in your read preference and MongoDB is unable to find replica set members with the specified tags. To avoid this error, include an empty dictionary ({}) at the end of the tag-set list. This instructs PyMongo to read from any member that matches the read-reference mode when it can't find matching tags.

DeprecationWarning: Count Is Deprecated

PyMongo no longer supports the count() method. Instead, use the count_documents() method from the Collection class.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Cursor' object has no attribute 'count'

MongoClient Fails ConfigurationError

Providing invalid keyword argument names causes the driver to raise this error.

Ensure that the keyword arguments you specify exist and are spelled correctly.

No Results When Querying for a Document by ObjectId in Web Applications

It's common in web applications to encode documents' ObjectIds in URLs, as shown in the following code example:

"/posts/50b3bda58a02fb9a84d8991e"

Your web framework passes the ObjectId part of the URL to your request handler as a string. You must convert the string to an ObjectId instance before passing it to the find_one() method.

The following code example shows how to perform this conversion in a Flask application. The process is similar for other web frameworks.

from pymongo import MongoClient
from bson.objectid import ObjectId
from flask import Flask, render_template
client = MongoClient()
app = Flask(__name__)
@app.route("/posts/<_id>")
def show_post(_id):
   # NOTE!: converting _id from string to ObjectId before passing to find_one
   post = client.db.posts.find_one({'_id': ObjectId(_id)})
   return render_template('post.html', post=post)
if __name__ == "__main__":
    app.run()

Query Works in the Shell But Not in PyMongo

After the _id field, which is always first, the key-value pairs in a BSON document can be in any order. The mongo shell preserves key order when reading and writing data, as shown by the fields "b" and "a" in the following code example:

// mongo shell
db.collection.insertOne( { "_id" : 1, "subdocument" : { "b" : 1, "a" : 1 } } )
// Returns: WriteResult({ "nInserted" : 1 })
db.collection.findOne()
// Returns: { "_id" : 1, "subdocument" : { "b" : 1, "a" : 1 } }

PyMongo represents BSON documents as Python dictionaries by default, and the order of keys in dictionaries is not defined. In Python, a dictionary declared with the "a" key first is the same as one with the "b" key first. In the following example, the keys are displayed in the same order regardless of their order in the print statement:

print({'a': 1.0, 'b': 1.0})
# Returns: {'a': 1.0, 'b': 1.0}
print({'b': 1.0, 'a': 1.0})
# Returns: {'a': 1.0, 'b': 1.0}

Similarly, Python dictionaries might not show keys in the order they are stored in BSON. The following example shows the result of printing the document inserted in a preceding example:

print(collection.find_one())
# Returns: {'_id': 1.0, 'subdocument': {'a': 1.0, 'b': 1.0}}

To preserve the order of keys when reading BSON, use the SON class, which is a dictionary that remembers its key order.

The following code example shows how to create a collection configured to use the SON class:

from bson import CodecOptions, SON
opts = CodecOptions(document_class=SON)
CodecOptions(document_class=...SON..., tz_aware=False, uuid_representation=UuidRepresentation.UNSPECIFIED, unicode_decode_error_handler='strict', tzinfo=None, type_registry=TypeRegistry(type_codecs=[], fallback_encoder=None), datetime_conversion=DatetimeConversion.DATETIME)
collection_son = collection.with_options(codec_options=opts)

When you find the preceding subdocument, the driver represents query results with SON objects and preserves key order:

print(collection_son.find_one())

SON([('_id', 1.0), ('subdocument', SON([('b', 1.0), ('a', 1.0)]))])

The subdocument's actual storage layout is now visible: "b" is before "a".

Because a Python dictionary's key order is not defined, you cannot predict how it will be serialized to BSON. However, MongoDB considers subdocuments equal only if their keys have the same order. If you use a Python dictionary to query on a subdocument, it may not match:

collection.find_one({'subdocument': {'b': 1.0, 'a': 1.0}}) is None

True

Because Python considers the two dictionaries the same, swapping the key order in your query makes no difference:

collection.find_one({'subdocument': {'b': 1.0, 'a': 1.0}}) is None

True

You can solve this in two ways. First, you can match the subdocument field-by-field:

collection.find_one({'subdocument.a': 1.0,
                   'subdocument.b': 1.0})

{'_id': 1.0, 'subdocument': {'a': 1.0, 'b': 1.0}}

The query matches any subdocument with an "a" of 1.0 and a "b" of 1.0, regardless of the order in which you specify them in Python, or the order in which they're stored in BSON. This query also now matches subdocuments with additional keys besides "a" and "b", whereas the previous query required an exact match.

The second solution is to use a ~bson.son.SON object to specify the key order:

query = {'subdocument': SON([('b', 1.0), ('a', 1.0)])}
collection.find_one(query)

{'_id': 1.0, 'subdocument': {'a': 1.0, 'b': 1.0}}

The driver preserves the key order you use when you create a ~bson.son.SON when serializing it to BSON and using it as a query. Thus, you can create a subdocument that exactly matches the subdocument in the collection.

'Cursor' Object Has No Attribute '_Cursor__killed'

PyMongo v3.8 or earlier raises a TypeError and an AttributeError if you supply invalid arguments to the Cursor constructor. The AttributeError is irrelevant, but the TypeError contains debugging information as shown in the following example:

Exception ignored in: <function Cursor.__del__ at 0x1048129d8>
...
AttributeError: 'Cursor' object has no attribute '_Cursor__killed'
...
TypeError: __init__() got an unexpected keyword argument '<argument>'

To fix this, ensure that you supply the correct keyword arguments. You can also upgrade to PyMongo v3.9 or later, which removes the irrelevant error.

"CursorNotFound cursor id not valid at server"

Cursors in MongoDB can timeout on the server if they've been open for a long time without any operations being performed on them. This can lead to a CursorNotFound exception when you try to iterate through the cursor.

'Cannot Do Exclusion on Field <field> in Inclusion Projection'

The driver returns an OperationFailure with this message if you attempt to include and exclude fields in a single projection. Ensure that your projection specifies only fields to include or fields to exclude.

DuplicateKeyException

If you perform a write operation that stores a duplicate value that violates a unique index, the driver raises a DuplicateKeyException, and MongoDB throws an error resembling the following:

E11000 duplicate key error index

ValueError: cannot encode native uuid.UUID with UuidRepresentation.UNSPECIFIED

This error results from trying to encode a native UUID object to a Binary object when the UUID representation is UNSPECIFIED, as shown in the following code example:

unspecified_collection.insert_one({'_id': 'bar', 'uuid': uuid4()})
Traceback (most recent call last):
...
ValueError: cannot encode native uuid.UUID with UuidRepresentation.UNSPECIFIED.
UUIDs can be manually converted to bson.Binary instances using bson.Binary.from_uuid()
or a different UuidRepresentation can be configured. See the documentation for
UuidRepresentation for more information.

Instead, you must explicitly convert a native UUID to a Binary object by using the Binary.from_uuid() method, as shown in the following example:

explicit_binary = Binary.from_uuid(uuid4(), UuidRepresentation.STANDARD)
unspec_collection.insert_one({'_id': 'bar', 'uuid': explicit_binary})

OverflowError When Decoding Dates Stored by Another Language's Driver

PyMongo decodes BSON datetime values to instances of Python's datetime.datetime class. Instances of datetime.datetime are limited to years between datetime.MINYEAR (1) and datetime.MAXYEAR (9999). Some MongoDB drivers can store BSON datetimes with year values far outside those supported by datetime.datetime.

There are a few ways to work around this issue. Starting with PyMongo 4.3, bson.decode can decode BSON datetime values in one of four ways. You can specify the conversion method by using datetime_conversion parameter of ~bson.codec_options.CodecOptions.

The default conversion option is ~bson.codec_options.DatetimeConversion.DATETIME, which will attempt to decode the value as a datetime.datetime, allowing ~builtin.OverflowError to occur for out-of-range dates. ~bson.codec_options.DatetimeConversion.DATETIME_AUTO alters this behavior to instead return ~bson.datetime_ms.DatetimeMS when representations are out-of-range, while returning ~datetime.datetime objects as before:

from datetime import datetime
from bson.datetime_ms import DatetimeMS
from bson.codec_options import DatetimeConversion
from pymongo import MongoClient
client = MongoClient(datetime_conversion=DatetimeConversion.DATETIME_AUTO)
client.db.collection.insert_one({"x": datetime(1970, 1, 1)})
client.db.collection.insert_one({"x": DatetimeMS(2**62)})
for x in client.db.collection.find():
   print(x)

{'_id': ObjectId('...'), 'x': datetime.datetime(1970, 1, 1, 0, 0)}
{'_id': ObjectId('...'), 'x': DatetimeMS(4611686018427387904)}

For other options, see the API documentation for the DatetimeConversion class.

Another option that does not involve setting datetime_conversion is to filter out document values outside of the range supported by ~datetime.datetime:

from datetime import datetime
coll = client.test.dates
cur = coll.find({'dt': {'$gte': datetime.min, '$lte': datetime.max}})

If you don't need the value of datetime, you can filter out just that field:

cur = coll.find({}, projection={'dt': False})

CERTIFICATE_VERIFY_FAILED

An error message similar to the following means that OpenSSL couldn't verify the server's certificate:

[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed

This often happens because OpenSSL can't access the system's root certificates, or because the certificates are out of date.

If you use Linux, ensure that you have the latest root certificate updates installed from your Linux vendor.

If you use macOS, and if you're running Python v3.7 or later that you downloaded from python.org, run the following command to install root certificates:

open "/Applications/Python <YOUR PYTHON VERSION>/Install Certificates.command"

If you use portable-pypy, you might need to set an environment variable to tell OpenSSL where to find root certificates. The following code example shows how to install the certifi module from PyPi and export the SSL_CERT_FILE environment variable:

$ pypy -m pip install certifi
$ export SSL_CERT_FILE=$(pypy -c "import certifi; print(certifi.where())")

TLSV1_ALERT_PROTOCOL_VERSION

An error message similar to the following means that the OpenSSL version used by Python doesn't support a new enough TLS protocol to connect to the server:

[SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version

Industry best practices recommend, and some regulations require, that older TLS protocols be disabled in some MongoDB deployments. Some deployments might disable TLS 1.0, while others might disable TLS 1.0 and TLS 1.1.

No application changes are required for PyMongo to use the newest TLS versions, but some operating system versions might not provide an OpenSSL version new enough to support them.

If you use macOS v10.12 (High Sierra) or earlier, install Python from python.org, homebrew, macports, or a similar source.

If you use Linux or another non-macOS Unix, use the following command to check your OpenSSL version:

$ openssl version

If the preceding command shows a version number less than 1.0.1, support for TLS 1.1 or newer isn't available. Upgrade to a newer version or contact your OS vendor for a solution.

To check the TLS version of your Python interpreter, install the requests module and execute the following code:

python -c "import requests; print(requests.get('https://www.howsmyssl.com/a/check', verify=False).json()['tls_version'])"

You should see TLS 1.1 or later.

Invalid Status Response

An error message similar to the following means that certificate revocation checking failed:

[('SSL routines', 'tls_process_initial_server_flight', 'invalid status response')]

For more details, see the OCSP section of this guide.

SSLV3_ALERT_HANDSHAKE_FAILURE

When using Python v3.10 or later with MongoDB versions earlier than v4.0, you might see errors similar to the following messages:

SSL handshake failed: localhost:27017: [SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:997)
SSL handshake failed: localhost:27017: EOF occurred in violation of protocol (_ssl.c:997)

The MongoDB Server logs might also show the following error:

2021-06-30T21:22:44.917+0100 E NETWORK  [conn16] SSL: error:1408A0C1:SSL routines:ssl3_get_client_hello:no shared cipher

Changes made to the ssl module in Python v3.10 might cause incompatibilities with MongoDB versions earlier than v4.0. To resolve this issue, try one or more of the following steps:

• Downgrade Python to v3.9 or earlier

• Upgrade MongoDB Server to v4.2 or later

• Install PyMongo with the OCSP option, which relies on PyOpenSSL

ServerSelectionTimeoutError

This error indicates that the client couldn't find an available server to run the operation within the given timeout:

pymongo.errors.ServerSelectionTimeoutError: No servers found yet, Timeout: -0.00202266700216569s, Topology Description: <TopologyDescription id: 63698e87cebfd22ab1bd2ae0, topology_type: Unknown, servers: [<ServerDescription ('localhost', 27017) server_type: Unknown, rtt: None>]>

NetworkTimeout

This error indicates either that the client couldn't establish a connection within the given timeout or that the operation was sent but the server didn't respond in time:

pymongo.errors.NetworkTimeout: localhost:27017: timed out

ExecutionTimeout

This error might indicate that the server cancelled the operation because it exceeded the given timeout. Even if PyMongo raises this exception, the operation might have partially completed on the server.

pymongo.errors.ExecutionTimeout: operation exceeded time limit, full error: {'ok': 0.0, 'errmsg': 'operation exceeded time limit', 'code': 50, 'codeName': 'MaxTimeMSExpired'}

It also might indicate that the client cancelled the operation because it wasn't possible to complete it within the given timeout:

pymongo.errors.ExecutionTimeout: operation would exceed time limit, remaining timeout:0.00196 <= network round trip time:0.00427

WTimeoutError

This error indicates that the server couldn't complete the requested write operation within the given timeout and following the specified write concern:

pymongo.errors.WTimeoutError: operation exceeded time limit, full error: {'code': 50, 'codeName': 'MaxTimeMSExpired', 'errmsg': 'operation exceeded time limit', 'errInfo': {'writeConcern': {'w': 1, 'wtimeout': 0}}}

BulkWriteError

This error indicates that the server couldn't complete an insert_many() or bulk_write() method within the given timeout and following the specified write concern:

pymongo.errors.BulkWriteError: batch op errors occurred, full error: {'writeErrors': [], 'writeConcernErrors': [{'code': 50, 'codeName': 'MaxTimeMSExpired', 'errmsg': 'operation exceeded time limit', 'errInfo': {'writeConcern': {'w': 1, 'wtimeout': 0}}}], 'nInserted': 2, 'nUpserted': 0, 'nMatched': 0, 'nModified': 0, 'nRemoved': 0, 'upserted': []}

Forking a Process Causes a Deadlock

A MongoClient instance spawns multiple threads to run background tasks, such as monitoring connected servers. These threads share state that is protected by instances of the threading.Lock class, which are themselves not fork-safe. PyMongo is subject to the same limitations as any other multithreaded code that uses the threading.Lock class, or any mutexes.

One of these limitations is that the locks become useless after calling the fork() method. When fork() executes, the driver copies all the parent process's locks to the child process in the same state as they were in the parent. If they are locked in the parent process, they are also locked in the child process. The child process created by fork() has only one thread, so any locks created by other threads in the parent process are never released in the child process. The next time the child process attempts to acquire one of these locks, deadlock occurs.

Starting in PyMongo version 4.3, after you call the os.fork() method, the driver uses the os.register_at_fork() method to reset its locks and other shared state in the child process. Although this reduces the likelihood of a deadlock, PyMongo depends on libraries that aren't fork-safe in multithreaded applications, including OpenSSL and getaddrinfo(3). Therefore, a deadlock can still occur.

The Linux manual page for fork(2) also imposes the following restriction:

After a fork() in a multithreaded program, the child can safely call only async-signal-safe functions (see signal-safety(7)) until such time as it calls execve(2).

Because PyMongo relies on functions that are not async-signal-safe, it can cause deadlocks or crashes when running in a child process.