MongoDB + Python, unexpected numeric types

Question

I have the following Python code:

from pymongo import MongoClient
db = MongoClient("mongodb://localhost/")["admin"]
collection = db["collection_test"]
collection.insert_one({"key1": 1000000000.1, "key2": 1.1})
doc = collection.find_one()
print(type(doc["key1"]))
print(type(doc["key2"]))

Which prints:

<class 'float'>
<class 'float'>

But based on the docs I expected the types to be

<class 'bson.decimal128.Decimal128'>

followed by something like 'bson.double'.

Why aren't I seeing a BSON Decimal128 and a BSON Double? Is type() not the correct way to get the types in this context?

Belly Buster · Accepted Answer · 2021-03-30 23:13:07Z

1

Python floats are stored as BSON doubles. The API converts them on insertion and converts them back when querying.

If you want to store and retrieve BSON Decimal128 values, use bson.decimal128.Decimal128()

from bson.decimal128 import Decimal128

db = MongoClient()["mydatabase"]
collection = db["collection_test"]

collection.insert_one({"key1": Decimal128("1000000000.1"), "key2": Decimal128("1.1")})
doc = collection.find_one()
print(type(doc["key1"]))
print(type(doc["key2"]))

prints:

<class 'bson.decimal128.Decimal128'>
<class 'bson.decimal128.Decimal128'>

Documentation (albeit somewhat confusing): https://pymongo.readthedocs.io/en/stable/api/bson/index.html

Also: be careful of using the admin database; it's a special database in MongoDB.

edited Mar 30, 2021 at 23:13

answered Mar 30, 2021 at 22:58

Belly Buster

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