Introduction and Scope

This page documents the use case of handling floating point arithmetic with at least 12 decimal places in MongoDb using Python. This is a special case, because the default datatype for floating point numbers in Python is the float datatype. Arithmetic operations with high precision floats i.e. 12 or more decimal places yields incorrect results when using the Python float data type.

Storing High Precision Floats in MongoDb

High precision floats should be stored using the MongoDb Decimal128 data type. This is a standard datatype for MongoDb.

Reading and Writing High Precision Floats to MongoDb using Python

The code below shows:

• How to create a MongoDb document with a Decmial128 value
• How to retrieve that value from MongoDb
• How to perform addition with that value in Python
• How to write the result of the addition back to MongoDb

from pymongo import MongoClient
from bson.decimal128 import Decimal128

# MongoDB server hostname
mongodb_server = "kermit.osoyalce.com"
# Default MongoDb port number
mongodb_port = 27017
# Example MongoDb database name
mongodb_db_name = "my_db"
# Example MongoDb collection name
mongodb_collection_name = "my_col"

# Open a connection to MongoDb
db_client = MongoClient(f"mongodb://{mongodb_server}:{mongodb_port}/")
# Get the database handle
mongo_db = db_client[mongo_db_name]
# Get the collection handle
mongo_collection = mongo_db[mongodb_collection_name]

# High precision floating point number as a string datatype
string_num = "0.123456789012"
# High precision floating point number as a Decimal128 datatype
decimal128_num = Decimal128(string_num)

# A MongoDb document in BSON format
some_doc = {
  'some_name': 'high_precision_example_doc',
  'some_num': decimal128_num
}

### Insert some_doc into MongoDb
mongo_collection.insert_one(some_document)

### Retrieve the value from MongoDb
cur_doc = mining_col.find_one({'some_name': 'high_precision_example_doc'})
cur_value = cur_doc['some_num']

### Do high precision floating point math without rounding errors

# We will add "0.000000000001" to the current value
# NOTE: Decimal128 does not support the addition (+) operator
# NOTE: MongodDb does not support the Python decmial datatype

# We need to convert Decimal128 datatype to the decimal datatype
# Then do the addition, then convert it back to Decimal128
new_num = Decimal128(cur_value.to_decimal() + Decimal128("0.000000000001").to_decimal())

### Update the value
# Query to retrieve the newly created document
my_query = {'some_name': 'high_precision_example_doc'}
# Specify which key/value pair to update
new_value = { "$set": { 'some_num': new_num }}

# Update the number in MongoDb
mongo_collection.update_one(my_query, new_value)