Docs Home → Develop Applications → Python Drivers → PyMongo
Unique Indexes
Overview
Unique indexes ensure that the indexed fields do not store duplicate values. By
default, MongoDB creates a unique index on the _id field during the creation
of a collection. To create a unique index, perform the following steps:
Specify the field or combination of fields that you want to prevent duplication on.
Set the
uniqueoption to``True``.
Sample Data
The examples in this guide use the sample_mflix.theaters collection
from the Atlas sample datasets. To learn how to create a
free MongoDB Atlas cluster and load the sample datasets, see the
Get Started with PyMongo.
Create a Unique Index
The following example creates a descending unique index on the theaterId field:
theaters.create_index("theaterId", unique=True)
For more information, see the Unique Indexes guide in the MongoDB Server manual.
Troubleshooting
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