DynamoDB UpdateItem in Python (boto3)

update_item changes specific attributes of one item (and creates it if absent), leaving everything else untouched. With the boto3 resource API you pass an UpdateExpression plus native values (Decimal for numbers).

Code

import boto3
from decimal import Decimal

dynamodb = boto3.resource("dynamodb", region_name="us-east-1")
table = dynamodb.Table("Music")

def update_song():
    response = table.update_item(
        Key={
            "Artist": "Arturo Sandoval",
            "SongTitle": "Cubano Chant",
        },
        UpdateExpression="SET Genre = :genre, #yr = :year ADD Awards :inc",
        ExpressionAttributeNames={"#yr": "Year"},
        ExpressionAttributeValues={
            ":genre": "Latin Jazz",
            ":year": Decimal("1994"),
            ":inc": Decimal("1"),
        },
        ReturnValues="ALL_NEW",
    )

    print(response["Attributes"])  # the item after the update
    return response["Attributes"]

if __name__ == "__main__":
    update_song()

Explanation

• Key — the full primary key of the item to update.
• UpdateExpression — one or more clauses:
  • SET assigns attributes (Genre = :genre). Year is reserved, so it's aliased to #yr.
  • ADD on a number is an atomic increment (ADD Awards :inc) — no read-modify-write race. REMOVE deletes an attribute; DELETE removes elements from a set.
• ExpressionAttributeValues — the :placeholder → value map. Numbers must be Decimal, not float.
• ReturnValues — "ALL_NEW" returns the full item after the update (also UPDATED_NEW, ALL_OLD, UPDATED_OLD, NONE).
• Upsert semantics — update_item creates the item if the key doesn't exist. Add a ConditionExpression (e.g. attribute_exists(Artist)) to update-only.

Do it visually

The DynamoDB Expression Builder builds SET / ADD / REMOVE clauses with the name/value maps and copies runnable code.

To edit items in a GUI — change a field, review the generated UpdateExpression, copy the boto3 code — download DynoTable.

Related guides

• DynamoDB update expressions — SET, ADD, REMOVE, DELETE, and idioms.
• Understanding ReturnValues — what each ReturnValues option gives you.