Dynamodb - update item

import boto3

dynamodb = boto3.resource('dynamodb')
table = dynamodb.Table('table')
table.update_item(
    Key={
        'key1': 'value1',
        'key2': 'value2'
    },
    UpdateExpression='SET key3 = :val1',
    ExpressionAttributeValues={
        ':val1': newvalue
    }
)