Example of update_item in dynamodb boto3

Question

Following the documentation, I\'m trying to create an update statement that will update or add if not exists only one attribute in a dynamodb table.

I\'m trying this

Answer 1

Details on dynamodb updates using boto3 seem incredibly sparse online, so I'm hoping these alternative solutions are useful.

get / put

import boto3

table = boto3.resource('dynamodb').Table('my_table')

# get item
response = table.get_item(Key={'pkey': 'asdf12345'})
item = response['Item']

# update
item['status'] = 'complete'

# put (idempotent)
table.put_item(Item=item)

actual update

import boto3

table = boto3.resource('dynamodb').Table('my_table')

table.update_item(
    Key={'pkey': 'asdf12345'},
    AttributeUpdates={
        'status': 'complete',
    },
)

Answer 2

The original code example:

response = table.update_item(
    Key={'ReleaseNumber': '1.0.179'},
    UpdateExpression='SET',
    ConditionExpression='Attr(\'ReleaseNumber\').eq(\'1.0.179\')',
    ExpressionAttributeNames={'attr1': 'val1'},
    ExpressionAttributeValues={'val1': 'false'}
)

Fixed:

response = table.update_item(
    Key={'ReleaseNumber': '1.0.179'},
    UpdateExpression='SET #attr1 = :val1',
    ConditionExpression=Attr('ReleaseNumber').eq('1.0.179'),
    ExpressionAttributeNames={'#attr1': 'val1'},
    ExpressionAttributeValues={':val1': 'false'}
)

In the marked answer it was also revealed that there is a Range Key so that should also be included in the Key. The update_item method must seek to the exact record to be updated, there's no batch updates, and you can't update a range of values filtered to a condition to get to a single record. The ConditionExpression is there to be useful to make updates idempotent; i.e. don't update the value if it is already that value. It's not like a sql where clause.

Regarding the specific error seen.

ExpressionAttributeNames is a list of key placeholders for use in the UpdateExpression, useful if the key is a reserved word.

From the docs, "An expression attribute name must begin with a #, and be followed by one or more alphanumeric characters". The error is because the code hasn't used an ExpressionAttributeName that starts with a # and also not used it in the UpdateExpression.

ExpressionAttributeValues are placeholders for the values you want to update to, and they must start with :

Answer 3

If you don't want to check parameter by parameter for the update I wrote a cool function that would return the needed parameters to perform a update_item method using boto3.

def get_update_params(body):
    """Given a dictionary we generate an update expression and a dict of values
    to update a dynamodb table.

    Params:
        body (dict): Parameters to use for formatting.

    Returns:
        update expression, dict of values.
    """
    update_expression = ["set "]
    update_values = dict()

    for key, val in body.items():
        update_expression.append(f" {key} = :{key},")
        update_values[f":{key}"] = val

    return "".join(update_expression)[:-1], update_values

Here is a quick example:

def update(body):
    a, v = get_update_params(body)
    response = table.update_item(
        Key={'uuid':str(uuid)},
        UpdateExpression=a,
        ExpressionAttributeValues=dict(v)
        )
    return response

Answer 4

Based on the official example, here's a simple and complete solution which could be used to manually update (not something I would recommend) a table used by a terraform S3 backend.

Let's say this is the table data as shown by the AWS CLI:

$ aws dynamodb scan --table-name terraform_lock --region us-east-1
{
    "Items": [
        {
            "Digest": {
                "S": "2f58b12ae16dfb5b037560a217ebd752"
            },
            "LockID": {
                "S": "tf-aws.tfstate-md5"
            }
        }
    ],
    "Count": 1,
    "ScannedCount": 1,
    "ConsumedCapacity": null
}

You could update it to a new digest (say you rolled back the state) as follows:

import boto3

dynamodb = boto3.resource('dynamodb', 'us-east-1')


try:
    table = dynamodb.Table('terraform_lock')
    response = table.update_item(
        Key={
            "LockID": "tf-aws.tfstate-md5"
        },
        UpdateExpression="set Digest=:newDigest",
        ExpressionAttributeValues={
            ":newDigest": "50a488ee9bac09a50340c02b33beb24b"
        },
        ReturnValues="UPDATED_NEW"
    )
except Exception as msg:
    print(f"Oops, could not update: {msg}")

Note the : at the start of ":newDigest": "50a488ee9bac09a50340c02b33beb24b" they're easy to miss or forget.

Answer 5

Found working example here, very important to list as Keys all the indexes of the table, this will require additional query before update, but it works.

response = table.update_item(
    Key={
        'ReleaseNumber': releaseNumber,
        'Timestamp': result[0]['Timestamp']
    },
    UpdateExpression="set Sanity = :r",
    ExpressionAttributeValues={
        ':r': 'false',
    },
    ReturnValues="UPDATED_NEW"
)