Lambda python boto3 update_item series error

Question

**Issue**
Lambda Python Issues executing an update_item.

**Code**
responseSeQ = table.update_item(	//this is line79
        Key={ 'ID': '1', 'Highest' : '1' },
        UpdateExpression='Set LargestSeQNo = SeqNo',
        ExpressionAttributeValues={ 'LargestSeQNo' : SeqNo }
    )

The table consists of:
- ID		// the PK
- Highest	// Sortkey
- LargestSeQNo	// String

**Objective**
LargestSeQNo only ever has numbers in it starting with 1.  The intent is to read this with get_item (works) then increment the number (works) then change the number back to a string and update the attribute in the table.  The variable SeqNo is that string variable in Python.

**Error**
  "errorMessage": "An error occurred (ValidationException) when calling the UpdateItem operation: ExpressionAttributeValues contains invalid key: Syntax error; key: "LargestSeQNo"",
  "errorType": "ClientError",
  "requestId": "be7af4e0-040d-4387-911d-bc6a3be9e47b",
  "stackTrace": [
   "  File "/var/task/lambda_function.py", line 79, in lambda_handler
    responseSeQ = table.update_item(
",

Syntax error; key: "LargestSeQNo" - In the code, LargestSeQNo is addressed as an attribute, not a key - ExpressionAttributeValues={ 'LargestSeQNo' : SeqNo }
SeqNo - the python variable, a string, containing the that to update in the DynamoDB table, attribute LargestSeQNo, a string

Help, please!

Accepted Answer

For `ExpressionAttributeNames`, map a placeholder to the actual attribute name, e.g., `{'#lsq': 'LargestSeQNo'}`, and for `ExpressionAttributeValues`, map a placeholder to the new value, e.g., `{':val1': SeqNo}`.

> ℹ️ **`ExpressionAttributeNames`** and **`ExpressionAttributeValues`** are dictionaries not string.

> ℹ️ Use the **`#`** character in an expression to dereference an attribute name.

> ℹ️ Use the **`: (colon)`** character in an expression to dereference an attribute value.

---

> 🔗 For more information about **`ExpressionAttributeNames`** and **`ExpressionAttributeValues`**  check [update_item in Boto3](https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/dynamodb/client/update_item.html).

Answer

First, the error message in your comment `"'str' object has no attribute 'update'"` tells me that you're calling a method `update` but I don't see that in your code anywhere. I can see `table.update_item` but that's not the same thing. Have you deployed your Lambda before you are testing it?

Second, to the error message in the original question: `ExpressionAttributeValues contains invalid key` you might try this instead:
```
responseSeQ = table.update_item( //line 79
    Key={ 'ID': '1', 'Highest' : '1' },
    ExpressionAttributeNames={'#LargestSequenceNumber':'LargestSeQNo'},
    UpdateExpression='set #LargestSequenceNumber = :SequenceNumber',
    ExpressionAttributeValues={ ':SequenceNumber':SeqNo }
)
```

Note that you can also (sometimes) go simpler depending on whether your attributes are DynamoDB keywords or not:
```
responseSeQ = table.update_item( //line 79
    Key={ 'ID': '1', 'Highest' : '1' },
    AttributeUpdate={'LargestSeQNo':{''Value':SeqNo'}}
)
```

Lambda python boto3 update_item series error

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