Dynalite does not throws if path token starts with underscore

[exoego]

Summary

DynamoDB and DynamoDB-local throws ValidationException if attribute name inConditionExpression starts with _.
However, Dynalite does not throw and PutItem succeed.

Expectation

Dynalite should throw, so users can identity invalid chars and escape them using ExpressionAttributeNames.

Repro

DynamoDB Local

docker run -d -p 8000:8000 amazon/dynamodb-local 

aws dynamodb create-table \
    --endpoint http://localhost:8000 \
    --table-name MusicCollection \
    --attribute-definitions AttributeName=Artist,AttributeType=S AttributeName=SongTitle,AttributeType=S \
    --key-schema AttributeName=Artist,KeyType=HASH AttributeName=SongTitle,KeyType=RANGE \
    --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

aws dynamodb put-item --endpoint http://localhost:8000 --table-name MusicCollection \
  --item '{"Artist": {"S": "Foo"}, "SongTitle": {"S": "Bar"}, "__typename": {"S": "SongItem"}}' \
  --condition-expression "__typename <> :__typename" \
  --expression-attribute-values '{":__typename":{"S":"SongItem"}}'

This wil throwAn error occurred (ValidationException) when calling the PutItem operation: Invalid ConditionExpression: Syntax error; token: "_", near: "__".

Dynalite

docker run -d -p 8000:8000  dimaqq/dynalite:latest

aws dynamodb create-table \
    --endpoint http://localhost:8000 \
    --table-name MusicCollection \
    --attribute-definitions AttributeName=Artist,AttributeType=S AttributeName=SongTitle,AttributeType=S \
    --key-schema AttributeName=Artist,KeyType=HASH AttributeName=SongTitle,KeyType=RANGE \
    --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

aws dynamodb put-item --endpoint http://localhost:8000 --table-name MusicCollection \
  --item '{"Artist": {"S": "Foo"}, "SongTitle": {"S": "Bar"}, "__typename": {"S": "SongItem"}}' \
  --condition-expression "__typename <> :__typename" \
  --expression-attribute-values '{":__typename":{"S":"SongItem"}}'

This will succeed, but it should fail.

[ryanblock]

I haven't yet checked, but I would imagine this would also be the same for other supported special characters, namely - and . (per the Supported data types and naming rules doc). Have you looked into those yet, by chance?