Handling Errors

You Will Learn:

• How errors are raised before a request is submitted

• How errors are reported for individual samples after submission

• How to check whether a result is valid before accessing it

Errors can occur at several stages of a ServiceX request: before submission, during transformation, or when retrieving results.

Errors Before Submission

Errors detected before submission raise an exception immediately and nothing is sent to the backend:

• If the request cannot be parsed, a ValidationError is raised.

• If the request contains invalid values, an appropriate exception (ValueError, RuntimeError, TypeError, or NameError) is raised.

• If the authentication information in the .servicex or servicex.yaml file is incorrect, an AuthorizationError is raised.

Errors During Transformation

Sometimes errors occur during the transformation on the backend. These errors occur when the python code cannot check for them, for example, trying to open a container in an xAOD that is not there. These errors are a little tricky to find the full log. Here are the steps to find the log:

Step 1:

Step 2:

Step 3:

Errors After Submission

When deliver() returns, each sample’s result is a GuardList. A GuardList behaves like a normal list when the sample succeeded, but raises a ReturnValueException if accessed after a failure. This allows successful results to be accessed while failed ones are flagged.

The validity of a result can be checked before accessing it:

results = deliver(spec)
for name, result in results.items():
    if result.valid():
        # process result
    else:
        # handle failure

Common causes of post-submission errors include canceled or failed transformations, incomplete file processing, and download failures (such as insufficient disk space or connection problems). In all cases, failed results are not cached — resubmitting will send a new transformation request.