Issue number:

• Bug: BaseBatchProcessingError.child_exceptions doesn't handle None case #1203

Summary

Following code raises a runtime error:

from aws_lambda_powertools.utilities.batch.exceptions import BaseBatchProcessingError

print(BaseBatchProcessingError())  # would also raise an error

Changes

Please provide a summary of what's being changed

Change:

• Allow for child_exceptions being None in format_exceptions which is used when the error is printed.

diff --git a/aws_lambda_powertools/utilities/batch/exceptions.py b/aws_lambda_powertools/utilities/batch/exceptions.py
index d90c25f1..65afd918 100644
--- a/aws_lambda_powertools/utilities/batch/exceptions.py
+++ b/aws_lambda_powertools/utilities/batch/exceptions.py
@@ -16,7 +16,7 @@ class BaseBatchProcessingError(Exception):

     def format_exceptions(self, parent_exception_str):
         exception_list = [f"{parent_exception_str}\n"]
-        for exception in self.child_exceptions:
+        for exception in self.child_exceptions or []:
             extype, ex, tb = exception
             formatted = "".join(traceback.format_exception(extype, ex, tb))
             exception_list.append(formatted)
diff --git a/tests/functional/test_utilities_batch.py b/tests/functional/test_utilities_batch.py
index a5e1e706..757d2ddc 100644
--- a/tests/functional/test_utilities_batch.py
+++ b/tests/functional/test_utilities_batch.py
@@ -908,3 +908,7 @@ def test_batch_processor_error_when_entire_batch_fails(sqs_event_factory, record

     # THEN raise BatchProcessingError
     assert "All records failed processing. " in str(e.value)
+
+
+def test_child_exceptions_is_none():
+    assert str(SQSBatchProcessingError()) == "\n"

User experience

Please share what the user experience looks like before and after this change

Following code would not raise any errors

from aws_lambda_powertools.utilities.batch.exceptions import BaseBatchProcessingError

err = BaseBatchProcessingError(msg="something")
err.format_exceptions("parent exception string")
print(err)

Other solutions considered

• Changing the typing of child_exceptions is not possible as msg has a default value.
• Setting child_exceptions to () would generate a typing error with MyPy
• Setting child_exceptions to [] would make the default argument mutable
• Changing both msg and child_exceptions to be required would be a breaking change
• Setting self. child_exceptions is [] when None would also change the type of child_exceptions

Checklist

If your change doesn't seem to apply, please leave them unchecked.

• Meet tenets criteria
• I have performed a self-review of my this change
• Changes are tested
• Changes are documented
• PR title follows conventional commit semantics

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