Poor error reporting for bases earlier than 24
Metadata
Current evaluation
Snapcraft 8.x crashes with a raw traceback (TypeError: 'NoneType' object is not iterable) instead of a friendly error when an empty/None grammar field (e.g. build-packages:) is used on core22 and earlier bases; core24 reports a proper error. Triaged as an enhancement, internal ticket SNAPCRAFT-1186 created, no fix yet.
Suggested action: keep open
Reason: A maintainer (@mr-cal) explicitly triaged this as a real UX bug ('this is not a good user experience... Snapcraft should be more robust when processing grammar') and it was synced to internal ticket SNAPCRAFT-1186, so it is acknowledged and valid. The root cause is still present in the code: snapcraft/parts/grammar.py passes unprocessed_grammar (which is None for an empty key) straight into craft_grammar's GrammarProcessor.process, which does `for section in grammar` and raises TypeError. It is not stale-irrelevant since core22 is still supported and the code path is unchanged.
Impact:
45
Quick Win:
27.0
Staleness:
55
Complexity:
40
Confidence:
70
Support Request:
10
Issue body
### Bug Description
If something horrifically unexpected happens in a YAML snapcraft will throw a traceback instead of reporting a proper error.
Specifically, snapcraft will usefully tell me that I have some bad snapcraft.yaml content and what was expected when building `base: core24` snaps. However, a traceback is thrown instead of I use `base: core22` snaps.
### To Reproduce
Install snapcraft 8.x (using 8.10.y myself) and:
```
mkdir t && cd t; snapcraft init
echo 'build-packages:' >> snap/snapcraft.yaml
snapcraft
sed -i 's/core24/core22/' snap/snapcraft.yaml
snapcraft
```
### Environment
Ubuntu 25.04, snapcraft 8.x, building with LXD.
### snapcraft.yaml
```yaml
name: test # you probably want to 'snapcraft register <name>'
base: core22 # the base snap is the execution environment for this snap
version: '0.1' # just for humans, typically '1.2+git' or '1.3.2'
summary: Single-line elevator pitch for your amazing snap # 79 char long summary
description: |
This is my-snap's description. You have a paragraph or two to tell the
most important story about your snap. Keep it under 100 words though,
we live in tweetspace and your description wants to look good in the snap
store.
grade: devel # must be 'stable' to release into candidate/stable channels
confinement: devmode # use 'strict' once you have the right plugs and slots
parts:
my-part:
# See 'snapcraft plugins'
plugin: nil
build-packages:
```
### Relevant log output
```shell
dilyn@Ares:~/test -> sc
Starting snapcraft, version 8.10.0.post45
Logging execution to '/home/dilyn/.local/state/snapcraft/log/snapcraft-20250714-150700.168675.log'
Bad snapcraft.yaml content:
- input should be a valid list (in field 'parts.snapcraft/core.build-packages')
dilyn@Ares:~/test -> hx snap/snapcraft.yaml
dilyn@Ares:~/test -> sc
Starting snapcraft, version 8.10.0.post45
Logging execution to '/home/dilyn/.local/state/snapcraft/log/snapcraft-20250714-150707.652601.log'
Running on amd64 for amd64Traceback (most recent call last):
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 467, in main
return app.run()
^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/craft_application/application.py", line 615, in run
return_code = self._run_inner()
^^^^^^^^^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 178, in _run_inner
return_code = super()._run_inner()
^^^^^^^^^^^^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/craft_application/application.py", line 561, in _run_inner
dispatcher = self._get_dispatcher()
^^^^^^^^^^^^^^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 405, in _get_dispatcher
self._check_for_classic_fallback()
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 317, in _check_for_classic_fallback
raise errors.ClassicFallback()
snapcraft.errors.ClassicFallback
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/snap/snapcraft/x19/bin/snapcraft", line 8, in <module>
sys.exit(main())
^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 470, in main
return cli.run()
^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/cli.py", line 256, in run
_run_dispatcher(dispatcher, global_args)
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/cli.py", line 229, in _run_dispatcher
dispatcher.run()
File "/snap/snapcraft/x19/lib/python3.12/site-packages/craft_cli/dispatcher.py", line 528, in run
return self._loaded_command.run(self._parsed_command_args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/commands/core22/lifecycle.py", line 272, in run
super().run(parsed_args)
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/commands/core22/lifecycle.py", line 145, in run
parts_lifecycle.run(self.name, parsed_args)
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/parts/lifecycle.py", line 95, in run
yaml_data_for_arch = yaml_utils.apply_yaml(yaml_data, build_on, build_for)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/parts/yaml_utils.py", line 203, in apply_yaml
yaml_data["parts"] = grammar.process_parts(
^^^^^^^^^^^^^^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/parts/grammar.py", line 77, in process_parts
process_part(part_yaml_data=part, processor=processor)
File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/parts/grammar.py", line 47, in process_part
processed_grammar: list[Any] | None = processor.process(
^^^^^^^^^^^^^^^^^^
File "/snap/snapcraft/x19/lib/python3.12/site-packages/craft_grammar/_processor.py", line 89, in process
for section in grammar:
TypeError: 'NoneType' object is not iterable
```
### Additional context
_No response_
Evaluation history
| Date | Model | Scores | Action | Summary |
|---|---|---|---|---|
| qwen/qwen3.8-27b |
Impact:
45
Quick Win:
27.0
Staleness:
55
Complexity:
40
Confidence:
70
Support Request:
10
|
keep open | Snapcraft 8.x crashes with a raw traceback (TypeError: 'NoneType' object is not iterable) instead of a friendly error when an empty/None grammar field (e.g. build-packages:) is used on core22 and earlier bases; core24 reports a proper error. Triaged as an enhancement, internal ticket SNAPCRAFT-1186 created, no fix yet. | |
| qwen/qwen3.6-35b-a3b |
Staleness:
85
Complexity:
45
Confidence:
75
Support Request:
10
|
keep open | Snapcraft throws a traceback instead of a clear error when using base: core22 with malformed YAML. Triaged as enhancement, acknowledged by maintainer, but inactive for ~1 year. | |
| qwen3.6-35b-a3b-mtp-q6 |
Staleness:
85
Complexity:
30
Confidence:
85
Support Request:
0
|
keep open | Snapcraft throws a traceback instead of a clear error when using base: core22 with malformed YAML. Triaged and acknowledged by a maintainer, but inactive for over a year. | |
| qwen3.6-35b-a3b-mtp-q6 |
Staleness:
60
Complexity:
35
Confidence:
85
Support Request:
5
|
keep open | Snapcraft throws a traceback instead of a clear error message when parsing malformed YAML with bases older than core24. Triaged and acknowledged as a UX enhancement, awaiting implementation to improve grammar validation error reporting. |
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