Asking bandit to use pyproject.toml without a bandit config section causes exception

[apirogov]

Describe the bug

Apparently bandit does not like it if asked to use pyproject.toml and not find a dedicated config section.

Reproduction steps

I use the following pre-commit hook to run bandit:

  - repo: https://github.com/PyCQA/bandit
    rev: '1.7.5'
    hooks:
    - id: bandit
      args: ['-q', '-c', 'pyproject.toml']
      additional_dependencies: [ '.[toml]' ]

with a pyproject.toml that has no configuration for bandit.

This results in:

bandit...................................................................Failed
- hook id: bandit
- exit code: 1

Traceback (most recent call last):
  File "/local/home/a.pirogov/.cache/pre-commit/repo_hijlu0c/py_env-python3/bin/bandit", line 8, in <module>
    sys.exit(main())
  File "/local/home/a.pirogov/.cache/pre-commit/repo_hijlu0c/py_env-python3/lib/python3.8/site-packages/bandit/cli/main.py", line 455, in main
    b_conf = b_config.BanditConfig(config_file=args.config_file)
  File "/local/home/a.pirogov/.cache/pre-commit/repo_hijlu0c/py_env-python3/lib/python3.8/site-packages/bandit/core/config.py", line 56, in __init__
    self._config = tomllib.load(f)["tool"]["bandit"]
KeyError: 'bandit'

Expected behavior

For most tools with some config files, if the config does not "override" behavior, it is using default values.

So I expect the same when running bandit with a pyproject.toml. A missing section in the toml to me feels the same as an empty config file.

Currently, I have to create an empty bandit config section in order to be able to keep the hook as it is (anticipating that IF I change the config, I will put it in the toml).

Bandit version

1.7.5 (Default)

Python version

3.8

Additional context

No response