unused argument schema_editor in django data migrations

[lokesh1729]

in django when writing data migrations, the function definition would like this

def update_something(apps, schema_editor):
    # do something

here, apps and schema_editor will be passed by django when applying migration. There are cases where either of them will not be used. In such cases pylint-django should not complain unused-argument error.

EXPECTED : it is expected that to have a exception for this case

It happened many times that folks in our team removes schema_editor when pylint complains with unused-argument and that result into errors.

here's pip freeze

appdirs==1.4.4
appnope==0.1.0
argh==0.26.2
aspy.yaml==1.3.0
astroid==2.4.1
attrs==19.3.0
Babel==2.8.0
backcall==0.1.0
better-exceptions==0.2.2
billiard==3.6.3.0
bleach==3.1.5
celery==4.4.1
certifi==2020.4.5.1
cfgv==2.0.1
chardet==3.0.4
coreapi==2.3.3
coreschema==0.0.4
coverage==5.1
decorator==4.4.2
defusedxml==0.6.0
distlib==0.3.0
Django==2.1.5
django-extensions==2.1.6
django-redis-cache==2.1.0
django-rest-swagger==2.1.2
djangorestframework==3.9.3
entrypoints==0.3
Faker==2.0.3
filelock==3.0.12
flower==0.9.3
gevent==1.4.0
greenlet==0.4.15
gunicorn==20.0.4
identify==1.4.15
idna==2.8
importlib-metadata==1.6.0
importlib-resources==1.5.0
ipdb==0.13.2
ipykernel==5.2.1
ipython==7.9.0
ipython-genutils==0.2.0
ipywidgets==7.5.1
isort==4.3.21
itypes==1.2.0
jedi==0.17.0
Jinja2==2.10.3
jsonschema==3.2.0
jupyter==1.0.0
jupyter-client==6.1.3
jupyter-console==6.1.0
jupyter-core==4.6.3
kombu==4.6.8
lazy-object-proxy==1.4.3
lxml==4.5.0
MarkupSafe==1.1.1
mccabe==0.6.1
mistune==0.8.4
more-itertools==8.2.0
msgpack==1.0.0
nbconvert==5.6.1
nbformat==5.0.6
neovim==0.3.1
nodeenv==1.3.5
notebook==6.0.3
openapi-codec==1.3.2
packaging==20.3
pandocfilters==1.4.2
parso==0.7.0
pathlib2==2.3.5
pathtools==0.1.2
pexpect==4.8.0
pickleshare==0.7.5
Pillow==6.2.0
pluggy==0.13.1
pre-commit==1.21.0
prometheus-client==0.7.1
prompt-toolkit==2.0.10
psycopg2-binary==2.8.4
ptyprocess==0.6.0
py==1.8.1
Pygments==2.6.1
pylint==2.5.3
pylint-django==2.0.15
pylint-plugin-utils==0.6
pynvim==0.4.1
pyparsing==2.4.7
PyPDF2==1.26.0
pyrsistent==0.16.0
pytest==5.4.2
pytest-cov==2.6.1
pytest-django==3.4.5
pytest-ordering==0.6
python-dateutil==2.8.1
python-pptx==0.6.18
pytz==2020.1
PyYAML==5.3.1
pyzmq==19.0.1
qtconsole==4.7.3
QtPy==1.9.0
redis==3.4.1
reportlab==3.5.28
requests==2.22.0
Send2Trash==1.5.0
simplejson==3.17.0
six==1.14.0
terminado==0.8.3
testpath==0.4.4
text-unidecode==1.3
toml==0.10.0
tornado==5.1.1
traitlets==4.3.3
typed-ast==1.4.1
uritemplate==3.0.1
urllib3==1.25.9
vine==1.3.0
virtualenv==20.0.20
watchdog==0.9.0
wcwidth==0.1.9
webencodings==0.5.1
widgetsnbextension==3.5.1
wrapt==1.12.1
XlsxWriter==1.2.8
zipp==1.2.0

[carlio]

So if you have a function like that:

def update_something(apps, schema_editor):
    Person = apps.get_model('yourappname', 'Person')
    # don't use schema_editor

There are several possibilities here:

def update_something(apps, _schema_editor):
   ...

Pylint does not warn about arguments starting with an underscore. You can still see that it is a schema_editor but pylint won't issue the warning. I think this is cleaner than calling it _ which you could also do.

Secondly you could explicitly ignore the warning on the line:

def update_something(apps, schema_editor):  # pylint: disable=unused-argument
  ...

In this way you can fix your specific problem without needing pylint-django to have a new release.

[lokesh1729]

@carlio : thanks for looking into this.... i know about the second option to disable a specific line, but adding that to every migration would be painful, but the first option makes sense. I didn't know about that... hence closing the issue

[carlio]

@lokesh1729 no worries, check out this section of the documentation: http://pylint.pycqa.org/en/latest/technical_reference/features.html#variables-checker-options

By default _something and ignored_something and unused_something will not generate errors, but you can configure that value in your .pylintrc to whatever you want.

[atodorov]

Reopening b/c pyint_django should be smart enough to ignore these unused arguments in a similar way we do for view methods.

[atodorov]

Fixed via #273