[pylint] Implement global-variable-undefined (W0601)

[tibor-reiss]

Add pylint rule global-variable-undefined (W0601)

See #970 for rules

Test Plan: cargo test

[github-actions]

ruff-ecosystem results

Linter (stable)

ℹ️ ecosystem check detected linter changes. (+9 -0 violations, +0 -0 fixes in 2 projects; 42 projects unchanged)

apache/airflow (+8 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --no-preview --select ALL

+ airflow/serialization/serialized_objects.py:1675:5: PLW0601 Global variable `HAS_KUBERNETES` is undefined at the module
+ airflow/settings.py:243:5: PLW0601 Global variable `Session` is undefined at the module
+ airflow/settings.py:244:5: PLW0601 Global variable `engine` is undefined at the module
+ airflow/settings.py:364:5: PLW0601 Global variable `engine` is undefined at the module
+ airflow/settings.py:365:5: PLW0601 Global variable `Session` is undefined at the module
+ airflow/settings.py:407:5: PLW0601 Global variable `engine` is undefined at the module
+ tests/executors/test_executor_loader.py:54:9: PLW0601 Global variable `ExecutorLoader` is undefined at the module
+ tests/listeners/class_listener.py:37:9: PLW0601 Global variable `stopped_component` is undefined at the module

milvus-io/pymilvus (+1 -0 violations, +0 -0 fixes)

+ _version_helper.py:92:9: PLW0601 Global variable `version` is undefined at the module

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
PLW0601	9	9	0	0	0

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+9 -0 violations, +0 -0 fixes in 2 projects; 42 projects unchanged)

apache/airflow (+8 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview --select ALL

+ airflow/serialization/serialized_objects.py:1675:5: PLW0601 Global variable `HAS_KUBERNETES` is undefined at the module
+ airflow/settings.py:243:5: PLW0601 Global variable `Session` is undefined at the module
+ airflow/settings.py:244:5: PLW0601 Global variable `engine` is undefined at the module
+ airflow/settings.py:364:5: PLW0601 Global variable `engine` is undefined at the module
+ airflow/settings.py:365:5: PLW0601 Global variable `Session` is undefined at the module
+ airflow/settings.py:407:5: PLW0601 Global variable `engine` is undefined at the module
+ tests/executors/test_executor_loader.py:54:9: PLW0601 Global variable `ExecutorLoader` is undefined at the module
+ tests/listeners/class_listener.py:37:9: PLW0601 Global variable `stopped_component` is undefined at the module

milvus-io/pymilvus (+1 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview

+ _version_helper.py:92:9: PLW0601 Global variable `version` is undefined at the module

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
PLW0601	9	9	0	0	0

[tusharsadhwani]

Does this break on wildcard imports?

from os import *

def foo():
    global system  # W0601 should not be raised
    system = lambda _: None

[tibor-reiss]

Does this break on wildcard imports?

from os import *

def foo():
    global system  # W0601 should not be raised
    system = lambda _: None

Yes, it does. I have not found a function in ruff which resolves star-imports, e.g. there is F403 and F405 which are raised for star-imports and warn about "...unable to detect..." and "...may be undefined...".

[MichaReiser]

I think there's a way to detect if there's a star import in which case you could avoid creating any diagnostics (to avoid false-positives).

The main open question to me are

• how this fits into ruff long term when Ruff catches more static analyzable errors (undefined variables in general)
• What's the motivation that this rule is specific to globals and doesn't apply to all variables.

[tibor-reiss]

@MichaReiser
Regarding star imports: when I do from os import *, pylint resolves this and e.g. has system in the namespace. Ruff is not able to do this (yet). For now, I simply left this out in this PR. What is possible though is to add something similar to F403/F405, i.e. if there is a start import, then raise "global variable undefined but maybe imported via star import". What's your preference?

[MichaReiser]

@tibor-reiss my preference would be that we don't flag any variables if a file contains a star import. I think there's already precedence for this in Ruff but I would need to search it to. @charliermarsh should know where to find it.