Ruff (and flake8) doesn't report a situation where a variable _may_ be unbound

[DMRobertson]

Consider the following snippet:

from random import random


def coin_toss():
    return random.randint(0, 2)


def maybe_runtime_error():
    if coin_toss():
        x = []

    # runtime error if coin_toss returned False, but not detected by the linter
    x = list(x)
    print(x)


def runtime_error():
    # always a runtime error, detected by linter
    y = list(y)
    print(y)

There are two problems with this snippet.

1. In maybe_runtime_error, the name x may not be bound to a variable when we come to evaluate list(x). This means we might get a runtime UnboundLocalError.
2. In runtime_error, the name y is never bound to a variable when we come to evaluate list(y). We'll always get a runtime UnboundLocalError when calling this function.

Problem (2) is spotted by ruff, but problem (1) is not.

$ ruff --isolated temp.py
temp.py:19:14: F821 Undefined name `y`
Found 1 error.

I was a little surprised by this! But flake8 has the same output:

$ flake8 --version && flake8 temp.py
6.0.0 (mccabe: 0.7.0, pycodestyle: 2.10.0, pyflakes: 3.0.1) CPython 3.10.9 on Linux
temp.py:19:14: F821 undefined name 'y'

I'm not sure if it's fair game for ruff to spot the problem with x---perhaps that's more the remit of a typechecker like mypy. (Though mypy didn't warn about this either when I tried it!) It'd be great if it was feasible to spot this at lint-time.

Configuration

[tool.ruff]
line-length = 88

# See https://github.com/charliermarsh/ruff/#pycodestyle
# for error codes. The ones we ignore are:
#  E731: do not assign a lambda expression, use a def
#  E501: Line too long (black enforces this for us)
#
# flake8-bugbear compatible checks. Its error codes are described at
# https://github.com/charliermarsh/ruff/#flake8-bugbear
#  B019: Use of functools.lru_cache or functools.cache on methods can lead to memory leaks
#  B023: Functions defined inside a loop must not use variables redefined in the loop
#  B024: Abstract base class with no abstract method.
ignore = [
    "B019",
    "B023",
    "B024",
    "E501",
    "E731",
]
select = [
    # pycodestyle checks.
    "E",
    "W",
    # pyflakes checks.
    "F",
    # flake8-bugbear checks.
    "B0",
    # flake8-comprehensions checks.
    "C4",
]

And as for the version:

$ ruff --version
ruff 0.0.240

[charliermarsh]

Appreciate the clear issue! Right now, (1) is outside of our capabilities -- although I'm surprised that Mypy doesn't flag this... Pyright does:

❯ pyright foo.py
/Users/crmarsh/workspace/ruff/foo.py
  /Users/crmarsh/workspace/ruff/foo.py:13:14 - error: "x" is possibly unbound (reportUnboundVariable)
1 error, 0 warnings, 0 informations
Completed in 0.467sec

We could potentially include some kind of "possibly unbound" code for those cases. I'd have to think on it 🤔

[DMRobertson]

I'm surprised that Mypy doesn't flag this

And me! For cross-referencing, it seems like the relevant issues are

• References to names before they're bound should generate an error (UnboundLocalError) python/mypy#686
• Not detecting UnboundLocalError python/mypy#2400
• Proposal: use binder (or similar) to track which attributes are defined. python/mypy#4019

[charliermarsh]

I'm gonna close for now just because it's not super actionable. I'd like Ruff to be able to handle cases like this eventually, but it'd be a fairly big project and probably outside of the scope of a bug fix. Hope that intent is clear. I appreciate the issue!

[frankdilo]

just wanted to drop in and say that I would find this extremely useful

it's really easy in long Python functions and code blocks when refactoring and moving variables inside nested conditionals or inline functions to nest a variable used somewhere else and only discover at runtime via a Sentry notification days later

[sascharo]

Is this still unsupported?

[clemente0731]

STILL ~
Ruff cannot detect the problem of "referenced before assignment" and shows all passes. In an if-else scenario, for example, a variable is declared in the else block, but it is not declared in the if block. Then, this corresponding variable is used again in a subsequent inevitable path.

[Garrett-R]

Note: this is the subject of this issue: #6902

STILL ~

It sounds like a pretty tough one, but I'm sure Ruff maintainers would welcome a PR for it.