[flake8-pyi] Implement PYI059 (generic-not-last-base-class)

[tusharsadhwani]

Summary

Implements Y059 from flake8-pyi: typing.Generic[] should be the last base in a class' bases list.

If Multiple Generic[] classes are found, we don't generate a fix for it.

Test Plan

cargo test and cargo insta review.

[tusharsadhwani]

Is it okay to add the autofix in the same PR?

[zanieb]

Just wondering, what happens if a class has two Generic[] bases?

[zanieb]

Is it okay to add the autofix in the same PR?

Yep!

[tusharsadhwani]

what happens if a class has two Generic[] bases?

It's a runtime error:

>>> import typing
>>> T = typing.TypeVar('T')
>>> U = typing.TypeVar('U')
>>> class C(typing.Generic[T], typing.Generic[U]):
...   ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/homebrew/Cellar/python@3.12/3.12.2_1/Frameworks/Python.framework/Versions/3.12/lib/python3.12/typing.py", line 1102, in _generic_init_subclass
    raise TypeError(
TypeError: Cannot inherit from Generic[...] multiple times.

[zanieb]

Good to know. We should make sure the rule doesn't behave poorly on that case regardless, i.e. it could cause an infinite fix loop.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+2 -0 violations, +0 -0 fixes in 1 projects; 43 projects unchanged)

rotki/rotki (+2 -0 violations, +0 -0 fixes)

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

+ rotkehlchen/db/filtering.py:577:25: PYI059 [*] `Generic[]` should always be the last base class
+ rotkehlchen/types.py:977:34: PYI059 [*] `Generic[]` should always be the last base class

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
PYI059	2	2	0	0	0

[tusharsadhwani]

We should make sure the rule doesn't behave poorly on that case regardless

Should multiple Generic[] case not raise the issue at all then?

[AlexWaygood]

Nice, thank you! Overall this looks good. Left some inline comments below :-)

[AlexWaygood]

Should multiple Generic[] case not raise the issue at all then?

I'd say probably not. The behaviour we implemented at flake8-pyi is to ignore this rule if Generic[] appeared multiple times in the bases tuple, and I think it makes sense to do the same thing here. If you have something like class Foo(int, Generic[T], Generic[S]): pass, I think it's honestly just confusing for a user if they get a message about Generic[] "not being the last base class". There's a much more significant issue in their code (you can't have more than one Generic[] in the bases tuple), and if they solve the more significant issue, then Generic[] not being the last base class would be naturally solved as a side effect of that.

(We shouldn't flag multiple Generic[]s in this check; that could possibly be a separate check.)

[AlexWaygood]

what happens for pathological cases like

class Foo(  # comment about the bracket
    # Part 1 of multiline comment 1
    # Part 2 of multiline comment 1
    Generic[T]  # comment about Generic[T]
    # another comment?
    ,  # comment about the comma?
    # part 1 of multiline comment 2
    # part 2 of multiline comment 2
    int,  # comment about int
    # yet another comment?
):  # and another one for good measure
    pass

[tusharsadhwani]

It will technically work, but comments before int and after Generic[T] get eaten up. That is fixable, but the fix is subjective.

What's ruff's general stance in such cases? I know libcst has capabilities to try and identify which comment groups belong to which expression. But the easier thing would be to avoid moving comments at all, and just preserve comments.

[AlexWaygood]

Different rules take different stances; we don't have a unified policy yet. Several of our autofixes will currently happily delete your comments (e.g. #9779); I think we all agree that this is bad, but we haven't yet decided how bad it is (see #9790). Other rules approach comments with a perhaps-excessive level of caution, either using the raw token stream for their autofix (RUF022, RUF023), or using libcst. Both are much slower than simple text replacements, I think libcst especially.

the fix is subjective.

absolutely! But I think that's always the case for something like this. I don't mind that much if some comments end up in the "wrong place", but I do think we should try to avoid deleting any comments if it's not too hard or costly to do.

[tusharsadhwani]

Trying to figure out how to get the start and end ranges of the comma after the Generic base. Tried using libcst to obtain the Comma itself, but does it have position information?

[zanieb]

I would probably just read one character at a time after the end of the generic base until you see a comma — does that make sense here? I'd avoid LibCST unless it's absolutely necessary (it's slow).

[AlexWaygood]

I would probably just read one character at a time after the end of the generic base until you see a comma — does that make sense here? I'd avoid LibCST unless it's absolutely necessary (it's slow).

Agree -- memchr is probably the best tool for that (you can grep in the crates/ruff_linter directory for existing uses). Other possibilities are using libCST (but as Zanie says, it's really slow), or using the raw tokens (also can be slow, and probably more complicated here than using memchr).

If you look at https://play.ruff.rs/b2b834e8-0247-47fe-995a-c0ade0e6b240, with the following snippet:

class Foo(int, str): pass

On the AST tab on the right, we can see:

• The ExprName node for the int base has range 10..13
• The ExprName node for the str base has range 15..18

On the tokens tab on the right, we can see that:

• The comma in between the two nodes has range 13..14

[tusharsadhwani]

First try at this, I've used .find() (which I suppose can be swapped with memchr) and .position() (which I'm not sure if it can be replaced)

[AlexWaygood]

I don't really understand what's going on here, I'm afraid :(

Is this the best name for this variable? Would you mind adding some comments to this function to explain how it works (preferably with some examples)?

[tusharsadhwani]

I'm trying to find the first index in the source code after Generic[...],, which doesn't contain a whitespace.

Basically, deleting just Generic[T] and the comma after it would leave a trailing whitespace around (usually a space).

Without the whitespace detection, this code:

class Foo(Generic[T], Bar):

would be autofixed into:

class Foo( Bar, Generic[T]):

i.e. The space before Bar was not deleted.

[tusharsadhwani]

I'm honestly not sure if this is the best way to write this, please feel free to refactor or comment on this.

[tusharsadhwani]

I think deleting a positional argument from a function call during autofix is common enough that once we finalize on the way it should be done this can be lifted to make a helper instead.

[AlexWaygood]

Ah... in fact...

ruff/crates/ruff_linter/src/fix/edits.rs

Lines 155 to 237 in 12b5c3a

	/// Generic function to remove arguments or keyword arguments in function
	/// calls and class definitions. (For classes `args` should be considered
	/// `bases`)
	///
	/// Supports the removal of parentheses when this is the only (kw)arg left.
	/// For this behavior, set `remove_parentheses` to `true`.
	pub(crate) fn remove_argument<T: Ranged>(
	argument: &T,
	arguments: &Arguments,
	parentheses: Parentheses,
	source: &str,
	) -> Result<Edit> {
	// Partition into arguments before and after the argument to remove.
	let (before, after): (Vec<_>, Vec<_>) = arguments
	.arguments_source_order()
	.map(|arg| arg.range())
	.filter(|range| argument.range() != *range)
	.partition(|range| range.start() < argument.start());
	if !after.is_empty() {
	// Case 1: argument or keyword is _not_ the last node, so delete from the start of the
	// argument to the end of the subsequent comma.
	let mut tokenizer = SimpleTokenizer::starts_at(argument.end(), source);
	// Find the trailing comma.
	tokenizer
	.find(|token| token.kind == SimpleTokenKind::Comma)
	.context("Unable to find trailing comma")?;
	// Find the next non-whitespace token.
	let next = tokenizer
	.find(|token| {
	token.kind != SimpleTokenKind::Whitespace && token.kind != SimpleTokenKind::Newline
	})
	.context("Unable to find next token")?;
	Ok(Edit::deletion(argument.start(), next.start()))
	} else if let Some(previous) = before.iter().map(Ranged::end).max() {
	// Case 2: argument or keyword is the last node, so delete from the start of the
	// previous comma to the end of the argument.
	let mut tokenizer = SimpleTokenizer::starts_at(previous, source);
	// Find the trailing comma.
	let comma = tokenizer
	.find(|token| token.kind == SimpleTokenKind::Comma)
	.context("Unable to find trailing comma")?;
	Ok(Edit::deletion(comma.start(), argument.end()))
	} else {
	// Case 3: argument or keyword is the only node, so delete the arguments (but preserve
	// parentheses, if needed).
	Ok(match parentheses {
	Parentheses::Remove => Edit::range_deletion(arguments.range()),
	Parentheses::Preserve => Edit::range_replacement("()".to_string(), arguments.range()),
	})
	}
	}
	/// Generic function to add arguments or keyword arguments to function calls.
	pub(crate) fn add_argument(
	argument: &str,
	arguments: &Arguments,
	comment_ranges: &CommentRanges,
	source: &str,
	) -> Edit {
	if let Some(last) = arguments.arguments_source_order().last() {
	// Case 1: existing arguments, so append after the last argument.
	let last = parenthesized_range(
	match last {
	ArgOrKeyword::Arg(arg) => arg.into(),
	ArgOrKeyword::Keyword(keyword) => (&keyword.value).into(),
	},
	arguments.into(),
	comment_ranges,
	source,
	)
	.unwrap_or(last.range());
	Edit::insertion(format!(", {argument}"), last.end())
	} else {
	// Case 2: no arguments. Add argument, without any trailing comma.
	Edit::insertion(argument.to_string(), arguments.start() + TextSize::from(1))
	}
	}

[AlexWaygood]

I'm so sorry, I completely forgot that we had those helpers!

[tusharsadhwani]

I also didn't find them, so partly on me :P it uses the Tokenizer which is pretty clever

[tusharsadhwani]

I'll go ahead and refactor it.

[AlexWaygood]

(Everything looks great to me now except the autofix code, which I'm still not quite sure about -- both in terms of code readability and in terms of the number of .expect() and .unwrap() calls in it :-)

[AlexWaygood]

@tusharsadhwani, I've just pushed 6fce0fd, which gets rid of the remaining .expect() and .unwrap() calls from generate_fix(). Instead, the errors are bubbled up by returning an anyhow::Result<Fix> from generate_fix(). In combination with the Diagnostic::try_set_fix() method, that means that, rather than panicking, we'll gracefully log the error and move on if we fail to construct a Fix. That's usually preferable for this kind of thing, since it's not 100% essential that a Fix is constructed, but panicking would be pretty bad.

[tusharsadhwani]

Okay, this is much cleaner.

[AlexWaygood]

Thanks! :D