Fix formatting of lambda star arguments (#6257)

## Summary
Previously, the ruff formatter was removing the star argument of
`lambda` expressions when formatting.

Given the following code snippet
```python
lambda *a: ()
lambda **b: ()
```
it would be formatted to
```python
lambda: ()
lambda: ()
```

We fix this by checking for the presence of `args`, `vararg` or `kwarg`
in the `lambda` expression, before we were only checking for the
presence of `args`.

Fixes #5894

## Test Plan

Add new tests cases.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/lambda.py

@@ -75,3 +75,19 @@ def f(
 
 # ...but we do preserve a trailing comma after the arguments
 a = lambda b,: 0
+
+lambda a,: 0
+lambda *args,: 0
+lambda **kwds,: 0
+lambda a, *args,: 0
+lambda a, **kwds,: 0
+lambda *args, b,: 0
+lambda *, b,: 0
+lambda *args, **kwds,: 0
+lambda a, *args, b,: 0
+lambda a, *, b,: 0
+lambda a, *args, **kwds,: 0
+lambda *args, b, **kwds,: 0
+lambda *, b, **kwds,: 0
+lambda a, *args, b, **kwds,: 0
+lambda a, *, b, **kwds,: 0

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/function.py

@@ -231,3 +231,21 @@ def f42(
     c,
 ):
     pass
+
+
+# Check trailing commas are permitted in funcdef argument list.
+def f(a, ): pass
+def f(*args, ): pass
+def f(**kwds, ): pass
+def f(a, *args, ): pass
+def f(a, **kwds, ): pass
+def f(*args, b, ): pass
+def f(*, b, ): pass
+def f(*args, **kwds, ): pass
+def f(a, *args, b, ): pass
+def f(a, *, b, ): pass
+def f(a, *args, **kwds, ): pass
+def f(*args, b, **kwds, ): pass
+def f(*, b, **kwds, ): pass
+def f(a, *args, b, **kwds, ): pass
+def f(a, *, b, **kwds, ): pass

crates/ruff_python_formatter/src/expression/expr_lambda.rs

@@ -22,7 +22,8 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
 
         write!(f, [text("lambda")])?;
 
-        if !parameters.args.is_empty() {
+        if !parameters.args.is_empty() || parameters.vararg.is_some() || parameters.kwarg.is_some()
+        {
             write!(
                 f,
                 [

crates/ruff_python_formatter/src/other/parameters.rs

@@ -278,9 +278,9 @@ pub(crate) struct ArgumentSeparator {
     pub(crate) following_start: TextSize,
 }
 
-/// Finds slash and star in `f(a, /, b, *, c)`
+/// Finds slash and star in `f(a, /, b, *, c)` or `lambda a, /, b, *, c: 1`.
 ///
-/// Returns slash and star
+/// Returns the location of the slash and star separators, if any.
 pub(crate) fn find_argument_separators(
     contents: &str,
     parameters: &Parameters,
@@ -347,14 +347,21 @@ pub(crate) fn find_argument_separators(
         } else {
             let mut tokens = SimpleTokenizer::new(contents, parameters.range).skip_trivia();
 
-            let lparen = tokens
+            let lparen_or_star = tokens
-                .next()
-                .expect("The function definition can't end here");
-            debug_assert!(lparen.kind() == SimpleTokenKind::LParen, "{lparen:?}");
-            let star = tokens
                 .next()
                 .expect("The function definition can't end here");
+
+            // In a function definition, the first token should always be a `(`; in a lambda
+            // definition, it _can't_ be a `(`.
+            let star = if lparen_or_star.kind == SimpleTokenKind::LParen {
+                tokens
+                    .next()
+                    .expect("The function definition can't end here")
+            } else {
+                lparen_or_star
+            };
             debug_assert!(star.kind() == SimpleTokenKind::Star, "{star:?}");
+
             Some(ArgumentSeparator {
                 preceding_end: parameters.range.start(),
                 separator: star.range,

crates/ruff_python_formatter/tests/snapshots/black_compatibility@simple_cases__power_op_spacing.py.snap

@@ -1,234 +0,0 @@
----
-source: crates/ruff_python_formatter/tests/fixtures.rs
-input_file: crates/ruff_python_formatter/resources/test/fixtures/black/simple_cases/power_op_spacing.py
----
-## Input
-
-```py
-def function(**kwargs):
-    t = a**2 + b**3
-    return t ** 2
-
-
-def function_replace_spaces(**kwargs):
-    t = a **2 + b** 3 + c ** 4
-
-
-def function_dont_replace_spaces():
-    {**a, **b, **c}
-
-
-a = 5**~4
-b = 5 ** f()
-c = -(5**2)
-d = 5 ** f["hi"]
-e = lazy(lambda **kwargs: 5)
-f = f() ** 5
-g = a.b**c.d
-h = 5 ** funcs.f()
-i = funcs.f() ** 5
-j = super().name ** 5
-k = [(2**idx, value) for idx, value in pairs]
-l = mod.weights_[0] == pytest.approx(0.95**100, abs=0.001)
-m = [([2**63], [1, 2**63])]
-n = count <= 10**5
-o = settings(max_examples=10**6)
-p = {(k, k**2): v**2 for k, v in pairs}
-q = [10**i for i in range(6)]
-r = x**y
-
-a = 5.0**~4.0
-b = 5.0 ** f()
-c = -(5.0**2.0)
-d = 5.0 ** f["hi"]
-e = lazy(lambda **kwargs: 5)
-f = f() ** 5.0
-g = a.b**c.d
-h = 5.0 ** funcs.f()
-i = funcs.f() ** 5.0
-j = super().name ** 5.0
-k = [(2.0**idx, value) for idx, value in pairs]
-l = mod.weights_[0] == pytest.approx(0.95**100, abs=0.001)
-m = [([2.0**63.0], [1.0, 2**63.0])]
-n = count <= 10**5.0
-o = settings(max_examples=10**6.0)
-p = {(k, k**2): v**2.0 for k, v in pairs}
-q = [10.5**i for i in range(6)]
-
-
-# WE SHOULD DEFINITELY NOT EAT THESE COMMENTS (https://github.com/psf/black/issues/2873)
-if hasattr(view, "sum_of_weights"):
-    return np.divide(  # type: ignore[no-any-return]
-        view.variance,  # type: ignore[union-attr]
-        view.sum_of_weights,  # type: ignore[union-attr]
-        out=np.full(view.sum_of_weights.shape, np.nan),  # type: ignore[union-attr]
-        where=view.sum_of_weights**2 > view.sum_of_weights_squared,  # type: ignore[union-attr]
-    )
-
-return np.divide(
-    where=view.sum_of_weights_of_weight_long**2 > view.sum_of_weights_squared,  # type: ignore
-)
-```
-
-## Black Differences
-
-```diff
---- Black
-+++ Ruff
-@@ -15,7 +15,7 @@
- b = 5 ** f()
- c = -(5**2)
- d = 5 ** f["hi"]
--e = lazy(lambda **kwargs: 5)
-+e = lazy(lambda: 5)
- f = f() ** 5
- g = a.b**c.d
- h = 5 ** funcs.f()
-@@ -34,7 +34,7 @@
- b = 5.0 ** f()
- c = -(5.0**2.0)
- d = 5.0 ** f["hi"]
--e = lazy(lambda **kwargs: 5)
-+e = lazy(lambda: 5)
- f = f() ** 5.0
- g = a.b**c.d
- h = 5.0 ** funcs.f()
-```
-
-## Ruff Output
-
-```py
-def function(**kwargs):
-    t = a**2 + b**3
-    return t**2
-
-
-def function_replace_spaces(**kwargs):
-    t = a**2 + b**3 + c**4
-
-
-def function_dont_replace_spaces():
-    {**a, **b, **c}
-
-
-a = 5**~4
-b = 5 ** f()
-c = -(5**2)
-d = 5 ** f["hi"]
-e = lazy(lambda: 5)
-f = f() ** 5
-g = a.b**c.d
-h = 5 ** funcs.f()
-i = funcs.f() ** 5
-j = super().name ** 5
-k = [(2**idx, value) for idx, value in pairs]
-l = mod.weights_[0] == pytest.approx(0.95**100, abs=0.001)
-m = [([2**63], [1, 2**63])]
-n = count <= 10**5
-o = settings(max_examples=10**6)
-p = {(k, k**2): v**2 for k, v in pairs}
-q = [10**i for i in range(6)]
-r = x**y
-
-a = 5.0**~4.0
-b = 5.0 ** f()
-c = -(5.0**2.0)
-d = 5.0 ** f["hi"]
-e = lazy(lambda: 5)
-f = f() ** 5.0
-g = a.b**c.d
-h = 5.0 ** funcs.f()
-i = funcs.f() ** 5.0
-j = super().name ** 5.0
-k = [(2.0**idx, value) for idx, value in pairs]
-l = mod.weights_[0] == pytest.approx(0.95**100, abs=0.001)
-m = [([2.0**63.0], [1.0, 2**63.0])]
-n = count <= 10**5.0
-o = settings(max_examples=10**6.0)
-p = {(k, k**2): v**2.0 for k, v in pairs}
-q = [10.5**i for i in range(6)]
-
-
-# WE SHOULD DEFINITELY NOT EAT THESE COMMENTS (https://github.com/psf/black/issues/2873)
-if hasattr(view, "sum_of_weights"):
-    return np.divide(  # type: ignore[no-any-return]
-        view.variance,  # type: ignore[union-attr]
-        view.sum_of_weights,  # type: ignore[union-attr]
-        out=np.full(view.sum_of_weights.shape, np.nan),  # type: ignore[union-attr]
-        where=view.sum_of_weights**2 > view.sum_of_weights_squared,  # type: ignore[union-attr]
-    )
-
-return np.divide(
-    where=view.sum_of_weights_of_weight_long**2 > view.sum_of_weights_squared,  # type: ignore
-)
-```
-
-## Black Output
-
-```py
-def function(**kwargs):
-    t = a**2 + b**3
-    return t**2
-
-
-def function_replace_spaces(**kwargs):
-    t = a**2 + b**3 + c**4
-
-
-def function_dont_replace_spaces():
-    {**a, **b, **c}
-
-
-a = 5**~4
-b = 5 ** f()
-c = -(5**2)
-d = 5 ** f["hi"]
-e = lazy(lambda **kwargs: 5)
-f = f() ** 5
-g = a.b**c.d
-h = 5 ** funcs.f()
-i = funcs.f() ** 5
-j = super().name ** 5
-k = [(2**idx, value) for idx, value in pairs]
-l = mod.weights_[0] == pytest.approx(0.95**100, abs=0.001)
-m = [([2**63], [1, 2**63])]
-n = count <= 10**5
-o = settings(max_examples=10**6)
-p = {(k, k**2): v**2 for k, v in pairs}
-q = [10**i for i in range(6)]
-r = x**y
-
-a = 5.0**~4.0
-b = 5.0 ** f()
-c = -(5.0**2.0)
-d = 5.0 ** f["hi"]
-e = lazy(lambda **kwargs: 5)
-f = f() ** 5.0
-g = a.b**c.d
-h = 5.0 ** funcs.f()
-i = funcs.f() ** 5.0
-j = super().name ** 5.0
-k = [(2.0**idx, value) for idx, value in pairs]
-l = mod.weights_[0] == pytest.approx(0.95**100, abs=0.001)
-m = [([2.0**63.0], [1.0, 2**63.0])]
-n = count <= 10**5.0
-o = settings(max_examples=10**6.0)
-p = {(k, k**2): v**2.0 for k, v in pairs}
-q = [10.5**i for i in range(6)]
-
-
-# WE SHOULD DEFINITELY NOT EAT THESE COMMENTS (https://github.com/psf/black/issues/2873)
-if hasattr(view, "sum_of_weights"):
-    return np.divide(  # type: ignore[no-any-return]
-        view.variance,  # type: ignore[union-attr]
-        view.sum_of_weights,  # type: ignore[union-attr]
-        out=np.full(view.sum_of_weights.shape, np.nan),  # type: ignore[union-attr]
-        where=view.sum_of_weights**2 > view.sum_of_weights_squared,  # type: ignore[union-attr]
-    )
-
-return np.divide(
-    where=view.sum_of_weights_of_weight_long**2 > view.sum_of_weights_squared,  # type: ignore
-)
-```
-
-

crates/ruff_python_formatter/tests/snapshots/format@expression__lambda.py.snap

@@ -81,6 +81,22 @@ def f(
 
 # ...but we do preserve a trailing comma after the arguments
 a = lambda b,: 0
+
+lambda a,: 0
+lambda *args,: 0
+lambda **kwds,: 0
+lambda a, *args,: 0
+lambda a, **kwds,: 0
+lambda *args, b,: 0
+lambda *, b,: 0
+lambda *args, **kwds,: 0
+lambda a, *args, b,: 0
+lambda a, *, b,: 0
+lambda a, *args, **kwds,: 0
+lambda *args, b, **kwds,: 0
+lambda *, b, **kwds,: 0
+lambda a, *args, b, **kwds,: 0
+lambda a, *, b, **kwds,: 0
 ```
 
 ## Output
@@ -162,6 +178,22 @@ def f(
 
 # ...but we do preserve a trailing comma after the arguments
 a = lambda b,: 0
+
+lambda a,: 0
+lambda *args,: 0
+lambda **kwds,: 0
+lambda a, *args,: 0
+lambda a, **kwds,: 0
+lambda *args, b,: 0
+lambda: 0
+lambda *args, **kwds,: 0
+lambda a, *args, b,: 0
+lambda a, *, b,: 0
+lambda a, *args, **kwds,: 0
+lambda *args, b, **kwds,: 0
+lambda *, b, **kwds,: 0
+lambda a, *args, b, **kwds,: 0
+lambda a, *, b, **kwds,: 0
 ```
 
 

crates/ruff_python_formatter/tests/snapshots/format@statement__function.py.snap

@@ -237,6 +237,24 @@ def f42(
     c,
 ):
     pass
+
+
+# Check trailing commas are permitted in funcdef argument list.
+def f(a, ): pass
+def f(*args, ): pass
+def f(**kwds, ): pass
+def f(a, *args, ): pass
+def f(a, **kwds, ): pass
+def f(*args, b, ): pass
+def f(*, b, ): pass
+def f(*args, **kwds, ): pass
+def f(a, *args, b, ): pass
+def f(a, *, b, ): pass
+def f(a, *args, **kwds, ): pass
+def f(*args, b, **kwds, ): pass
+def f(*, b, **kwds, ): pass
+def f(a, *args, b, **kwds, ): pass
+def f(a, *, b, **kwds, ): pass
 ```
 
 ## Output
@@ -513,6 +531,118 @@ def f42(
     c,
 ):
     pass
+
+
+# Check trailing commas are permitted in funcdef argument list.
+def f(
+    a,
+):
+    pass
+
+
+def f(
+    *args,
+):
+    pass
+
+
+def f(
+    **kwds,
+):
+    pass
+
+
+def f(
+    a,
+    *args,
+):
+    pass
+
+
+def f(
+    a,
+    **kwds,
+):
+    pass
+
+
+def f(
+    *args,
+    b,
+):
+    pass
+
+
+def f(
+    *,
+    b,
+):
+    pass
+
+
+def f(
+    *args,
+    **kwds,
+):
+    pass
+
+
+def f(
+    a,
+    *args,
+    b,
+):
+    pass
+
+
+def f(
+    a,
+    *,
+    b,
+):
+    pass
+
+
+def f(
+    a,
+    *args,
+    **kwds,
+):
+    pass
+
+
+def f(
+    *args,
+    b,
+    **kwds,
+):
+    pass
+
+
+def f(
+    *,
+    b,
+    **kwds,
+):
+    pass
+
+
+def f(
+    a,
+    *args,
+    b,
+    **kwds,
+):
+    pass
+
+
+def f(
+    a,
+    *,
+    b,
+    **kwds,
+):
+    pass
 ```