Allow no whitespace between lambda keyword and params in certain cases

Python accepts code where `lambda` follows a `*`, so this PR relaxes validation rules for Lambdas.

Raised in #930.

libcst/_nodes/expression.py

@@ -1983,6 +1983,25 @@ def _visit_and_replace_children(self, visitor: CSTVisitorT) -> "Parameters":
  star_kwarg=visit_optional(self, "star_kwarg", self.star_kwarg, visitor),
  )
 
+ def _safe_to_join_with_lambda(self) -> bool:
+ """
+ Determine if Parameters need a space after the `lambda` keyword. Returns True
+ iff it's safe to omit the space between `lambda` and these Parameters.
+
+ See also `BaseExpression._safe_to_use_with_word_operator`.
+
+ For example: `lambda*_: pass`
+ """
+ if len(self.posonly_params) != 0:
+ return False
+
+ # posonly_ind can't appear if above condition is false
+
+ if len(self.params) > 0 and self.params[0].star not in {"*", "**"}:
+ return False
+
+ return True
+
  def _codegen_impl(self, state: CodegenState) -> None: # noqa: C901
  # Compute the star existence first so we can ask about whether
  # each element is the last in the list or not.
@@ -2115,6 +2134,7 @@ def _validate(self) -> None:
  if (
  isinstance(whitespace_after_lambda, BaseParenthesizableWhitespace)
  and whitespace_after_lambda.empty
+ and not self.params._safe_to_join_with_lambda()
  ):
  raise CSTValidationError(
  "Must have at least one space after lambda when specifying params"

libcst/_nodes/tests/test_lambda.py

@@ -303,30 +303,6 @@ def test_valid(
  ),
  "at least one space after lambda",
  ),
- (
- lambda: cst.Lambda(
- cst.Parameters(star_arg=cst.Param(cst.Name("arg"))),
- cst.Integer("5"),
- whitespace_after_lambda=cst.SimpleWhitespace(""),
- ),
- "at least one space after lambda",
- ),
- (
- lambda: cst.Lambda(
- cst.Parameters(kwonly_params=(cst.Param(cst.Name("arg")),)),
- cst.Integer("5"),
- whitespace_after_lambda=cst.SimpleWhitespace(""),
- ),
- "at least one space after lambda",
- ),
- (
- lambda: cst.Lambda(
- cst.Parameters(star_kwarg=cst.Param(cst.Name("arg"))),
- cst.Integer("5"),
- whitespace_after_lambda=cst.SimpleWhitespace(""),
- ),
- "at least one space after lambda",
- ),
  (
  lambda: cst.Lambda(
  cst.Parameters(
@@ -944,6 +920,38 @@ class LambdaParserTest(CSTNodeTest):
  ),
  "( lambda : 5 )",
  ),
+ # No space between lambda and params
+ (
+ cst.Lambda(
+ cst.Parameters(star_arg=cst.Param(cst.Name("args"), star="*")),
+ cst.Integer("5"),
+ whitespace_after_lambda=cst.SimpleWhitespace(""),
+ ),
+ "lambda*args: 5",
+ ),
+ (
+ cst.Lambda(
+ cst.Parameters(star_kwarg=cst.Param(cst.Name("kwargs"), star="**")),
+ cst.Integer("5"),
+ whitespace_after_lambda=cst.SimpleWhitespace(""),
+ ),
+ "lambda**kwargs: 5",
+ ),
+ (
+ cst.Lambda(
+ cst.Parameters(
+ star_arg=cst.ParamStar(
+ comma=cst.Comma(
+ cst.SimpleWhitespace(""), cst.SimpleWhitespace("")
+ )
+ ),
+ kwonly_params=[cst.Param(cst.Name("args"), star="")],
+ ),
+ cst.Integer("5"),
+ whitespace_after_lambda=cst.SimpleWhitespace(""),
+ ),
+ "lambda*,args: 5",
+ ),
  )
  )
  def test_valid(