From 744d6fd3cd72ca1edbc1f9ba43a4a86a317643aa Mon Sep 17 00:00:00 2001
From: Zsolt Dollenstein <zsol.zsol@gmail.com>
Date: Sat, 27 May 2023 20:21:24 +0100
Subject: [PATCH] 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        | 20 +++++++++++
 libcst/_nodes/tests/test_lambda.py | 56 +++++++++++++++++-------------
 2 files changed, 52 insertions(+), 24 deletions(-)

diff --git a/libcst/_nodes/expression.py b/libcst/_nodes/expression.py
index be0589bb4..a011e5f41 100644
--- a/libcst/_nodes/expression.py
+++ b/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"
diff --git a/libcst/_nodes/tests/test_lambda.py b/libcst/_nodes/tests/test_lambda.py
index f956ee03e..dbe5d51bf 100644
--- a/libcst/_nodes/tests/test_lambda.py
+++ b/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(