Don't increase split penalty on 'lambda' keyword

If a lambda expression is an argument to a function (common), then we
should be able to split before it without hinderance.

CHANGELOG

@@ -13,6 +13,9 @@
 ### Fixed
 - `BLANK_LINE_BEFORE_NESTED_CLASS_OR_DEF=False` wasn't honored because the
   number of newlines was erroneously calculated beforehand.
+- Lambda expressions shouldn't have an increased split penalty applied to the
+  'lambda' keyword. This prevents them from being properly formatted when they're
+  arguments to functions.
 
 ## [0.27.0] 2019-04-07
 ### Added

yapf/yapflib/format_decision_state.py

@@ -950,7 +950,7 @@ def _ContainerFitsOnStartLine(self, opening):
 
     Arguments:
       opening: (FormatToken) The unwrapped line we're currently processing.
-      
+
     Returns:
       True if the container fits on the start line.
     """

yapf/yapflib/split_penalty.py

@@ -129,9 +129,9 @@ def Visit_lambdef(self, node):  # pylint: disable=invalid-name
             break
 
     if allow_multiline_lambdas:
-      _SetStronglyConnected(node)
+      _SetExpressionPenalty(node, STRONGLY_CONNECTED)
     else:
-      _SetVeryStronglyConnected(node)
+      _SetExpressionPenalty(node, VERY_STRONGLY_CONNECTED)
 
   def Visit_parameters(self, node):  # pylint: disable=invalid-name
     # parameters ::= '(' [typedargslist] ')'

yapftests/reformatter_buganizer_test.py

@@ -28,6 +28,17 @@ class BuganizerFixes(yapf_test_helper.YAPFTest):
   def setUpClass(cls):
     style.SetGlobalStyle(style.CreateChromiumStyle())
 
+  def testB26521719(self):
+    code = """\
+class _():
+
+  def _(self):
+    self.stubs.Set(some_type_of_arg, 'ThisIsAStringArgument',
+                   lambda *unused_args, **unused_kwargs: fake_resolver)
+"""
+    uwlines = yapf_test_helper.ParseAndUnwrap(code)
+    self.assertCodeEqual(code, reformatter.Reformat(uwlines))
+
   def testB122541552(self):
     code = """\
 # pylint: disable=g-explicit-bool-comparison,singleton-comparison

yapftests/split_penalty_test.py

@@ -139,7 +139,7 @@ class B(A):
       """)
     tree = self._ParseAndComputePenalties(code)
     self._CheckPenalties(tree, [
-        ('lambda', VERY_STRONGLY_CONNECTED),
+        ('lambda', None),
         ('a', VERY_STRONGLY_CONNECTED),
         (',', VERY_STRONGLY_CONNECTED),
         ('b', VERY_STRONGLY_CONNECTED),
@@ -180,7 +180,7 @@ def testStronglyConnected(self):
         (',', None),
         ('y', None),
         ('(', UNBREAKABLE),
-        ('lambda', VERY_STRONGLY_CONNECTED),
+        ('lambda', STRONGLY_CONNECTED),
         ('a', VERY_STRONGLY_CONNECTED),
         (':', VERY_STRONGLY_CONNECTED),
         ('23', VERY_STRONGLY_CONNECTED),

yapftests/subtype_assigner_test.py

@@ -159,7 +159,7 @@ def testBitwiseOperators(self):
          ('3', [format_token.Subtype.NONE]),
          (')', [format_token.Subtype.NONE]),
          ('>>', {format_token.Subtype.BINARY_OPERATOR}),
-         ('1', [format_token.Subtype.NONE]), ],
+         ('1', [format_token.Subtype.NONE]),],
     ])  # yapf: disable
 
   def testArithmeticOperators(self):
@@ -201,7 +201,7 @@ def testArithmeticOperators(self):
          (')', [format_token.Subtype.NONE]),
          ('//', {format_token.Subtype.BINARY_OPERATOR,
                  format_token.Subtype.M_EXPR_OPERATOR}),
-         ('1', [format_token.Subtype.NONE]), ],
+         ('1', [format_token.Subtype.NONE]),],
     ])  # yapf: disable
 
   def testSubscriptColon(self):
@@ -234,7 +234,7 @@ def testFunctionCallWithStarExpression(self):
          ('*', {format_token.Subtype.UNARY_OPERATOR,
                 format_token.Subtype.VARARGS_STAR}),
          ('b', [format_token.Subtype.NONE]),
-         (']', [format_token.Subtype.NONE]), ],
+         (']', [format_token.Subtype.NONE]),],
     ])  # yapf: disable