bpo-37757: Disallow PEP 572 cases that expose implementation details (GH-15131)

- drop TargetScopeError in favour of raising SyntaxError directly
  as per the updated PEP 572
- comprehension iteration variables are explicitly local, but
  named expression targets in comprehensions are nonlocal or
  global. Raise SyntaxError as specified in PEP 572
- named expression targets in the outermost iterable of a
  comprehension have an ambiguous target scope. Avoid resolving
  that question now by raising SyntaxError. PEP 572
  originally required this only for cases where the bound name
  conflicts with the iteration variable in the comprehension,
  but CPython can't easily restrict the exception to that case
  (as it doesn't know the target variable names when visiting
  the outermost iterator expression)

1 files changed, 7 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-08-05-14-22-59.bpo-37757.lRv5HX.rst b/Misc/NEWS.d/next/Core and Builtins/2019-08-05-14-22-59.bpo-37757.lRv5HX.rst
new file mode 100644
index 0000000..258df0d
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-08-05-14-22-59.bpo-37757.lRv5HX.rst
@@ -0,0 +1,7 @@
+:pep:`572`: As described in the PEP, assignment expressions now raise
+:exc:`SyntaxError` when their interaction with comprehension scoping results
+in an ambiguous target scope.
+
+The ``TargetScopeError`` subclass originally proposed by the PEP has been
+removed in favour of just raising regular syntax errors for the disallowed
+cases.