bpo-41848: Allow simple lambdas in if clauses of comprehensions

Grammar/python.gram

@@ -332,6 +332,10 @@ expression[expr_ty] (memo):
     | disjunction
     | lambdef
 
+expression_no_cond[expr_ty]:
+    | disjunction
+    | 'lambda' a=[lambda_params] ':' b=expression_no_cond { _Py_Lambda((a) ? a : CHECK(_PyPegen_empty_arguments(p)), b, EXTRA) }
+
 lambdef[expr_ty]:
     | 'lambda' a=[lambda_params] ':' b=expression { _Py_Lambda((a) ? a : CHECK(_PyPegen_empty_arguments(p)), b, EXTRA) }
 
@@ -521,9 +525,9 @@ kvpair[KeyValuePair*]: a=expression ':' b=expression { _PyPegen_key_value_pair(p
 for_if_clauses[asdl_comprehension_seq*]:
     | a[asdl_comprehension_seq*]=for_if_clause+ { a }
 for_if_clause[comprehension_ty]:
-    | ASYNC 'for' a=star_targets 'in' ~ b=disjunction c[asdl_expr_seq*]=('if' z=disjunction { z })* {
+    | ASYNC 'for' a=star_targets 'in' ~ b=disjunction c[asdl_expr_seq*]=('if' z=expression_no_cond { z })* {
         CHECK_VERSION(6, "Async comprehensions are", _Py_comprehension(a, b, c, 1, p->arena)) }
-    | 'for' a=star_targets 'in' ~ b=disjunction c[asdl_expr_seq*]=('if' z=disjunction { z })* {
+    | 'for' a=star_targets 'in' ~ b=disjunction c[asdl_expr_seq*]=('if' z=expression_no_cond { z })* {
         _Py_comprehension(a, b, c, 0, p->arena) }
     | invalid_for_target
 

Lib/test/test_coroutines.py

@@ -2005,6 +2005,19 @@ async def f():
             run_async(f()),
             ([], {1: 1, 2: 2, 3: 3}))
 
+    def test_comp_9(self):
+        async def gen():
+            yield 1
+            yield 2
+
+        async def f():
+            l = [i async for i in gen() if lambda: None]
+            return [i for i in l]
+
+        self.assertEqual(
+            run_async(f()),
+            ([], [1, 2]))
+
     def test_copy(self):
         async def func(): pass
         coro = func()

Lib/test/test_listcomps.py

@@ -139,6 +139,20 @@
     >>> test_func()
     [2, 2, 2, 2, 2]
 
+########### Tests for different syntactic corner cases ############
+
+    >>> [x for x in [] if lambda: x]
+    []
+
+    >>> [x for x in [] if lambda: x or b]
+    []
+
+    >>> [x for x in [True, True, False] if x or True]
+    [True, True, False]
+
+    >>> [y for x in [1, 1, 0] if (y := x + 1)]
+    [2, 2, 1]
+
 """
 
 

Lib/test/test_syntax.py

@@ -746,6 +746,32 @@
 Traceback (most recent call last):
 SyntaxError: only single target (not list) can be annotated
 
+>>> [x for x in range(5) if a:=b]
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+>>> [x for x in range(5) if if a else b]
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+>>> [x for x in range(5) if lambda: a:=b]
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+>>> [x for x in range(5) if lambda: True if True else False]
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
+>>> [x async for x in range(5) if a:=b]
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+>>> [x async for x in range(5) if if a else b]
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+>>> [x async for x in range(5) if lambda: a:=b]
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+>>> [x async for x in range(5) if lambda: True if True else False]
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
 Corner-cases that used to fail to raise the correct error:
 
     >>> def f(*, x=lambda __debug__:0): pass

Misc/NEWS.d/next/Core and Builtins/2020-09-24-09-50-52.bpo-41848.ifb8m3.rst

@@ -0,0 +1 @@
+Allow simple lambda expressions in the if clauses of the comprehensions.