Fix internal error on list/dict comprehension with walrus operator in global scope (#9062)

* Add tests for dicts using assignment expression

* Confirm the symbol table retrieved is not None

It may be None in the case of a list comprehension being declared
in a class scope (not in a function of a class). This, however,
is not valid python syntax.

* Add tests for assignment expression in class scope

Author: dhood

mypy/semanal.py

@@ -4542,9 +4542,18 @@ def current_symbol_table(self, escape_comprehensions: bool = False) -> SymbolTab
         if self.is_func_scope():
             assert self.locals[-1] is not None
             if escape_comprehensions:
+                assert len(self.locals) == len(self.is_comprehension_stack)
+                # Retrieve the symbol table from the enclosing non-comprehension scope.
                 for i, is_comprehension in enumerate(reversed(self.is_comprehension_stack)):
                     if not is_comprehension:
-                        names = self.locals[-1 - i]
+                        if i == len(self.locals) - 1:  # The last iteration.
+                            # The caller of the comprehension is in the global space.
+                            names = self.globals
+                        else:
+                            names_candidate = self.locals[-1 - i]
+                            assert names_candidate is not None, \
+                                "Escaping comprehension from invalid scope"
+                            names = names_candidate
                         break
                 else:
                     assert False, "Should have at least one non-comprehension scope"

test-data/unit/check-python38.test

@@ -198,6 +198,27 @@ if a := 2:
 while b := "x":
     reveal_type(b)  # N: Revealed type is 'builtins.str'
 
+l = [y2 := 1, y2 + 2, y2 + 3]
+reveal_type(y2)  # N: Revealed type is 'builtins.int'
+reveal_type(l)  # N: Revealed type is 'builtins.list[builtins.int*]'
+ 
+filtered_data = [y3 for x in l if (y3 := a) is not None]
+reveal_type(filtered_data)  # N: Revealed type is 'builtins.list[builtins.int*]'
+reveal_type(y3)  # N: Revealed type is 'builtins.int'
+
+d = {'a': (a2 := 1), 'b': a2 + 1, 'c': a2 + 2}
+reveal_type(d)  # N: Revealed type is 'builtins.dict[builtins.str*, builtins.int*]'
+reveal_type(a2)  # N: Revealed type is 'builtins.int'
+
+d2 = {(prefix := 'key_') + 'a': (start_val := 1), prefix + 'b': start_val + 1, prefix + 'c': start_val + 2}
+reveal_type(d2)  # N: Revealed type is 'builtins.dict[builtins.str*, builtins.int*]'
+reveal_type(prefix)  # N: Revealed type is 'builtins.str'
+reveal_type(start_val)  # N: Revealed type is 'builtins.int'
+
+filtered_dict = {k: new_v for k, v in [('a', 1), ('b', 2), ('c', 3)] if (new_v := v + 1) == 2}
+reveal_type(filtered_dict)  # N: Revealed type is 'builtins.dict[builtins.str*, builtins.int*]'
+reveal_type(new_v)  # N: Revealed type is 'builtins.int'
+
 def f(x: int = (c := 4)) -> int:
     if a := 2:
         reveal_type(a)  # N: Revealed type is 'builtins.int'
@@ -218,6 +239,19 @@ def f(x: int = (c := 4)) -> int:
     reveal_type(filtered_data)  # N: Revealed type is 'builtins.list[builtins.int*]'
     reveal_type(y3)  # N: Revealed type is 'builtins.int'
 
+    d = {'a': (a2 := 1), 'b': a2 + 1, 'c': a2 + 2}
+    reveal_type(d)  # N: Revealed type is 'builtins.dict[builtins.str*, builtins.int*]'
+    reveal_type(a2)  # N: Revealed type is 'builtins.int'
+
+    d2 = {(prefix := 'key_') + 'a': (start_val := 1), prefix + 'b': start_val + 1, prefix + 'c': start_val + 2}
+    reveal_type(d2)  # N: Revealed type is 'builtins.dict[builtins.str*, builtins.int*]'
+    reveal_type(prefix)  # N: Revealed type is 'builtins.str'
+    reveal_type(start_val)  # N: Revealed type is 'builtins.int'
+
+    filtered_dict = {k: new_v for k, v in [('a', 1), ('b', 2), ('c', 3)] if (new_v := v + 1) == 2}
+    reveal_type(filtered_dict)  # N: Revealed type is 'builtins.dict[builtins.str*, builtins.int*]'
+    reveal_type(new_v)  # N: Revealed type is 'builtins.int'
+
     # https://www.python.org/dev/peps/pep-0572/#exceptional-cases
     (y4 := 3)
     reveal_type(y4)  # N: Revealed type is 'builtins.int'
@@ -304,6 +338,28 @@ def check_narrow(x: Optional[int], s: List[int]) -> None:
     if isinstance((y := x), int):
         reveal_type(y)  # N: Revealed type is 'builtins.int'
 
+class AssignmentExpressionsClass:
+    x = (y := 1) + (z := 2)
+    reveal_type(z)  # N: Revealed type is 'builtins.int'
+
+    l = [x2 := 1, 2, 3]
+    reveal_type(x2)  # N: Revealed type is 'builtins.int'
+
+    def __init__(self) -> None:
+        reveal_type(self.z)  # N: Revealed type is 'builtins.int'
+
+        l = [z2 := 1, z2 + 2, z2 + 3]
+        reveal_type(z2)  # N: Revealed type is 'builtins.int'
+        reveal_type(l)  # N: Revealed type is 'builtins.list[builtins.int*]'
+
+        filtered_data = [z3 for x in l if (z3 := 1) is not None]
+        reveal_type(filtered_data)  # N: Revealed type is 'builtins.list[builtins.int*]'
+        reveal_type(z3)  # N: Revealed type is 'builtins.int'
+
+# Assignment expressions from inside the class should not escape the class scope.
+reveal_type(x2)  # E: Name 'x2' is not defined  # N: Revealed type is 'Any'
+reveal_type(z2)  # E: Name 'z2' is not defined  # N: Revealed type is 'Any'
+
 [builtins fixtures/isinstancelist.pyi]
 
 [case testWalrusPartialTypes]