Fix type inference with iterable initializer

mypy/checker.py

@@ -526,10 +526,12 @@ def infer_variable_type(self, names: List[Var], lvalues: List[Node],
  self.msg.incompatible_value_count_in_assignment(
  len(names), len(init_type.items), context)
  elif (isinstance(init_type, Instance) and
- cast(Instance, init_type).type.fullname() ==
- 'builtins.list'):
- # Initializer with an array type.
- item_type = cast(Instance, init_type).args[0]
+ is_subtype(init_type,
+ self.named_generic_type('builtins.Iterable',
+ [AnyType()]))):
+ # Initializer with an iterable type.
+ item_type = self.iterable_item_type(cast(Instance,
+ init_type))
  for i in range(len(names)):
  self.set_inferred_type(names[i], lvalues[i], item_type)
  elif isinstance(init_type, AnyType):
@@ -614,18 +616,15 @@ def check_multi_assignment(self, lvalue_types: List[Type],
  [AnyType()])) and
  isinstance(rvalue_type, Instance)):
  # Rvalue is iterable.
- iterable = map_instance_to_supertype(
- cast(Instance, rvalue_type),
- self.lookup_typeinfo('builtins.Iterable'))
- item_type = iterable.args[0]
+ item_type = self.iterable_item_type(cast(Instance, rvalue_type))
  for k in range(len(lvalue_types)):
  self.check_single_assignment(lvalue_types[k],
  index_lvalues[k],
  self.temp_node(item_type),
  context, msg)
  else:
  self.fail(msg, context)
- 
+
  def check_single_assignment(self,
  lvalue_type: Type, index_lvalue: IndexExpr,
  rvalue: Node, context: Context,
@@ -1158,6 +1157,12 @@ def fail(self, msg: str, context: Context) -> None:
  """Produce an error message."""
  self.msg.fail(msg, context)
 
+ def iterable_item_type(self, instance: Instance) -> Type:
+ iterable = map_instance_to_supertype(
+ instance,
+ self.lookup_typeinfo('builtins.Iterable'))
+ return iterable.args[0]
+
 
 def map_type_from_supertype(typ: Type, sub_info: TypeInfo,
  super_info: TypeInfo) -> Type:

mypy/test/data/check-inference.test

@@ -174,8 +174,8 @@ class A: pass
 main: In function "f":
 
 
--- Inferring local variable types in multiple definition
--- -----------------------------------------------------
+-- Inferring variable types in multiple definition
+-- -----------------------------------------------
 
 
 [case testInferringLvarTypesInMultiDef]
@@ -270,6 +270,17 @@ def f(d: Any) -> None:
  a.x
  b.x
 
+[case testInferringTypesFromIterable]
+from typing import Iterable
+class Nums(Iterable[int]):
+ def __iter__(self): pass
+ def __next__(self): pass
+a, b = Nums()
+a = b = 1
+a = '' # E: Incompatible types in assignment
+b = '' # E: Incompatible types in assignment
+[builtins fixtures/for.py]
+
 
 -- Type variable inference for generic functions
 -- ---------------------------------------------