Special case assignment to '_': always infer 'Any'

Fixes python#465

Author: Guido van Rossum

mypy/checker.py

@@ -2016,6 +2016,10 @@ def infer_variable_type(self, name: Var, lvalue: Lvalue,
             # Make the type more general (strip away function names etc.).
             init_type = strip_type(init_type)
 
+            # Special case if target is named '_' -- the variable gets type Any.
+            if isinstance(lvalue, NameExpr) and lvalue.name == '_':
+                init_type = AnyType(TypeOfAny.special_form)
+
             self.set_inferred_type(name, lvalue, init_type)
 
     def infer_partial_type(self, name: Var, lvalue: Lvalue, init_type: Type) -> bool:

mypy/types.py

@@ -281,6 +281,7 @@ class TypeOfAny(Enum):
     from_error = 'from_error'
     # Is this a type that can't be represented in mypy's type system? For instance, type of
     # call to NewType...). Even though these types aren't real Anys, we treat them as such.
+    # Also used for variables named '_'.
     special_form = 'special_form'
     # Does this Any come from interaction with another Any?
     from_another_any = 'from_another_any'