Support negative indexes in tuple slices (#1620)

Also fix the defaults for start, end, and stride to `None` to correctly match the Python slice semantics -- in particular, if the stride is negative then the default start and end are effectively `length - 1` and `0` respectively rather than vice versa.

Fixes #886, except for the part that's been separated out as #899.

Author: axitkhurana

mypy/checkexpr.py

@@ -1136,31 +1136,47 @@ def visit_index_expr_helper(self, e: IndexExpr) -> Type:
             return result
 
     def visit_tuple_slice_helper(self, left_type: TupleType, slic: SliceExpr):
-        begin = 0
-        end = len(left_type.items)
-        stride = 1
+        begin = None  # type: int
+        end = None  # type: int
+        stride = None  # type:int
+
         if slic.begin_index:
-            if isinstance(slic.begin_index, IntExpr):
-                begin = slic.begin_index.value
-            else:
-                self.chk.fail(messages.TUPLE_SLICE_MUST_BE_AN_INT_LITERAL, slic.begin_index)
+            begin = self._get_value(slic.begin_index)
+            if begin is None:
+                self.chk.fail(
+                    messages.TUPLE_SLICE_MUST_BE_AN_INT_LITERAL,
+                    slic.begin_index)
                 return AnyType()
+
         if slic.end_index:
-            if isinstance(slic.end_index, IntExpr):
-                end = slic.end_index.value
-            else:
-                self.chk.fail(messages.TUPLE_SLICE_MUST_BE_AN_INT_LITERAL, slic.end_index)
+            end = self._get_value(slic.end_index)
+            if end is None:
+                self.chk.fail(
+                    messages.TUPLE_SLICE_MUST_BE_AN_INT_LITERAL,
+                    slic.end_index)
                 return AnyType()
+
         if slic.stride:
-            if isinstance(slic.stride, IntExpr):
-                stride = slic.stride.value
-            else:
-                self.chk.fail(messages.TUPLE_SLICE_MUST_BE_AN_INT_LITERAL, slic.stride)
+            stride = self._get_value(slic.stride)
+            if stride is None:
+                self.chk.fail(
+                    messages.TUPLE_SLICE_MUST_BE_AN_INT_LITERAL,
+                    slic.stride)
                 return AnyType()
 
         return TupleType(left_type.items[begin:end:stride], left_type.fallback,
                     left_type.line, left_type.implicit)
 
+    def _get_value(self, index: Node) -> Optional[int]:
+        if isinstance(index, IntExpr):
+            return index.value
+        elif isinstance(index, UnaryExpr):
+            if index.op == '-':
+                operand = index.expr
+                if isinstance(operand, IntExpr):
+                    return -1 * operand.value
+        return None
+
     def visit_cast_expr(self, expr: CastExpr) -> Type:
         """Type check a cast expression."""
         source_type = self.accept(expr.expr, context=AnyType())

mypy/test/data/check-tuples.test

@@ -169,9 +169,11 @@ b = t1[(0)] # E: Incompatible types in assignment (expression has type "A", vari
 
 a = t1[0]
 b = t1[1]
+b = t1[-1]
 a = t1[(0)]
 x = t3[0:3] # type (A, B, C)
 y = t3[0:5:2] # type (A, C, E)
+x = t3[:-2] # type (A, B, C)
 
 class A: pass
 class B: pass