Allow assignment to an empty tuple (python#5617)

tyehle · web-flow · commit 48f2b10d55a7 · 2020-10-17T21:27:50.000-07:00
diff --git a/mypy/semanal.py b/mypy/semanal.py
@@ -2640,9 +2640,6 @@ def analyze_lvalue(self,
                 self.fail('Unexpected type declaration', lval)
             lval.accept(self)
         elif isinstance(lval, TupleExpr):
-            items = lval.items
-            if len(items) == 0 and isinstance(lval, TupleExpr):
-                self.fail("can't assign to ()", lval)
             self.analyze_tuple_or_list_lvalue(lval, explicit_type)
         elif isinstance(lval, StarExpr):
             if nested:
diff --git a/test-data/unit/README.md b/test-data/unit/README.md
@@ -7,7 +7,8 @@ Quick Start
 
 To add a simple unit test for a new feature you developed, open or create a
 `test-data/unit/check-*.test` file with a name that roughly relates to the
-feature you added.
+feature you added. If you added a new `check-*.test` file, add it to the list
+of files in `mypy/test/testcheck.py`.
 
 Add the test in this format anywhere in the file:
 
diff --git a/test-data/unit/check-tuples.test b/test-data/unit/check-tuples.test
@@ -1458,3 +1458,15 @@ x7, x8, y7, y8 = *points2, *points3 # E: Contiguous iterable with same type expe
 
 x9, y9, x10, y10, z5 = *points2, 1, *points2 # E: Contiguous iterable with same type expected
 [builtins fixtures/tuple.pyi]
+
+[case testAssignEmptyPy36]
+# flags: --python-version 3.6
+() = []
+
+[case testAssignEmptyPy27]
+# flags: --python-version 2.7
+() = []  # E: can't assign to ()
+
+[case testAssignEmptyBogus]
+() = 1  # E: 'Literal[1]?' object is not iterable
+[builtins fixtures/tuple.pyi]
diff --git a/test-data/unit/semanal-errors.test b/test-data/unit/semanal-errors.test
@@ -377,11 +377,6 @@ main:1: error: can't assign to literal
 [out]
 main:1: error: can't assign to literal
 
-[case testInvalidLvalues5]
-() = 1
-[out]
-main:1: error: can't assign to ()
-
 [case testInvalidLvalues6]
 x = y = z = 1  # ok
 x, (y, 1) = 1
