bpo-35169: Improve error messages for forbidden assignments.

Author: serhiy-storchaka

Lib/test/test_dictcomps.py

@@ -73,11 +73,11 @@ def test_local_visibility(self):
         self.assertEqual(v, "Local variable")
 
     def test_illegal_assignment(self):
-        with self.assertRaisesRegex(SyntaxError, "can't assign"):
+        with self.assertRaisesRegex(SyntaxError, "cannot assign"):
             compile("{x: y for y, x in ((1, 2), (3, 4))} = 5", "<test>",
                     "exec")
 
-        with self.assertRaisesRegex(SyntaxError, "can't assign"):
+        with self.assertRaisesRegex(SyntaxError, "cannot assign"):
             compile("{x: y for y, x in ((1, 2), (3, 4))} += 5", "<test>",
                     "exec")
 

Lib/test/test_generators.py

@@ -1865,12 +1865,12 @@ def printsolution(self, x):
 >>> def f(): (yield bar) = y
 Traceback (most recent call last):
   ...
-SyntaxError: can't assign to yield expression
+SyntaxError: cannot assign to yield expression
 
 >>> def f(): (yield bar) += y
 Traceback (most recent call last):
   ...
-SyntaxError: can't assign to yield expression
+SyntaxError: cannot assign to yield expression
 
 
 Now check some throw() conditions:

Lib/test/test_genexps.py

@@ -137,12 +137,12 @@
     >>> (y for y in (1,2)) = 10
     Traceback (most recent call last):
        ...
-    SyntaxError: can't assign to generator expression
+    SyntaxError: cannot assign to generator expression
 
     >>> (y for y in (1,2)) += 10
     Traceback (most recent call last):
        ...
-    SyntaxError: can't assign to generator expression
+    SyntaxError: cannot assign to generator expression
 
 
 ########### Tests borrowed from or inspired by test_generators.py ############

Lib/test/test_syntax.py

@@ -33,35 +33,55 @@
 
 >>> None = 1
 Traceback (most recent call last):
-SyntaxError: can't assign to keyword
+SyntaxError: cannot assign to None
+
+>>> obj.True = 1
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
+>>> True = 1
+Traceback (most recent call last):
+SyntaxError: cannot assign to True
+
+>>> obj.__debug__ = 1
+Traceback (most recent call last):
+SyntaxError: cannot assign to __debug__
+
+>>> __debug__ = 1
+Traceback (most recent call last):
+SyntaxError: cannot assign to __debug__
 
 >>> f() = 1
 Traceback (most recent call last):
-SyntaxError: can't assign to function call
+SyntaxError: cannot assign to function call
 
 >>> del f()
 Traceback (most recent call last):
-SyntaxError: can't delete function call
+SyntaxError: cannot delete function call
 
 >>> a + 1 = 2
 Traceback (most recent call last):
-SyntaxError: can't assign to operator
+SyntaxError: cannot assign to operator
 
 >>> (x for x in x) = 1
 Traceback (most recent call last):
-SyntaxError: can't assign to generator expression
+SyntaxError: cannot assign to generator expression
 
 >>> 1 = 1
 Traceback (most recent call last):
-SyntaxError: can't assign to literal
+SyntaxError: cannot assign to literal
 
 >>> "abc" = 1
 Traceback (most recent call last):
-SyntaxError: can't assign to literal
+SyntaxError: cannot assign to literal
 
 >>> b"" = 1
 Traceback (most recent call last):
-SyntaxError: can't assign to literal
+SyntaxError: cannot assign to literal
+
+>>> ... = 1
+Traceback (most recent call last):
+SyntaxError: cannot assign to Ellipsis
 
 >>> `1` = 1
 Traceback (most recent call last):
@@ -74,15 +94,31 @@
 
 >>> (a, "b", c) = (1, 2, 3)
 Traceback (most recent call last):
-SyntaxError: can't assign to literal
+SyntaxError: cannot assign to literal
+
+>>> (a, True, c) = (1, 2, 3)
+Traceback (most recent call last):
+SyntaxError: cannot assign to True
+
+>>> (a, __debug__, c) = (1, 2, 3)
+Traceback (most recent call last):
+SyntaxError: cannot assign to __debug__
+
+>>> (a, *True, c) = (1, 2, 3)
+Traceback (most recent call last):
+SyntaxError: cannot assign to True
+
+>>> (a, *__debug__, c) = (1, 2, 3)
+Traceback (most recent call last):
+SyntaxError: cannot assign to __debug__
 
 >>> [a, b, c + 1] = [1, 2, 3]
 Traceback (most recent call last):
-SyntaxError: can't assign to operator
+SyntaxError: cannot assign to operator
 
 >>> a if 1 else b = 1
 Traceback (most recent call last):
-SyntaxError: can't assign to conditional expression
+SyntaxError: cannot assign to conditional expression
 
 From compiler_complex_args():
 
@@ -255,36 +291,39 @@
 
 >>> f(lambda x: x[0] = 3)
 Traceback (most recent call last):
-SyntaxError: lambda cannot contain assignment
+SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
 
 The grammar accepts any test (basically, any expression) in the
 keyword slot of a call site.  Test a few different options.
 
 >>> f(x()=2)
 Traceback (most recent call last):
-SyntaxError: keyword can't be an expression
+SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
 >>> f(a or b=1)
 Traceback (most recent call last):
-SyntaxError: keyword can't be an expression
+SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
 >>> f(x.y=1)
 Traceback (most recent call last):
-SyntaxError: keyword can't be an expression
+SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
 >>> f((x)=2)
 Traceback (most recent call last):
-SyntaxError: keyword can't be an expression
+SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
 
 
 More set_context():
 
 >>> (x for x in x) += 1
 Traceback (most recent call last):
-SyntaxError: can't assign to generator expression
+SyntaxError: cannot assign to generator expression
 >>> None += 1
 Traceback (most recent call last):
-SyntaxError: can't assign to keyword
+SyntaxError: cannot assign to None
+>>> __debug__ += 1
+Traceback (most recent call last):
+SyntaxError: cannot assign to __debug__
 >>> f() += 1
 Traceback (most recent call last):
-SyntaxError: can't assign to function call
+SyntaxError: cannot assign to function call
 
 
 Test continue in finally in weird combinations.
@@ -481,15 +520,15 @@
    ...   pass
    Traceback (most recent call last):
      ...
-   SyntaxError: can't assign to function call
+   SyntaxError: cannot assign to function call
 
    >>> if 1:
    ...   pass
    ... elif 1:
    ...   x() = 1
    Traceback (most recent call last):
      ...
-   SyntaxError: can't assign to function call
+   SyntaxError: cannot assign to function call
 
    >>> if 1:
    ...   x() = 1
@@ -499,7 +538,7 @@
    ...   pass
    Traceback (most recent call last):
      ...
-   SyntaxError: can't assign to function call
+   SyntaxError: cannot assign to function call
 
    >>> if 1:
    ...   pass
@@ -509,7 +548,7 @@
    ...   pass
    Traceback (most recent call last):
      ...
-   SyntaxError: can't assign to function call
+   SyntaxError: cannot assign to function call
 
    >>> if 1:
    ...   pass
@@ -519,7 +558,7 @@
    ...   x() = 1
    Traceback (most recent call last):
      ...
-   SyntaxError: can't assign to function call
+   SyntaxError: cannot assign to function call
 
 Make sure that the old "raise X, Y[, Z]" form is gone:
    >>> raise X, Y
@@ -539,21 +578,33 @@
 
 >>> {1, 2, 3} = 42
 Traceback (most recent call last):
-SyntaxError: can't assign to literal
+SyntaxError: cannot assign to set display
+
+>>> {1: 2, 3: 4} = 42
+Traceback (most recent call last):
+SyntaxError: cannot assign to dict display
+
+>>> f'{x}' = 42
+Traceback (most recent call last):
+SyntaxError: cannot assign to f-string expression
+
+>>> f'{x}-{y}' = 42
+Traceback (most recent call last):
+SyntaxError: cannot assign to f-string expression
 
 Corner-cases that used to fail to raise the correct error:
 
     >>> def f(*, x=lambda __debug__:0): pass
     Traceback (most recent call last):
-    SyntaxError: assignment to keyword
+    SyntaxError: cannot assign to __debug__
 
     >>> def f(*args:(lambda __debug__:0)): pass
     Traceback (most recent call last):
-    SyntaxError: assignment to keyword
+    SyntaxError: cannot assign to __debug__
 
     >>> def f(**kwargs:(lambda __debug__:0)): pass
     Traceback (most recent call last):
-    SyntaxError: assignment to keyword
+    SyntaxError: cannot assign to __debug__
 
     >>> with (lambda *:0): pass
     Traceback (most recent call last):
@@ -563,11 +614,11 @@
 
     >>> def f(**__debug__): pass
     Traceback (most recent call last):
-    SyntaxError: assignment to keyword
+    SyntaxError: cannot assign to __debug__
 
     >>> def f(*xx, __debug__): pass
     Traceback (most recent call last):
-    SyntaxError: assignment to keyword
+    SyntaxError: cannot assign to __debug__
 
 """
 

Misc/NEWS.d/next/Core and Builtins/2018-11-05-21-19-05.bpo-35169._FyPI2.rst

@@ -0,0 +1 @@
+Improved error messages for forbidden assignments.