improve error message when trying to take address of Python variable/expression (see #1768)

Cython/Compiler/ExprNodes.py

@@ -9990,7 +9990,10 @@ class AmpersandNode(CUnopNode):
                 self.error("Taking address of non-lvalue (type %s)" % argtype)
             return self
         if argtype.is_pyobject:
-            self.error("Cannot take address of Python variable")
+            self.error("Cannot take address of Python %s" % (
+                "variable '%s'" % self.operand.name if self.operand.is_name else
+                "object attribute '%s'" % self.operand.attribute if self.operand.is_attribute else
+                "object"))
             return self
         if not argtype.is_cpp_class or not self.type:
             self.type = PyrexTypes.c_ptr_type(argtype)

tests/errors/e_addressof.pyx

+# mode: error
+
+cdef class Ext:
+    cdef int a
+    cdef object o
+
+def f(int a):
+    cdef Ext e = Ext()
+    x = &a  # ok
+
+    cdef object o = &a  # pointer != object
+
+    po1 = &o        # pointer to Python variable
+    po2 = &o.xyz    # pointer to Python expression
+    po3 = &e.o      # pointer to Python object
+    po4 = &e.a      # ok (C attribute)
+
+    po5 = &(o + 1)  # pointer to non-lvalue Python expression
+    po6 = &(a + 1)  # pointer to non-lvalue C expression
+
+
+_ERRORS="""
+11:20: Cannot convert 'int *' to Python object
+13:10: Cannot take address of Python variable 'o'
+14:10: Cannot take address of Python object attribute 'xyz'
+15:10: Cannot take address of Python object attribute 'o'
+18:10: Taking address of non-lvalue (type Python object)
+19:10: Taking address of non-lvalue (type long)
+"""