Trac #19733: Assignment of special methods does not work in Cython

In Cython, you cannot do
{{{
cdef class Foo:
    __str__ = whatever
}}}
and expect it to work.

I asked upstream about this: [https://groups.google.com/forum/#!topic
/cython-users/aE7Tn-XLZnI]

-------

When deriving a `class` (not `cdef class`) this kind of assignment
suddenly works::
{{{
cdef class Base(object):
    def my_str(self):
        return "good"
    __str__ = my_str

class Derived(Base):
    pass
}}}
gives
{{{
sage: str(Base())
'<_home_jdemeyer__sage_temp_tamiyo_8430_tmp__MunIM_spyx_0.Base object at
0x7fb54ae544d0>'
sage: str(Derived())
'good'
}}}

This also explains why the `__len__ = cardinality` in
`src/sage/rings/finite_rings/finite_field_base.pyx` works. It doesn't
work on the level of
`sage.rings.finite_rings.finite_field_base.FiniteField`, but it works at
the level of the actual finite field classes (which are Python classes).

URL: http://trac.sagemath.org/19733
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Dima Pasechnik

src/sage/libs/gap/context_managers.py

@@ -13,7 +13,7 @@
 
     sage: with libgap.global_context('FooBar', 'test'):
     ....:     print libgap.get_global('FooBar')
-    "test"
+    test
 
 Afterward, the global variable reverts to the previous value::
 
@@ -27,7 +27,7 @@
     ....:     raise ValueError(libgap.get_global('FooBar'))
     Traceback (most recent call last):
     ...
-    ValueError: "test"
+    ValueError: test
     sage: print libgap.get_global('FooBar')
     123
 """

src/sage/libs/gap/element.pyx

@@ -1861,22 +1861,27 @@ cdef class GapElement_String(GapElement):
         sage: s = libgap('string')
         sage: type(s)
         <type 'sage.libs.gap.element.GapElement_String'>
+        sage: s
+        "string"
+        sage: print s
+        string
     """
-
-    def sage(self):
+    def __str__(self):
         r"""
-        Return the Sage equivalent of the :class:`GapElement`
+        Convert this :class:`GapElement_String` to a Python string.
 
         OUTPUT:
 
-        A Python list.
+        A Python string.
 
         EXAMPLES::
 
             sage: s = libgap.eval(' "string" '); s
             "string"
             sage: type(_)
             <type 'sage.libs.gap.element.GapElement_String'>
+            sage: str(s)
+            'string'
             sage: s.sage()
             'string'
             sage: type(_)
@@ -1887,7 +1892,7 @@ cdef class GapElement_String(GapElement):
         libgap_exit()
         return s
 
-    __str__ = sage
+    sage = __str__
 
 ############################################################################
 ### GapElement_Function ####################################################