Issue1985

Jython 2.5.3 behaves differently than CPython (2.5, 2.6, 2.7) with following code:

----

class ReadonlyDescriptor(object):
    def __get__(self, instance, owner):
        return 123

class A(object):
    __slots__ = []
    attr = ReadonlyDescriptor()

A().attr = 2  # Jython 2.5.3: TypeError: readonly attribute in Jython 2.5.3
              # CPython 2.5:  AttributeError: 'A' object attribute 'attr' is read-only

Fixing this would require breaking some tests we take from cpython.

The code reads

/////////
    public void readonlyAttributeError(String name) {
        // XXX: Should be an AttributeError but CPython throws TypeError for read only
        // member descriptors (in structmember.c::PyMember_SetOne), which is expected by a
        // few tests. fixed in py3k: http://bugs.python.org/issue1687163
        throw Py.TypeError("readonly attribute");
    }
/////

The problem is described in detail in this bug

Inconsistent Exceptions for Readonly Attributes
http://bugs.python.org/issue1687163

So we won't be able to fix this in 2.x without breaking tests.

What problems does this create for you?

Can you change your exception handling slightly to work around it? i.e.

try:
    A().attr = 2
except (AttributeError, TypeError):
    print "That was a read-only attribute"

or even

try:
    A().attr = 2
except TypeError, t:
    if t.message == "readonly attribute":
        raise AttributeError(t)

TypeError vs AttributeError is one of those grey areas where implementations can differ. Alan's workaround is the right way to go.