irregular behavior within class using __setitem__

[capnstabn]

BPO	1317376
Nosy	@ncoghlan

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2005-10-08.09:44:35.000>
created_at = <Date 2005-10-08.07:27:33.000>
labels = ['interpreter-core', 'invalid']
title = 'irregular behavior within class using __setitem__'
updated_at = <Date 2005-10-08.09:44:35.000>
user = 'https://bugs.python.org/capnstabn'

bugs.python.org fields:

activity = <Date 2005-10-08.09:44:35.000>
actor = 'capnstabn'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2005-10-08.07:27:33.000>
creator = 'capnstabn'
dependencies = []
files = []
hgrepos = []
issue_num = 1317376
keywords = []
message_count = 6.0
messages = ['26536', '26537', '26538', '26539', '26540', '26541']
nosy_count = 2.0
nosy_names = ['ncoghlan', 'capnstabn']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1317376'
versions = ['Python 2.3']

[capnstabn]

i found two bugs vaguely similar to this posted but i
think they were different bugs in theory, and both were
deemed 'features' anyway so ... :p

basically the class is a modified list, being used as a
queue

what happens is, when hotswapping the __setitem__
functionality, unless explicitly calling __setitem__, the
old __setitem__ continues to be called

this is very odd and as far as i can tell the behavior is
undocumented

class Queue(list):
def __setitem__(self, a, v):
print "old setitem"
def mysetitem(self, i, v):
print "new setitem"
def use_mysetitem(self):
self.__setitem__ = self.mysetitem

x = Queue()

x[1] = 2

x.__setitem__(2, 3)

x.use_mysetitem()
x[3] = 4

x.__setitem__(4, 5)

[ncoghlan]

Logged In: YES
user_id=1038590

This problem doesn't exist in Python 2.4:

Py> class Queue(list):
... def __setitem__(self, a, v):
... print "old setitem"
... def mysetitem(self, i, v):
... print "new setitem"
... def use_mysetitem(self):
... self.__setitem__ = self.mysetitem
...
Py> x = Queue()
Py>
Py> x[1] = 2
old setitem
Py>
Py> x.__setitem__(2, 3)
old setitem
Py>
Py> x.use_mysetitem()
Py> x[3] = 4
old setitem
Py>
Py> x.__setitem__(4, 5)
new setitem

[capnstabn]

Logged In: YES
user_id=1126596

err, yeh it does 8)
the last TWO prints should display new setitem

[ncoghlan]

Logged In: YES
user_id=1038590

Ah, sorry. I was looking at the wrong part of the printout.

[ncoghlan]

Logged In: YES
user_id=1038590

And now that I've read the bug report properly, I'm closing
it as "Not a bug" :)

To affect the operators you need to alter the slot on the
*class*, not on the instance, but your "use_mysetitem"
method only affects the current instance - it shadows the
slot in the instance dictionary, but the interpreter is only
interested in the slots in the class itself.

To make the method work, it needs to be a class method:

  def use_mysetitem(cls):
    cls.__setitem__ = cls.mysetitem
  use_mysetitem = classmethod(use_mysetitem)

Of course, it will then affect *all* instances of the class
once the change is made.

You can get per-instance swapping by writing the slot method
in such a way that it delegates to a normal method, which
you can then alter on a per-instance basis. The wisdom of
actually *doing* that is debatable though ;)

[capnstabn]

Logged In: YES
user_id=1126596

omg! you're lucky people on freenode spared you a half hour
of explaining that lol!

and after almost 4 hours of completely going around the world
just to take one step backwards...

[05:38] <squ1g3e> what if i want to change the class within
an instance method
[05:38] <squ1g3e> coz the whole method isnt a class method
[05:38] <TFK> Then you can use self.__class__

haha thanks!