slots vs dict

**Marcus von Appen** · Jul 18 '05, 10:51 AM

Re: __slots__ vs __dict__

Jean Brouwers <JBrouwers@Prop hICy.com> writes:
[color=blue]
> Classes using __slots__ seem to be quite a bit smaller and faster
> to instantiate than regular Python classes using __dict__.[/color]
[snip]

Yes, but instances usually will not have a Class.__dict__ attribute anymore.
Thus the following code throws an exception on binding new attributes on a
per-instance basis:

# start
class Foo (object):
__slots__ = "_test"

def __init__ (self):
self._test = None

if __name__ == "__main__":
f = Foo ()
f.testvar = "test"
return
# end

Rebinding __slots__ in __setattr__ fails with a bus error on my system (it
should not be possible anyways, because __slots__ is a tuple):

# start
class Foo (object):
__slots__ = "_test", "_test2"

def __init__ (self):
self._test = 1
self._test2 = 2

def __setattr__ (self, name, value):
# just test a simple rebinding
self.__slots__ = self.__slots__
return

if __name__ == "__main__":
f = Foo ()
f.testvar = "test"
return
# end

--- gdb backtrace

(gdb) r foo.py
....
Program received signal SIGBUS, Bus error.
0x08070050 in PyDict_GetItem ()
(gdb) bt
#0 0x08070050 in PyDict_GetItem ()
#1 0x08080ab7 in _PyType_Lookup ()
.....

Regards
Marcus

--
We don't understand the software, and sometimes we don't understand the
hardware, but we can *see* the blinking lights!

**Andrew Bennetts** · Jul 18 '05, 10:51 AM

Re: __slots__ vs __dict__

On Thu, May 13, 2004 at 12:07:54AM +0200, Marcus von Appen wrote:[color=blue]
> Jean Brouwers <JBrouwers@Prop hICy.com> writes:
>[color=green]
> > Classes using __slots__ seem to be quite a bit smaller and faster
> > to instantiate than regular Python classes using __dict__.[/color]
> [snip]
>
> Yes, but instances usually will not have a Class.__dict__ attribute anymore.
> Thus the following code throws an exception on binding new attributes on a
> per-instance basis:
>
> # start
> class Foo (object):
> __slots__ = "_test"
>
> def __init__ (self):
> self._test = None
>
> if __name__ == "__main__":
> f = Foo ()
> f.testvar = "test"
> return
> # end[/color]

This code is a syntax error for me -- the final return isn't in a function.
[color=blue]
> Rebinding __slots__ in __setattr__ fails with a bus error on my system (it
> should not be possible anyways, because __slots__ is a tuple):
>
> # start
> class Foo (object):
> __slots__ = "_test", "_test2"
>
> def __init__ (self):
> self._test = 1
> self._test2 = 2
>
> def __setattr__ (self, name, value):
> # just test a simple rebinding
> self.__slots__ = self.__slots__
> return
>
> if __name__ == "__main__":
> f = Foo ()
> f.testvar = "test"
> return
> # end[/color]

Also a syntax error. I also don't get a bus error, just infinite recursion
(because assigning to self.__slots__ calls __setattr__, but __slots__ isn't
an attribute of the instance). What version of Python, and what platform?
I'm guessing you're on something like FreeBSD where Python has had trouble
coping gracefully with infinite recursion.

-Andrew.

**Marcus von Appen** · Jul 18 '05, 10:51 AM

Re: __slots__ vs __dict__

Andrew Bennetts <andrew-pythonlist@puzz ling.org> writes:

[...][color=blue][color=green]
>> # start
>> class Foo (object):
>> __slots__ = "_test"
>>
>> def __init__ (self):
>> self._test = None
>>
>> if __name__ == "__main__":
>> f = Foo ()
>> f.testvar = "test"
>> return
>> # end[/color]
>
> This code is a syntax error for me -- the final return isn't in a function.[/color]

Sorry for that messy code - the return is absolutely wrong (no idea, why I put
it there after typing the example in).
[color=blue][color=green]
>> Rebinding __slots__ in __setattr__ fails with a bus error on my system (it
>> should not be possible anyways, because __slots__ is a tuple):
>>
>> # start
>> class Foo (object):
>> __slots__ = "_test", "_test2"
>>
>> def __init__ (self):
>> self._test = 1
>> self._test2 = 2
>>
>> def __setattr__ (self, name, value):
>> # just test a simple rebinding
>> self.__slots__ = self.__slots__
>> return
>>
>> if __name__ == "__main__":
>> f = Foo ()
>> f.testvar = "test"
>> return
>> # end[/color]
>
> Also a syntax error. I also don't get a bus error, just infinite recursion
> (because assigning to self.__slots__ calls __setattr__, but __slots__ isn't
> an attribute of the instance). What version of Python, and what platform?[/color]

Typed in a wrong line again here. Change it to something appropriate like

def __setattr__ (self, name, value):
# just test a simple rebinding
self.__slots__. __add__ (tuple (name))
return
[color=blue]
> I'm guessing you're on something like FreeBSD where Python has had trouble
> coping gracefully with infinite recursion.[/color]

Right, but this was caused by typing in wrong code. Sorry for that.

Regards
Marcus

--
We don't understand the software, and sometimes we don't understand the
hardware, but we can *see* the blinking lights!

**Andrew Bennetts** · Jul 18 '05, 10:51 AM

Re: __slots__ vs __dict__

On Thu, May 13, 2004 at 06:55:39AM +0200, Marcus von Appen wrote:[color=blue]
> Andrew Bennetts <andrew-pythonlist@puzz ling.org> writes:
>
> [...][color=green][color=darkred]
> >>
> >> # start
> >> class Foo (object):
> >> __slots__ = "_test", "_test2"
> >>
> >> def __init__ (self):
> >> self._test = 1
> >> self._test2 = 2
> >>
> >> def __setattr__ (self, name, value):
> >> # just test a simple rebinding
> >> self.__slots__ = self.__slots__
> >> return
> >>
> >> if __name__ == "__main__":
> >> f = Foo ()
> >> f.testvar = "test"
> >> return
> >> # end[/color]
> >
> > Also a syntax error. I also don't get a bus error, just infinite recursion
> > (because assigning to self.__slots__ calls __setattr__, but __slots__ isn't
> > an attribute of the instance). What version of Python, and what platform?[/color]
>
> Typed in a wrong line again here. Change it to something appropriate like
>
> def __setattr__ (self, name, value):
> # just test a simple rebinding
> self.__slots__. __add__ (tuple (name))
> return[/color]

Now your example runs just fine (i.e. the script terminates normally, and
nothing happens). I still don't see any error. (And why do you keep
putting a redundant return at the end of your functions?)

Are you sure this is the same code that you get a crash with? What version
of Python are you using? (I've tested with 2.2.3 and 2.3.3).

-Andrew.

**Marcus von Appen** · Jul 18 '05, 10:52 AM

Re: __slots__ vs __dict__

Andrew Bennetts <andrew-pythonlist@puzz ling.org> writes:

[...]
[color=blue]
> Now your example runs just fine (i.e. the script terminates normally, and
> nothing happens). I still don't see any error.[/color]

I thought you would inspect the objects yourself. Let me show you another (now
working) example, which will explain it:

------
class DictClass (object):
test = "DictTest"

def __init__ (self):
pass

class SlotClass (object):
__slots__ = "test"

def __init__ (self):
self.test = "SlotTest"

def print_obj (obj):
# inspect .test and show the object attributes
print obj.test
print dir (obj)

# bind new attribute and inspect the object attributes
obj.new_test = "Test"
print obj.new_test
print dir (obj)
print "------"
return

if __name__ == "__main__":
dict_obj = DictClass ()
slot_obj = SlotClass ()

print_obj (dict_obj)
print_obj (slot_obj)

---

You will get something like the following output:

----------------
DictTest
['__class__', '__delattr__', '__dict__', ....., '__weakref__', 'test']

Test
['__class__', '__delattr__', '__dict__', ...., '__weakref__', 'new_test',
'test']

------

SlotTest
['__class__', '__delattr__', ...., '__slots__', '__str__', 'test']

Traceback (most recent call last):
File "foo.py", line 31, in ?
print_obj (slot_obj)
File "foo.py", line 19, in print_obj
obj.new_test = "Test"
AttributeError: 'SlotClass' object has no attribute 'new_test'
----------------

As you will notice, a __slot__ed object/class has no __weakref__ nor __dict__
attribute.
Thus binding new object attributes will fail with an AttributeError.

So let's try it again with using __setattr__ in SlotClass:

class SlotClass (object):
test = "SlotTest"
test2 = "SlotTest2" # just for making a tuple creation easier
__slots__ = test, test2

def __init__ (self):
pass

def __setattr__ (self, name, value):
# create a dict here to add its key to the __slot__ tuple
self.__slots__. __add__ (tuple ((name)))
return

def print_obj (obj):
# inspect .test and show the object attributes
print obj.test
print dir (obj)

# bind new attribute and inspect the object attributes
obj.new_test = "Test"
print obj.new_test
print dir (obj)
print "------"
return

if __name__ == "__main__":

#dict_obj = DictClass ()
slot_obj = SlotClass ()

#print_obj (dict_obj)
print_obj (slot_obj)

----

As you will see, you get an AttributeError again.tuple ((name))was not
concatenated to __slots__.
I think that should explain enough about the advantage and disadvantage of
__slots__ here.

(And why do you keep[color=blue]
> putting a redundant return at the end of your functions?)[/color]

I'm used to it :-).
[color=blue]
> Are you sure this is the same code that you get a crash with? What version
> of Python are you using? (I've tested with 2.2.3 and 2.3.3).[/color]

No, I put messy code without _really_ thinking about it in my first post.
I did not realize that self.__slots__ = self._slots in __setattr__ will end
up in a recursion...

Regards
Marcus

--
We don't understand the software, and sometimes we don't understand the
hardware, but we can *see* the blinking lights!

slots vs dict

slots vs dict

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