__slots__ and copy again: why does it work?

[fortepianissimo]

I remember from painful experience that copy.copy() won't really copy
__slots__ members. But I have trouble explaning why the following code
works:

--- START---
#!/usr/bin/env python

import copy


class Foo (object):
__slots__ = 'i'

def __init__ (self):
self.i = 10


class Bar (Foo):
__slots__ = 'j'

def __init__ (self):
self.j = 20


f1 = Foo()
f2 = copy.copy(f1)

print f2.i # why does it work?


b1 = Bar()
b2 = copy.copy(b1)

print b2.j # why does it work?
print b2.i # doesn't work, as expected

--- END---


Any insight is welcome!

 

[fortepianissimo]

I should've mentioned this was tested on Python 2.4.2.

 

[fortepianissimo]

More weird observations: the following code does not work until you
change the name of the member 'longer' to a one-char name, for example,
'j':


--- START ---
#!/usr/bin/env python

import copy


class Foo (object):
__slots__ = 'i'


class Bar (Foo):
__slots__ = 'longer'
#__slots__ = 'j'


b1 = Bar()
b1.longer = 22
#b1.j = 22

b2 = copy.copy(b1)


# doesn't work in Python 2.4.2
# BUT if 'longer' is changed to 'j' in the entire file, then it works!
print b2.longer
#print b2.j

--- END ---


I've tried different names and concluded that as long as I used one
character the code works. Anything longer than one character bombs.

Why?

 

[fortepianissimo]

Mystery solved - when there's only one slot I should've used __slots__
= ('i', ). Duh!

So in short, __slots__ and copy.copy() work fine in Python 2.4.2.

 

[limodou]

Mystery solved - when there's only one slot I should've used __slots__
= ('i', ). Duh!

So in short, __slots__ and copy.copy() work fine in Python 2.4.2.

Hard works, but very useful

 

[fortepianissimo]

To be complete, the first code snippet, when modified as follows, works
fine in Python 2.4.2:

--- START ---
#!/usr/bin/env python

import copy

class Foo (object):
__slots__ = ('i', )

def __init__ (self):
self.i = 10

class Bar (Foo):
__slots__ = ('j', )

def __init__ (self):
super(Bar, self).__init__()
self.j = 20

f1 = Foo()
f2 = copy.copy(f1)

print f2.i # works

b1 = Bar()
b2 = copy.copy(b1)

print b2.j # works
print b2.i # works
--- END ---

The last line didn't work last time because b2.i was never initialized.