Hello,
I was wondering if someone could explain the following situation to me
please: class C(object):
def f(self):
pass c = C()
c.f
<bound method C.f of <__main__.C object at 0x00B25950>> C.__dict__['f'].__get__(c,C)
<bound method C.f of <__main__.C object at 0x00B25950>> c.f == C.__dict__['f'].__get__(c,C)
True c.f is C.__dict__['f'].__get__(c,C)
False
Why do c.f and C.__dict__['f'].__get__(c,C) compare as equal under ==
but not under *is* ?
Thanks,
John 4  1244 
Yup ?!? Weird ... especially as: id(c.f) == id(C.__dict__['f'].__get__(c,C))
True
I was pretty sure that 'id(a) == id(b)' iff 'a is b' ...
I thought initially that you had two *copies* of the
same method bot obviously it's not true ...
SB
On Wed, May 04, 2005 at 09:14:18AM -0700, Sébastien Boisgérault wrote:
Yup ?!? Weird ... especially as:
id(c.f) == id(C.__dict__['f'].__get__(c,C)) True
Here, c.f is discarded by the time the right-hand-side of == is
executed. So the object whose id() is being calculated on the
right-hand-side could turn out to be the same, since the two objects
have disjoint lifetimes.
Here are some more cases of the same thing: id([]) == id([])
1 id([]) == id([1])
1
Jeff
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john wrote: Why do c.f and C.__dict__['f'].__get__(c,C) compare as equal under ==
but not under *is* ?
These variations are equivalent. Every attribute access gives you a new
bound method: class C(object):
.... def f(self): pass
.... c = C()
c.f is c.f
False c.f == c.f
True
'a is b' is true only when the names a and b are bound to the same object.
For bound methods, c.f is c.f == True would mean that Python would have to
cache them, and I don't see a potential advantage of that.
'a == b' on the other hand does what the programmer specifies:
class Str(str):
.... def __eq__(self, other): return True
.... Str("black") == Str("blue")
True
It makes sense for bound methods to compare equal if they refer to the same
function and instance (though I don't know if the class is checked, too).
Peter
Jeff Epler wrote: >> id(c.f) == id(C.__dict__['f'].__get__(c,C))
True
Here, c.f is discarded by the time the right-hand-side of == is
executed. So the object whose id() is being calculated on the
right-hand-side could turn out to be the same, since the two objects
have disjoint lifetimes.
Understood. I guess I was influenced by C++ where a temporary survives
up to the end of the complete statement in which it was created. Right
?
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