references and buffer()
Hi all, was looking at references in python... a = 10 b = a id(a) 153918788 id(b) 153918788 where a and b point to the same id. now is this id an address ? can one dereference a value based on address alone in python? is id similar to the address of a variable or a class ? read abt buffers as a refernce to a buffer object. actually i tried passing list and dict types to buffer function, but only with string type i could createa buffer reference, y = 'GATCGTACC' x = buffer(y, 0,8) x read-only buffer for 0xbf4cd3b8, size 8, offset 2 at 0xbf4cf0e0 print x TCGTACC id(y) -1085484104 id(x) -1085476384 now the ids of y and x are not the same - why ? In which situatons are buffers used against the strings ? can i create a write buffer instead of readonly buffer ? what exactly are buffer object types ? and how can i instantiate them ? regards, KM -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
km wrote: Hi all, was looking at references in python... a = 10 b = a id(a) 153918788 id(b) 153918788 where a and b point to the same id. now is this id an address ? no, it's the object identity, and all it tells you is that both names point to the same object. can one dereference a value based on address alone in python? no. is id similar to the address of a variable or a class ? in the CPython implementation, it's the address where the object is stored. but that's an implementation detail. /F -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
On 10/8/06, km [EMAIL PROTECTED] wrote:
Hi all,
was looking at references in python...
a = 10
b = a
id(a)
153918788
id(b)
153918788
where a and b point to the same id. now is this id an address ?
The id may be considered similar to an address in C, etc. slightly
different but conceptually close.
can one dereference a value based on address alone in python?
Not to my knowledge. Generally speaking, you wouldn't want to anyway.
is id similar to the address of a variable or a class ?
Exactly. Similar. Not the same, but similar.
read abt buffers as a refernce to a buffer object.
actually i tried passing list and dict types to buffer function, but only
with string type i could createa buffer reference,
y = 'GATCGTACC'
x = buffer(y, 0,8)
x
read-only buffer for 0xbf4cd3b8, size 8, offset 2 at 0xbf4cf0e0
print x
TCGTACC
id(y)
-1085484104
id(x)
-1085476384
now the ids of y and x are not the same - why ?
You assigned two different object values to these names. In Python, a
name is just a name. It can point at any object. This relation becomes
very clear with mutable objects.
x = {'foo':42, 'bar':69}
id(x)
1076761980
y = x
y['baz'] = 36
id(y)
1076761980
y
{'baz': 36, 'foo': 42, 'bar': 69}
x
{'baz': 36, 'foo': 42, 'bar': 69}
When I wrote y = x, all I did was make the variable y point to the
dictionary object x is also pointing at. Hence they point to the same
object. Things would be different if I decided to copy an object
instead:
x = {'foo':42, 'bar':69}
import copy
y = copy.deepcopy(x)
y['baz'] = 36
id(x)
1076761164
id(y)
1076890180
x
{'foo': 42, 'bar': 69}
y
{'baz': 36, 'foo': 42, 'bar': 69}
Since your name is Sri Krishna, an avatar of Vishnu, the concept
should be familiar. Variables are like avatars; they represent the
object (whether this is a humble dictionary or Vishnu the Preserver)
for the user, and serve as a bridge between the computer and the user,
as an avatar of Vishnu is a bridge between the physical world and the
supernatural.
Dasha Avatar -- one god. Think about it.
what exactly are buffer object types ? and how can i instantiate them ?
If you want to get a line of user input, the basic function is raw_input().
-- Theerasak
--
http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
Hi all, in the CPython implementation, it's the address where the object isstored.but that's an implementation detail. ok so can i point a vairiable to an address location just as done in C language ? y = 'ATGCATGC' x = buffer(y) del(y) x read-only buffer for 0xbf4cf0e0, size -1, offset 0 at 0xbf4cf240 print x ATGCATGC now even when i delete y, why is that x still exists ? thats true even in the case of vairable assignment which states it a a reference ! a = 10 b = a del(a) b 10 i always thought if u modify the referred variable/buffer object it should be reflected in the referenced variables/buffers objects . am i wrong ? does it mean that references in python are not true references ? regards, KM -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
Fredrik Lundh wrote: [snip] is id similar to the address of a variable or a class ? in the CPython implementation, it's the address where the object is stored. but that's an implementation detail. Just as an obscure sidenote, in PyPy it is ~address some of the time. This is due to the fact that since PyPy can use the Boehm garbage collector. The Boehm collector is conservative and therefore has to assume that everything in RAM could be a pointer. Now if somebody stores the id of an object the GC could not distinguish this from a real pointer if id returned the address, which would keep the object alive. /sidenode Cheers, Carl Friedrich Bolz -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
km wrote: ok so can i point a vairiable to an address location just as done in C language ? no. there are no C-style variables in Python; just objects and names bound to objects. Python variables are names, not memory locations. y = 'ATGCATGC' x = buffer(y) del(y) x read-only buffer for 0xbf4cf0e0, size -1, offset 0 at 0xbf4cf240 print x ATGCATGC now even when i delete y, why is that x still exists ? because it's an object. thats true even in the case of vairable assignment which states it a a reference ! a = 10 b = a del(a) b 10 i always thought if u modify the referred variable/buffer object it should be reflected in the referenced variables/buffers objects . am i wrong ? reset your brain: http://effbot.org/zone/python-objects.htm /F -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
On 10/8/06, km [EMAIL PROTECTED] wrote: Hi all, in the CPython implementation, it's the address where the object is stored. but that's an implementation detail. ok so can i point a vairiable to an address location just as done in C language ? y = 'ATGCATGC' x = buffer(y) del(y) x read-only buffer for 0xbf4cf0e0, size -1, offset 0 at 0xbf4cf240 print x ATGCATGC now even when i delete y, why is that x still exists ? Say that you copy the contents of file foo into file bar and delete the original foo. Of course file bar still exists in this case. Not much of a difference; I haven't seen buffer objects yet (I am also new to Python), but the initialization for the buffer probably copies whatever is in y somewhere. thats true even in the case of vairable assignment which states it a a reference ! a = 10 b = a del(a) b 10 i always thought if u modify the referred variable/buffer object it should be reflected in the referenced variables/buffers objects You didn't modify the object that the variable /refers to/. Furthermore, numbers are immutable anyway. To continue with the Hindu god analogy, Vishnu did not cease to exist when any of his avatars passed from the physical world; it is no different with objects in Python. IOW a -- 10 b -/ Delete the 'a' reference and: b -- 10 Or: class god(object): ... pass ... vishnu = god() matsya = vishnu kurma = vishnu varaha = vishnu narasimha = vishnu # Etc ... del narasimha matsya __main__.god object at 0x402e3c6c kurma __main__.god object at 0x402e3c6c What is a little different is this: if there are no references left to an object (such as variables), the object the references point to will eventually be deleted. Variables are one way to have a reference to an object. References to an object may also exist in a list, hash, or other data type. a = 'foo' b = [1,2,3,a] del(a) b [1, 2, 3, 'foo'] am i wrong ? does it mean that references in python are not true references ? Python is object-oriented in either sense of the word (think Lisp sense then think Smalltalk sense). These are true references. -- Theerasak -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
km wrote: Hi all, in the CPython implementation, it's the address where the object is stored. but that's an implementation detail. ok so can i point a vairiable to an address location just as done in C language ? y = 'ATGCATGC' x = buffer(y) del(y) x read-only buffer for 0xbf4cf0e0, size -1, offset 0 at 0xbf4cf240 print x ATGCATGC now even when i delete y, why is that x still exists ? Because assignment is a *binding* of a name, in some namespace, to an object. x still exists because it hasn't been deleted. Because it is a reference to the object formerly bound to the name y, that object still also exists. thats true even in the case of vairable assignment which states it a a reference ! a = 10 b = a del(a) b 10 i always thought if u modify the referred variable/buffer object it should be reflected in the referenced variables/buffers objects . am i wrong ? Yes, you are wrong. Think of Python names as pure references. does it mean that references in python are not true references ? No, it doesn't mean that. Python bindings are exactly true references (think pointer in C), the storage for the referred objects is allocated from a heap and has a lifetime as long as the last reference to it and possibly, depending on the storage allocation and garbage collection strategy of the specific implementation, rather longer. Once all references have been deleted it can no longer be reached from inside Python code, however. The original CPython implementation uses reference counting to control storage reclamation, but other implementations used other strategies. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://holdenweb.blogspot.com Recent Ramblings http://del.icio.us/steve.holden -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
Hi all,Say that you copy the contents of file foo into file bar and delete the original foo. Of course file bar still exists in this case. Notmuch of a difference; I haven't seen buffer objects yet (I am also newto Python), but the initialization for the buffer probably copieswhatever is in y somewhere. that means when u refer to an object with different names (variable), it referes to the same object- fine. but is it that the original object stays in memory until it is Garbage Collected ? is it that del() deletes the link of variable to the object and not the object ? and thats why u can access it from other variables ? You didn't modify the object that the variable /refers to/.Furthermore, numbers are immutable anyway. To continue with the Hindu god analogy, Vishnu did not cease to exist when any of his avatarspassed from the physical world; it is no different with objects inPython. vishnu analogy is a bit complicated as it is a manifestation of divine energy in terms of earthly object(avatar). Its clearly not a reference. each avatar is himself (vishnu). It is the same energy people around have too (coz of manifestation). ofcourse they dont realise coz of ego (id in python) and so the object class (divine energy) is the same - unlike python where we have different classes derived from object class. IOWa -- 10b -/Delete the 'a' reference and:b -- 10 got it! What is a little different is this: if there are no references left toan object (such as variables), the object the references point to will eventually be deleted. Variables are one way to have a reference to anobject. References to an object may also exist in a list, hash, orother data type. so the object exists until there are no references to it and will be Garbage Collected immediately? regards, KM -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
On 10/8/06, km [EMAIL PROTECTED] wrote: Hi all, Say that you copy the contents of file foo into file bar and delete the original foo. Of course file bar still exists in this case. Not much of a difference; I haven't seen buffer objects yet (I am also new to Python), but the initialization for the buffer probably copies whatever is in y somewhere. that means when u refer to an object with different names (variable), it referes to the same object- fine. but is it that the original object stays in memory until it is Garbage Collected ? Exactly. is it that del() deletes the link of variable to the object and not the object ? and thats why u can access it from other variables ? Exactly. You didn't modify the object that the variable /refers to/. Furthermore, numbers are immutable anyway. To continue with the Hindu god analogy, Vishnu did not cease to exist when any of his avatars passed from the physical world; it is no different with objects in Python. vishnu analogy is a bit complicated as it is a manifestation of divine energy in terms of earthly object(avatar). Its clearly not a reference. each avatar is himself (vishnu). It is the same energy people around have too (coz of manifestation). ofcourse they dont realise coz of ego (id in python) and so the object class (divine energy) is the same - unlike python where we have different classes derived from object class. Congratulations, you understand both Hinduism and Python better than I do now. :) c.f. http://www.swami-krishnananda.org/brdup/brhad_III-09.html Kati references, Yajnavalkya, iti? (It is worth noting that a subclass is an instance of its superclass, both in terms of interface and implementation.) so the object exists until there are no references to it and will be Garbage Collected immediately? Python uses two garbage collection schemes together. It uses reference counting (when number of references goes to zero, remove object from memory) and mark-and-sweep (comb through process memory methodically looking for objects that are no longer accessible). This is what allows it to collect cyclic structures, such as trees whose nodes links to their parents and vice versa. GC intercedes at various intervals when convenient. I don't think it would be immediate though. -- Theerasak -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
Hi all, Congratulations, you understand both Hinduism and Python better than Ido now.:) c.f.http://www.swami-krishnananda.org/brdup/brhad_III-09.htmlKati references, Yajnavalkya, iti? the answer lies in a single line as pronounced by sri adi sankaracharya - aham bramhasmi sivoha sivoham , which is still not implemented in python. infact not in any other language! which is a limitatin of computers (and comp languages) - they simply arent intelligent. aham, tvam sah: cha madhye kim bhedam bhavati ? bhedam nasti ! Python uses two garbage collection schemes together. It uses referencecounting (when number of references goes to zero, remove object from memory) and mark-and-sweep (comb through process memory methodicallylooking for objects that are no longer accessible). This is whatallows it to collect cyclic structures, such as trees whose nodeslinks to their parents and vice versa. why is that python doesnt implement direct memory addressing provided a reference to an object exists ? GC intercedes at various intervals when convenient. I don't think itwould be immediate though. what is the interval and what is its effect on the performance of python interpreter ? regards, KM -- http://mail.python.org/mailman/listinfo/python-list
Re: references and buffer()
km wrote: why is that python doesnt implement direct memory addressing provided a reference to an object exists ? because Python is a portable high-level language. if you want assembler, you shouldn't use Python. did you read the reset your brain article ? /F -- http://mail.python.org/mailman/listinfo/python-list
