Re: [Zope] persistence and dictionaries
Ok, here are some of the offending bits out of my boringplus product .... which I can send again if people want. I have another product that keeps a dictionary of cookies so I can see how many times someone uses the back button to access the same forms page again. This worked as expected, where I had to issue an _p_changed=1 to get them to persist. But I am doing a similar thing below, or so I thought, once with a dictionary in the product and once in a dictionary within a class that does not inherit Persistent. This is what is confusing. Through restarts etc, everything perisists class Boring( OFS.SimpleItem.Item, # A simple Principia object. Not Folderish. Persistent, # Make us persistent. Yaah! Acquisition.Implicit, # Uh, whatever. AccessControl.Role.RoleManager # Security manager. ): def __init__(self, id, title=''): self._things = {} self._more = myClass.MoreStuff() def manage_edit(self, title, REQUEST=None): if REQUEST is not None: if REQUEST.has_key('thing'): self._things[REQUEST['thing']] = REQUEST['thing_value'] self.addToMore(REQUEST['thing'],REQUEST['thing_value']) def addToMore(self,name,value): self._more.addStuff(name,value) return "" class MoreStuff: def __init__(self): self._stuff = {'animal':'monkey'} def addStuff(self,stuff_name,stuff_item): On Sat, 09 Dec 2000, Jonothan Farr wrote:
Maybe I'm mistaken, but it seems like you can put an instance of an object that doesn't inherit from Persistent into the ZODB just fine, but its contents won't persist, so you'll always end up with a copy of the object as it was first added to the database.
--jfarr
----- Original Message ----- From: "Chris McDonough" <chrism@digicool.com> To: <matt.bion@eudoramail.com> Cc: <zope@zope.org> Sent: Friday, December 08, 2000 11:57 AM Subject: Re: [Zope] persistence and dictionaries
Huh. If they do, it's by chance only. I'd be hard-pressed to explain it.
Do they inherit from *anything*?
----- Original Message ----- From: "Matt" <matt.bion@eudoramail.com> To: "Chris McDonough" <chrism@digicool.com> Cc: <zope@zope.org> Sent: Friday, December 08, 2000 2:30 PM Subject: Re: [Zope] persistence and dictionaries
Chris, this was my original confusion .... the two places below where you say
"You can put instances which do not inherit from Persistence.Persistent in your database. They just won't "stick". They'll hang around until the server is restarted or for an undetermined amount of time during normal operations."
"No. It'll work for "a while" or until the server is restarted. :-)"
actually do persist after restarts ... that's what confused me, they wouldn't go away and they should!!
regards Matt
Chris McDonough wrote:
Thanks for the reply, that is really useful. There are a couple of things though that still don't add up. Firstly, you say below, as do all the ZODB documents that "Custom" classes can certainly persist, they just need to mix in the "Persistence.Persistent" class as a base class. Well, in my example I attached in my first email, my product certainly has Persistence.Persistent, but my second class that I add to this one does not, yet it still persists. There was an email sometime ago on the mailing list that told someone that this was why their product instances disappearing from the ZODB. (the ref for the original email is : http://www.egroups.com/message/zope/44263 ... I can't find the reply again.)
You can put instances which do not inherit from Persistence.Persistent in your database. They just won't "stick". They'll hang around until the server is restarted or for an undetermined amount of time during normal operations.
So my current understanding would be that any classes you want to add in do not need to derive from Persistence.Persistent, and if it is pickleable then all should be fine if you call on instances of that object within you product.
No. It'll work for "a while" or until the server is restarted. :-)
The next part that worried me came from the "python product tutorial" http://www.zope.org/Members/hathawsh/PythonProductTutorial
This stated that the class dictionary self.votes = {} needed to be changed to self._votes = Globals.PersistentMapping() so that updates to it persist. Hence my query about dictionaries.
This was for convenience, I'd imagine.
I also noticed your comment about __setstate__ . What is it about this that is dangerous.
Nothing implicitly dangerous, but it can get confusing if you have multiple revisions of your product and you use variables caused by __setstate__. Also, once you add a __setstate__ which modifies the object in-place, there's a likelihood that it can never go away (you're can never be sure if all instances have been updated).
Recently I built a product out of some python classes I wrapped around 4DOM, and since 4DOM documents do not seem to persist(well the document does, but it loses all its children), then I persisted them to the local file system, since I needed to do that anyway for what I was doing.
Setstate
seemed
to work nicely to bring them back, though watching its behaviour I noticed that it was called very often by zope.
Sure, that works... although at that point you're creating your own object database. :-)
Chris McDonough wrote:
All pickleable Python primitive types (strings,
dictionaries, lists, Nones,
integers, floats, longs, etc.) can live in the ZODB. They can persist just like instances that inherit from the Persistent class.
I think you read a little too much in to the fact that you need to "treat mutable objects immutably" when working with them in the ZODB. This statement doesn't mean that these kinds of objects can't be saved in the ZODB, it just means you need to treat them specially when putting them in the database.
For instance, if you were doing this inside of an external method:
def amethod(self): self.mydict = {} self.mydict['a'] = 1
(where self is the persistent object that is usually the external method's "container")
It wouldn't work as you expected. Although you'd see an 'a' in mydict for a little while in further accesses to it, 'mydict' would eventaully show up as an empty dictionary on the first access of it after it was expired from the RAM cache (after it was 'ghosted'), because the last thing that the ZODB "saw" (via the __setattr__ on 'self' and a subsequent transaction) was you setting a empty dictionary.
Persistent objects (like "self" in the above example) are only smart enough to notice changes to themselves that happen through their __setattr__ (e.g. self.mydict = {} calls self's __setattr__). Mutating the attribute 'mydict' above "in-place" (via self.mydict['a'] = 1) does not trigger self's __setattr__, so the ZODB never notices that "mydict" got changed.
There are two ways to handle this. The first is to treat mutable attributes "immutably" via assigning to a temporary variable and then making sure the persistent container's __setattr__ gets called:
def amethod(self): dict = {} dict['a'] = 1 self.mydict = dict # trigger persistence mechanism implicitly
The second is to use the _p_changed attribute of the persistent object on which the primitive is set. This explcitly tells the persistence system to include the object on which it's set into the current transaction:
def amethod(self): self.mydict = {} self.mydict['a'] = 1 self._p_changed = 1 # trigger persistence mechanism manually
Variations on this theme extend to list methods too (e.g. list.append, list.pop, etc.)
"Custom" classes can certainly persist, they just need to mix in the "Persistence.Persistent" class as a base class.
As long as you obey this (slightly annoying, but necessary) rule, you shouldn't need to use PersistentMapping (it's really just a convenient wrapper that does this "magic" for you) or make any other wrappers for other mutable objects.
I don't know why you're using __setstate__, but ummmm.. I won't go there. :-) I didn't look at your product, but I don't think I need to to answer your question...
> Hi I am trying to get a handle on how I should handle peristence in my > python products. I have read all the ZODB documents and all the Product > tutorials, which all led me to believe that 1) mutable objects such as > lists and dictionaries are not persistent and that updates to these will > not be implicitly saved into the ZODB, and 2) that custom classes would > certainly not persist. That all seemed fine, and I used persitent > mapping and __setstate__ to fill things back in where necessary. But > then I decided to demonstrate that persitence does break as suggested. > > I used boring product, added a dicitonary and a custom class that > contains it's own dictionary.... let the user update name : value pairs > for both, and print the contents through index.dtml > > The problem is that everything persists fine !!!! through
restarts,
> everything? > > Why does it work??? shouldn't it not? > > I have attached the modified boring product .... boringplus ..... it's > dead simple to follow if you have made products before. > > Any explanation would be really nice. > > regards > Matt > >
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Oops. You snipped the contents of MoreStuff.addStuff(). No way to tell what's going on without that! --jfarr ----- Original Message ----- From: "Matt" <matt.bion@eudoramail.com> To: "Jonothan Farr" <jfarr@real.com>; <zope@zope.org> Sent: Friday, December 08, 2000 2:39 PM Subject: Re: [Zope] persistence and dictionaries
Ok, here are some of the offending bits out of my boringplus product .... which I can send again if people want. I have another product that keeps a dictionary of cookies so I can see how many times someone uses the back button to access the same forms page again. This worked as expected, where I had to issue an _p_changed=1 to get them to persist. But I am doing a similar thing below, or so I thought, once with a dictionary in the product and once in a dictionary within a class that does not inherit Persistent. This is what is confusing. Through restarts etc, everything perisists
class Boring( OFS.SimpleItem.Item, # A simple Principia object. Not Folderish. Persistent, # Make us persistent. Yaah! Acquisition.Implicit, # Uh, whatever. AccessControl.Role.RoleManager # Security manager. ):
def __init__(self, id, title=''): self._things = {} self._more = myClass.MoreStuff()
def manage_edit(self, title, REQUEST=None): if REQUEST is not None: if REQUEST.has_key('thing'): self._things[REQUEST['thing']] = REQUEST['thing_value'] self.addToMore(REQUEST['thing'],REQUEST['thing_value'])
def addToMore(self,name,value): self._more.addStuff(name,value) return ""
class MoreStuff:
def __init__(self): self._stuff = {'animal':'monkey'}
def addStuff(self,stuff_name,stuff_item):
On Sat, 09 Dec 2000, Jonothan Farr wrote:
Maybe I'm mistaken, but it seems like you can put an instance of an object that doesn't inherit from Persistent into the ZODB just fine, but its contents won't persist, so you'll always end up with a copy of the object as it was first added to the database.
--jfarr
----- Original Message ----- From: "Chris McDonough" <chrism@digicool.com> To: <matt.bion@eudoramail.com> Cc: <zope@zope.org> Sent: Friday, December 08, 2000 11:57 AM Subject: Re: [Zope] persistence and dictionaries
Huh. If they do, it's by chance only. I'd be hard-pressed to explain it.
Do they inherit from *anything*?
----- Original Message ----- From: "Matt" <matt.bion@eudoramail.com> To: "Chris McDonough" <chrism@digicool.com> Cc: <zope@zope.org> Sent: Friday, December 08, 2000 2:30 PM Subject: Re: [Zope] persistence and dictionaries
Chris, this was my original confusion .... the two places below where you say
"You can put instances which do not inherit from Persistence.Persistent in your database. They just won't "stick". They'll hang around until the server is restarted or for an undetermined amount of time during normal operations."
"No. It'll work for "a while" or until the server is restarted. :-)"
actually do persist after restarts ... that's what confused me, they wouldn't go away and they should!!
regards Matt
Chris McDonough wrote:
Thanks for the reply, that is really useful. There are a couple of things though that still don't add up. Firstly, you say below, as do all the ZODB documents that "Custom" classes can certainly persist, they just need to mix in the "Persistence.Persistent" class as a base class. Well, in my example I attached in my first email, my product certainly has Persistence.Persistent, but my second class that I add to this one does not, yet it still persists. There was an email sometime ago on the mailing list that told someone that this was why their product instances disappearing from the ZODB. (the ref for the original email is : http://www.egroups.com/message/zope/44263 ... I can't find the reply again.)
You can put instances which do not inherit from Persistence.Persistent in your database. They just won't "stick". They'll hang around until the server is restarted or for an undetermined amount of time during normal operations.
So my current understanding would be that any classes you want to add in do not need to derive from Persistence.Persistent, and if it is pickleable then all should be fine if you call on instances of that object within you product.
No. It'll work for "a while" or until the server is restarted. :-)
The next part that worried me came from the "python product tutorial" http://www.zope.org/Members/hathawsh/PythonProductTutorial
This stated that the class dictionary self.votes = {} needed to be changed to self._votes = Globals.PersistentMapping() so that updates to it persist. Hence my query about dictionaries.
This was for convenience, I'd imagine.
I also noticed your comment about __setstate__ . What is it about this that is dangerous.
Nothing implicitly dangerous, but it can get confusing if you have multiple revisions of your product and you use variables caused by __setstate__. Also, once you add a __setstate__ which modifies the object in-place, there's a likelihood that it can never go away (you're can never be sure if all instances have been updated).
Recently I built a product out of some python classes I wrapped around 4DOM, and since 4DOM documents do not seem to persist(well the document does, but it loses all its children), then I persisted them to the local file system, since I needed to do that anyway for what I was doing.
Setstate
seemed
to work nicely to bring them back, though watching its behaviour I noticed that it was called very often by zope.
Sure, that works... although at that point you're creating your own object database. :-)
Chris McDonough wrote:
> All pickleable Python primitive types (strings,
dictionaries, lists, Nones,
> integers, floats, longs, etc.) can live in the ZODB. They can persist just > like instances that inherit from the Persistent class. > > I think you read a little too much in to the fact that you need to "treat > mutable objects immutably" when working with them in the ZODB. This > statement doesn't mean that these kinds of objects can't be saved in the > ZODB, it just means you need to treat them specially when putting them in > the database. > > For instance, if you were doing this inside of an external method: > > def amethod(self): > self.mydict = {} > self.mydict['a'] = 1 > > (where self is the persistent object that is usually the external method's > "container") > > It wouldn't work as you expected. Although you'd see an 'a' in mydict for a > little while in further accesses to it, 'mydict' would eventaully show up as > an empty dictionary on the first access of it after it was expired from the > RAM cache (after it was 'ghosted'), because the last thing that the ZODB > "saw" (via the __setattr__ on 'self' and a subsequent transaction) was you > setting a empty dictionary. > > Persistent objects (like "self" in the above example) are only smart enough > to notice changes to themselves that happen through their __setattr__ (e.g. > self.mydict = {} calls self's __setattr__). Mutating the attribute 'mydict' > above "in-place" (via self.mydict['a'] = 1) does not trigger self's > __setattr__, so the ZODB never notices that "mydict" got changed. > > There are two ways to handle this. The first is to treat mutable attributes > "immutably" via assigning to a temporary variable and then making sure the > persistent container's __setattr__ gets called: > > def amethod(self): > dict = {} > dict['a'] = 1 > self.mydict = dict # trigger persistence mechanism implicitly > > The second is to use the _p_changed attribute of the persistent object on > which the primitive is set. This explcitly tells the persistence system to > include the object on which it's set into the current transaction: > > def amethod(self): > self.mydict = {} > self.mydict['a'] = 1 > self._p_changed = 1 # trigger persistence mechanism manually > > Variations on this theme extend to list methods too (e.g. list.append, > list.pop, etc.) > > "Custom" classes can certainly persist, they just need to mix in the > "Persistence.Persistent" class as a base class. > > As long as you obey this (slightly annoying, but necessary) rule, you > shouldn't need to use PersistentMapping (it's really just a convenient > wrapper that does this "magic" for you) or make any other wrappers for other > mutable objects. > > I don't know why you're using __setstate__, but ummmm.. I won't go there. > :-) I didn't look at your product, but I don't think I need to to answer > your question... > > > Hi I am trying to get a handle on how I should handle peristence in my > > python products. I have read all the ZODB documents and all the Product > > tutorials, which all led me to believe that 1) mutable objects such as > > lists and dictionaries are not persistent and that updates to these will > > not be implicitly saved into the ZODB, and 2) that custom classes would > > certainly not persist. That all seemed fine, and I used persitent > > mapping and __setstate__ to fill things back in where necessary. But > > then I decided to demonstrate that persitence does break as suggested. > > > > I used boring product, added a dicitonary and a custom class that > > contains it's own dictionary.... let the user update name : value pairs > > for both, and print the contents through index.dtml > > > > The problem is that everything persists fine !!!! through
restarts,
> > everything? > > > > Why does it work??? shouldn't it not? > > > > I have attached the modified boring product .... boringplus ..... it's > > dead simple to follow if you have made products before. > > > > Any explanation would be really nice. > > > > regards > > Matt > > > >
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um, nope, that's all there is to that class .... I'll attach the whole product Matt Jonothan Farr wrote:
Oops. You snipped the contents of MoreStuff.addStuff(). No way to tell what's going on without that! --jfarr
----- Original Message ----- From: "Matt" <matt.bion@eudoramail.com> To: "Jonothan Farr" <jfarr@real.com>; <zope@zope.org> Sent: Friday, December 08, 2000 2:39 PM Subject: Re: [Zope] persistence and dictionaries
Ok, here are some of the offending bits out of my boringplus product .... which I can send again if people want. I have another product that keeps a dictionary of cookies so I can see how many times someone uses the back button to access the same forms page again. This worked as expected, where I had to issue an _p_changed=1 to get them to persist. But I am doing a similar thing below, or so I thought, once with a dictionary in the product and once in a dictionary within a class that does not inherit Persistent. This is what is confusing. Through restarts etc, everything perisists
class Boring( OFS.SimpleItem.Item, # A simple Principia object. Not Folderish. Persistent, # Make us persistent. Yaah! Acquisition.Implicit, # Uh, whatever. AccessControl.Role.RoleManager # Security manager. ):
def __init__(self, id, title=''): self._things = {} self._more = myClass.MoreStuff()
def manage_edit(self, title, REQUEST=None): if REQUEST is not None: if REQUEST.has_key('thing'): self._things[REQUEST['thing']] = REQUEST['thing_value'] self.addToMore(REQUEST['thing'],REQUEST['thing_value'])
def addToMore(self,name,value): self._more.addStuff(name,value) return ""
class MoreStuff:
def __init__(self): self._stuff = {'animal':'monkey'}
def addStuff(self,stuff_name,stuff_item):
On Sat, 09 Dec 2000, Jonothan Farr wrote:
Maybe I'm mistaken, but it seems like you can put an instance of an object that doesn't inherit from Persistent into the ZODB just fine, but its contents won't persist, so you'll always end up with a copy of the object as it was first added to the database.
--jfarr
----- Original Message ----- From: "Chris McDonough" <chrism@digicool.com> To: <matt.bion@eudoramail.com> Cc: <zope@zope.org> Sent: Friday, December 08, 2000 11:57 AM Subject: Re: [Zope] persistence and dictionaries
Huh. If they do, it's by chance only. I'd be hard-pressed to explain it.
Do they inherit from *anything*?
----- Original Message ----- From: "Matt" <matt.bion@eudoramail.com> To: "Chris McDonough" <chrism@digicool.com> Cc: <zope@zope.org> Sent: Friday, December 08, 2000 2:30 PM Subject: Re: [Zope] persistence and dictionaries
Chris, this was my original confusion .... the two places below where you say
"You can put instances which do not inherit from Persistence.Persistent in your database. They just won't "stick". They'll hang around until the server is restarted or for an undetermined amount of time during normal operations."
"No. It'll work for "a while" or until the server is restarted. :-)"
actually do persist after restarts ... that's what confused me, they wouldn't go away and they should!!
regards Matt
Chris McDonough wrote:
> Thanks for the reply, that is really useful. There are a couple of things > though that still don't add up. Firstly, you say below, as do all the ZODB > documents that "Custom" classes can certainly persist, they just need to mix in > the "Persistence.Persistent" class as a base class. Well, in my example I > attached in my first email, my product certainly has Persistence.Persistent, > but my second class that I add to this one does not, yet it still persists. > There was an email sometime ago on the mailing list that told someone that this > was why their product instances disappearing from the ZODB. > (the ref for the original email is : http://www.egroups.com/message/zope/44263 > ... I can't find the reply again.) >
You can put instances which do not inherit from Persistence.Persistent in your database. They just won't "stick". They'll hang around until the server is restarted or for an undetermined amount of time during normal operations.
> So my current understanding would be that any classes you want to add in do not > need to derive from Persistence.Persistent, and if it is pickleable then all > should be fine if you call on instances of that object within you product.
No. It'll work for "a while" or until the server is restarted. :-)
> The next part that worried me came from the "python product tutorial" > http://www.zope.org/Members/hathawsh/PythonProductTutorial > > This stated that the class dictionary self.votes = {} needed to be changed to > self._votes = Globals.PersistentMapping() so that updates to it persist. > Hence my query about dictionaries.
This was for convenience, I'd imagine.
> I also noticed your comment about __setstate__ . What is it about this that is > dangerous.
Nothing implicitly dangerous, but it can get confusing if you have multiple revisions of your product and you use variables caused by __setstate__. Also, once you add a __setstate__ which modifies the object in-place, there's a likelihood that it can never go away (you're can never be sure if all instances have been updated).
> Recently I built a product out of some python classes I wrapped > around 4DOM, and since 4DOM documents do not seem to persist(well the document > does, but it loses all its children), then I persisted them to the local file > system, since I needed to do that anyway for what I was doing.
Setstate
seemed > to work nicely to bring them back, though watching its behaviour I noticed that > it was called very often by zope.
Sure, that works... although at that point you're creating your own object database. :-)
> > Chris McDonough wrote: > > > All pickleable Python primitive types (strings, dictionaries, lists, Nones, > > integers, floats, longs, etc.) can live in the ZODB. They can persist just > > like instances that inherit from the Persistent class. > > > > I think you read a little too much in to the fact that you need to "treat > > mutable objects immutably" when working with them in the ZODB. This > > statement doesn't mean that these kinds of objects can't be saved in the > > ZODB, it just means you need to treat them specially when putting them in > > the database. > > > > For instance, if you were doing this inside of an external method: > > > > def amethod(self): > > self.mydict = {} > > self.mydict['a'] = 1 > > > > (where self is the persistent object that is usually the external method's > > "container") > > > > It wouldn't work as you expected. Although you'd see an 'a' in mydict for a > > little while in further accesses to it, 'mydict' would eventaully show up as > > an empty dictionary on the first access of it after it was expired from the > > RAM cache (after it was 'ghosted'), because the last thing that the ZODB > > "saw" (via the __setattr__ on 'self' and a subsequent transaction) was you > > setting a empty dictionary. > > > > Persistent objects (like "self" in the above example) are only smart enough > > to notice changes to themselves that happen through their __setattr__ (e.g. > > self.mydict = {} calls self's __setattr__). Mutating the attribute 'mydict' > > above "in-place" (via self.mydict['a'] = 1) does not trigger self's > > __setattr__, so the ZODB never notices that "mydict" got changed. > > > > There are two ways to handle this. The first is to treat mutable attributes > > "immutably" via assigning to a temporary variable and then making sure the > > persistent container's __setattr__ gets called: > > > > def amethod(self): > > dict = {} > > dict['a'] = 1 > > self.mydict = dict # trigger persistence mechanism implicitly > > > > The second is to use the _p_changed attribute of the persistent object on > > which the primitive is set. This explcitly tells the persistence system to > > include the object on which it's set into the current transaction: > > > > def amethod(self): > > self.mydict = {} > > self.mydict['a'] = 1 > > self._p_changed = 1 # trigger persistence mechanism manually > > > > Variations on this theme extend to list methods too (e.g. list.append, > > list.pop, etc.) > > > > "Custom" classes can certainly persist, they just need to mix in the > > "Persistence.Persistent" class as a base class. > > > > As long as you obey this (slightly annoying, but necessary) rule, you > > shouldn't need to use PersistentMapping (it's really just a convenient > > wrapper that does this "magic" for you) or make any other wrappers for other > > mutable objects. > > > > I don't know why you're using __setstate__, but ummmm.. I won't go there. > > :-) I didn't look at your product, but I don't think I need to to answer > > your question... > > > > > Hi I am trying to get a handle on how I should handle peristence in my > > > python products. I have read all the ZODB documents and all the Product > > > tutorials, which all led me to believe that 1) mutable objects such as > > > lists and dictionaries are not persistent and that updates to these will > > > not be implicitly saved into the ZODB, and 2) that custom classes would > > > certainly not persist. That all seemed fine, and I used persitent > > > mapping and __setstate__ to fill things back in where necessary. But > > > then I decided to demonstrate that persitence does break as suggested. > > > > > > I used boring product, added a dicitonary and a custom class that > > > contains it's own dictionary.... let the user update name : value pairs > > > for both, and print the contents through index.dtml > > > > > > The problem is that everything persists fine !!!! through
restarts,
> > > everything? > > > > > > Why does it work??? shouldn't it not? > > > > > > I have attached the modified boring product .... boringplus ..... it's > > > dead simple to follow if you have made products before. > > > > > > Any explanation would be really nice. > > > > > > regards > > > Matt > > > > > > > >
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I tried a few things out it seems the following is happening : the methods that seemed to be activating persistence when I thought they shouldn't are called through my handle to the manage_edit ... i.e. in the product I sent this was anything called within def manage_edit(self, title, REQUEST=None): blah If I call the same methods from another method in the product, and then call that method from say a dtmlMethod then persotence failed unless I use _p_changed. This would go along with what Chris mentioned. So it seems that def manage_edit has a special behaviour, possible linked to it's binding to manage_main = HTMLFile('boringEdit', globals()) # Management Interface which is linked to manage_options = ( {'label': 'Edit', 'action': 'manage_main'}, hopefully someone has an answer to that. regards Matt Jonothan Farr wrote:
Oops. You snipped the contents of MoreStuff.addStuff(). No way to tell what's going on without that! --jfarr
----- Original Message ----- From: "Matt" <matt.bion@eudoramail.com> To: "Jonothan Farr" <jfarr@real.com>; <zope@zope.org> Sent: Friday, December 08, 2000 2:39 PM Subject: Re: [Zope] persistence and dictionaries
Ok, here are some of the offending bits out of my boringplus product .... which I can send again if people want. I have another product that keeps a dictionary of cookies so I can see how many times someone uses the back button to access the same forms page again. This worked as expected, where I had to issue an _p_changed=1 to get them to persist. But I am doing a similar thing below, or so I thought, once with a dictionary in the product and once in a dictionary within a class that does not inherit Persistent. This is what is confusing. Through restarts etc, everything perisists
class Boring( OFS.SimpleItem.Item, # A simple Principia object. Not Folderish. Persistent, # Make us persistent. Yaah! Acquisition.Implicit, # Uh, whatever. AccessControl.Role.RoleManager # Security manager. ):
def __init__(self, id, title=''): self._things = {} self._more = myClass.MoreStuff()
def manage_edit(self, title, REQUEST=None): if REQUEST is not None: if REQUEST.has_key('thing'): self._things[REQUEST['thing']] = REQUEST['thing_value'] self.addToMore(REQUEST['thing'],REQUEST['thing_value'])
def addToMore(self,name,value): self._more.addStuff(name,value) return ""
class MoreStuff:
def __init__(self): self._stuff = {'animal':'monkey'}
def addStuff(self,stuff_name,stuff_item):
On Sat, 09 Dec 2000, Jonothan Farr wrote:
Maybe I'm mistaken, but it seems like you can put an instance of an object that doesn't inherit from Persistent into the ZODB just fine, but its contents won't persist, so you'll always end up with a copy of the object as it was first added to the database.
--jfarr
----- Original Message ----- From: "Chris McDonough" <chrism@digicool.com> To: <matt.bion@eudoramail.com> Cc: <zope@zope.org> Sent: Friday, December 08, 2000 11:57 AM Subject: Re: [Zope] persistence and dictionaries
Huh. If they do, it's by chance only. I'd be hard-pressed to explain it.
Do they inherit from *anything*?
----- Original Message ----- From: "Matt" <matt.bion@eudoramail.com> To: "Chris McDonough" <chrism@digicool.com> Cc: <zope@zope.org> Sent: Friday, December 08, 2000 2:30 PM Subject: Re: [Zope] persistence and dictionaries
Chris, this was my original confusion .... the two places below where you say
"You can put instances which do not inherit from Persistence.Persistent in your database. They just won't "stick". They'll hang around until the server is restarted or for an undetermined amount of time during normal operations."
"No. It'll work for "a while" or until the server is restarted. :-)"
actually do persist after restarts ... that's what confused me, they wouldn't go away and they should!!
regards Matt
Chris McDonough wrote:
> Thanks for the reply, that is really useful. There are a couple of things > though that still don't add up. Firstly, you say below, as do all the ZODB > documents that "Custom" classes can certainly persist, they just need to mix in > the "Persistence.Persistent" class as a base class. Well, in my example I > attached in my first email, my product certainly has Persistence.Persistent, > but my second class that I add to this one does not, yet it still persists. > There was an email sometime ago on the mailing list that told someone that this > was why their product instances disappearing from the ZODB. > (the ref for the original email is : http://www.egroups.com/message/zope/44263 > ... I can't find the reply again.) >
You can put instances which do not inherit from Persistence.Persistent in your database. They just won't "stick". They'll hang around until the server is restarted or for an undetermined amount of time during normal operations.
> So my current understanding would be that any classes you want to add in do not > need to derive from Persistence.Persistent, and if it is pickleable then all > should be fine if you call on instances of that object within you product.
No. It'll work for "a while" or until the server is restarted. :-)
> The next part that worried me came from the "python product tutorial" > http://www.zope.org/Members/hathawsh/PythonProductTutorial > > This stated that the class dictionary self.votes = {} needed to be changed to > self._votes = Globals.PersistentMapping() so that updates to it persist. > Hence my query about dictionaries.
This was for convenience, I'd imagine.
> I also noticed your comment about __setstate__ . What is it about this that is > dangerous.
Nothing implicitly dangerous, but it can get confusing if you have multiple revisions of your product and you use variables caused by __setstate__. Also, once you add a __setstate__ which modifies the object in-place, there's a likelihood that it can never go away (you're can never be sure if all instances have been updated).
> Recently I built a product out of some python classes I wrapped > around 4DOM, and since 4DOM documents do not seem to persist(well the document > does, but it loses all its children), then I persisted them to the local file > system, since I needed to do that anyway for what I was doing.
Setstate
seemed > to work nicely to bring them back, though watching its behaviour I noticed that > it was called very often by zope.
Sure, that works... although at that point you're creating your own object database. :-)
> > Chris McDonough wrote: > > > All pickleable Python primitive types (strings, dictionaries, lists, Nones, > > integers, floats, longs, etc.) can live in the ZODB. They can persist just > > like instances that inherit from the Persistent class. > > > > I think you read a little too much in to the fact that you need to "treat > > mutable objects immutably" when working with them in the ZODB. This > > statement doesn't mean that these kinds of objects can't be saved in the > > ZODB, it just means you need to treat them specially when putting them in > > the database. > > > > For instance, if you were doing this inside of an external method: > > > > def amethod(self): > > self.mydict = {} > > self.mydict['a'] = 1 > > > > (where self is the persistent object that is usually the external method's > > "container") > > > > It wouldn't work as you expected. Although you'd see an 'a' in mydict for a > > little while in further accesses to it, 'mydict' would eventaully show up as > > an empty dictionary on the first access of it after it was expired from the > > RAM cache (after it was 'ghosted'), because the last thing that the ZODB > > "saw" (via the __setattr__ on 'self' and a subsequent transaction) was you > > setting a empty dictionary. > > > > Persistent objects (like "self" in the above example) are only smart enough > > to notice changes to themselves that happen through their __setattr__ (e.g. > > self.mydict = {} calls self's __setattr__). Mutating the attribute 'mydict' > > above "in-place" (via self.mydict['a'] = 1) does not trigger self's > > __setattr__, so the ZODB never notices that "mydict" got changed. > > > > There are two ways to handle this. The first is to treat mutable attributes > > "immutably" via assigning to a temporary variable and then making sure the > > persistent container's __setattr__ gets called: > > > > def amethod(self): > > dict = {} > > dict['a'] = 1 > > self.mydict = dict # trigger persistence mechanism implicitly > > > > The second is to use the _p_changed attribute of the persistent object on > > which the primitive is set. This explcitly tells the persistence system to > > include the object on which it's set into the current transaction: > > > > def amethod(self): > > self.mydict = {} > > self.mydict['a'] = 1 > > self._p_changed = 1 # trigger persistence mechanism manually > > > > Variations on this theme extend to list methods too (e.g. list.append, > > list.pop, etc.) > > > > "Custom" classes can certainly persist, they just need to mix in the > > "Persistence.Persistent" class as a base class. > > > > As long as you obey this (slightly annoying, but necessary) rule, you > > shouldn't need to use PersistentMapping (it's really just a convenient > > wrapper that does this "magic" for you) or make any other wrappers for other > > mutable objects. > > > > I don't know why you're using __setstate__, but ummmm.. I won't go there. > > :-) I didn't look at your product, but I don't think I need to to answer > > your question... > > > > > Hi I am trying to get a handle on how I should handle peristence in my > > > python products. I have read all the ZODB documents and all the Product > > > tutorials, which all led me to believe that 1) mutable objects such as > > > lists and dictionaries are not persistent and that updates to these will > > > not be implicitly saved into the ZODB, and 2) that custom classes would > > > certainly not persist. That all seemed fine, and I used persitent > > > mapping and __setstate__ to fill things back in where necessary. But > > > then I decided to demonstrate that persitence does break as suggested. > > > > > > I used boring product, added a dicitonary and a custom class that > > > contains it's own dictionary.... let the user update name : value pairs > > > for both, and print the contents through index.dtml > > > > > > The problem is that everything persists fine !!!! through
restarts,
> > > everything? > > > > > > Why does it work??? shouldn't it not? > > > > > > I have attached the modified boring product .... boringplus ..... it's > > > dead simple to follow if you have made products before. > > > > > > Any explanation would be really nice. > > > > > > regards > > > Matt > > > > > > > >
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participants (2)
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Jonothan Farr -
Matt