Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Programming > Python > pass object or use self.object?

Reply
Thread Tools

pass object or use self.object?

 
 
Tim Arnold
Guest
Posts: n/a
 
      04-06-2010
Hi,
I have a few classes that manipulate documents. One is really a
process that I use a class for just to bundle a bunch of functions
together (and to keep my call signatures the same for each of my
manipulator classes).

So my question is whether it's bad practice to set things up so each
method operates on self.document or should I pass document around from
one function to the next?
pseudo code:

class ManipulatorA(object):
def process(self, document):
document = self.do_one_thing(document)
document = self.do_another_thing(document)
# bunch of similar lines
return document

or

class ManipulatorA(object):
def process(self, document):
self.document = document
self.do_one_thing() # operates on self.document
self.do_another_thing()
# bunch of similar lines
return self.document

I ask because I've been told that the first case is easier to
understand. I never thought of it before, so I'd appreciate any
comments.
thanks,
--Tim
 
Reply With Quote
 
 
 
 
Bruno Desthuilliers
Guest
Posts: n/a
 
      04-06-2010
Tim Arnold a écrit :
> Hi,
> I have a few classes that manipulate documents. One is really a
> process that I use a class for just to bundle a bunch of functions
> together (and to keep my call signatures the same for each of my
> manipulator classes).
>
> So my question is whether it's bad practice to set things up so each
> method operates on self.document or should I pass document around from
> one function to the next?


As far as I'm concerned, I strongly prefer passing the document around.
Makes thing clear, avoids useless preconditions (is self.document set
???) and race conditions (if two threads have to share the Manipulator
instance), makes the code easier to understand / maintain / refactor IMHO.

Also remember that modules are objects too, so - depending on parts of
your code we don't see here - you may even maintain your API without
having to use a "class as module".

My 2 cents
 
Reply With Quote
 
 
 
 
Jean-Michel Pichavant
Guest
Posts: n/a
 
      04-06-2010
Tim Arnold wrote:
> Hi,
> I have a few classes that manipulate documents. One is really a
> process that I use a class for just to bundle a bunch of functions
> together (and to keep my call signatures the same for each of my
> manipulator classes).
>
> So my question is whether it's bad practice to set things up so each
> method operates on self.document or should I pass document around from
> one function to the next?
> pseudo code:
>
> class ManipulatorA(object):
> def process(self, document):
> document = self.do_one_thing(document)
> document = self.do_another_thing(document)
> # bunch of similar lines
> return document
>
> or
>
> class ManipulatorA(object):
> def process(self, document):
> self.document = document
> self.do_one_thing() # operates on self.document
> self.do_another_thing()
> # bunch of similar lines
> return self.document
>
> I ask because I've been told that the first case is easier to
> understand. I never thought of it before, so I'd appreciate any
> comments.
> thanks,
> --Tim
>

Usually, when using classes as namespace, functions are declared as
static (or as classmethod if required).
e.g.


class Foo:
@classmethod
def process(cls, document):
print 'process of'
cls.foo(document)

@staticmethod
def foo(document):
print document

In [5]: Foo.process('my document')
process of
my document


There is no more question about self, 'cause there is no more self. You
don't need to create any instance of Foo neither.

JM

 
Reply With Quote
 
Lie Ryan
Guest
Posts: n/a
 
      04-06-2010
On 04/06/10 23:52, Tim Arnold wrote:
> Hi,
> I have a few classes that manipulate documents. One is really a
> process that I use a class for just to bundle a bunch of functions
> together (and to keep my call signatures the same for each of my
> manipulator classes).
>
> So my question is whether it's bad practice to set things up so each
> method operates on self.document or should I pass document around from
> one function to the next?
> pseudo code:
>
> class ManipulatorA(object):
> def process(self, document):
> document = self.do_one_thing(document)
> document = self.do_another_thing(document)
> # bunch of similar lines
> return document
>
> or
>
> class ManipulatorA(object):
> def process(self, document):
> self.document = document
> self.do_one_thing() # operates on self.document
> self.do_another_thing()
> # bunch of similar lines
> return self.document


Since in function in python is a first-class object, you can instead do
something like:

def process(document):
# note: document should encapsulate its own logic
document.do_one_thing()
document.do_another_thing()

And when you need some complex logic, you can easily elevate your
function to a class:

class Appender(object):
def __init__(self, text):
self.text = text
def __call__(self, document):
mtext = self.manipulate(document, text)
document.append(mtext)

and I think for your purpose, the mixin pattern could cleanly separate
manipulation and document while still obeying object-oriented pattern
that document is self-sufficient:

# language with only single-inheritance can only dream to do this
class Appendable(object):
def append(self, text):
self.text += text
class Savable(object):
def save(self, fileobj):
fileobj.write(self.text)
class Openable(object):
def open(self, fileobj):
self.text = fileobj.read()
class Document(Appendable, Savable, Openable):
def __init__(self):
self.text = ''
 
Reply With Quote
 
Bruno Desthuilliers
Guest
Posts: n/a
 
      04-07-2010
Lie Ryan a écrit :
(snip)

> Since in function in python is a first-class object, you can instead do
> something like:
>
> def process(document):
> # note: document should encapsulate its own logic
> document.do_one_thing()


Obvious case of encapsulation abuse here. Should a file object
encapsulate all the csv parsing logic ? (and the html parsing, xml
parsing, image manipulation etc...) ? Should a "model" object
encapsulate the presentation logic ? I could go on for hours here...

>
> and I think for your purpose, the mixin pattern could cleanly separate
> manipulation and document while still obeying object-oriented pattern
> that document is self-sufficient:
>
> # language with only single-inheritance can only dream to do this
>
> class Appendable(object):
> def append(self, text):
> self.text += text
> class Savable(object):
> def save(self, fileobj):
> fileobj.write(self.text)
> class Openable(object):
> def open(self, fileobj):
> self.text = fileobj.read()
> class Document(Appendable, Savable, Openable):
> def __init__(self):
> self.text = ''


Anyone having enough experience with Zope2 knows why this sucks big time.
 
Reply With Quote
 
Tim Arnold
Guest
Posts: n/a
 
      04-07-2010
On Apr 6, 11:19*am, Jean-Michel Pichavant <(E-Mail Removed)>
wrote:
> Tim Arnold wrote:
> > Hi,
> > I have a few classes that manipulate documents. One is really a
> > process that I use a class for just to bundle a bunch of functions
> > together (and to keep my call signatures the same for each of my
> > manipulator classes).

>
> > So my question is whether it's bad practice to set things up so each
> > method operates on self.document or should I pass document around from
> > one function to the next?
> > pseudo code:

>
> > class ManipulatorA(object):
> > * * def process(self, document):
> > * * * * document = self.do_one_thing(document)
> > * * * * document = self.do_another_thing(document)
> > * * * * # bunch of similar lines
> > * * * * return document

>
> > or

>
> > class ManipulatorA(object):
> > * * def process(self, document):
> > * * * * self.document = document
> > * * * * self.do_one_thing() # operates on self.document
> > * * * * self.do_another_thing()
> > * * * * # bunch of similar lines
> > * * * * return self.document

>
> > I ask because I've been told that the first case is easier to
> > understand. I never thought of it before, so I'd appreciate any
> > comments.
> > thanks,
> > --Tim

>
> Usually, when using classes as namespace, functions are declared as
> static (or as classmethod if required).
> e.g.
>
> class Foo:
> * * @classmethod
> * * def process(cls, document):
> * * * * print 'process of'
> * * * * cls.foo(document)
>
> * * @staticmethod
> * * def foo(document):
> * * * * print document
>
> In [5]: Foo.process('my document')
> process of
> my document
>
> There is no more question about self, 'cause there is no more self. You
> don't need to create any instance of Foo neither.
>
> JM


Thanks for the input. I had always wondered about static methods; I'd
ask myself "why don't they just write a function in the first place?"

Now I see why. My situation poses a problem that I guess static
methods were invented to solve. And it settles the question about
using self.document since there is no longer any self. And as Bruno
says, it's easier to understand and refactor.

thanks,
--Tim
 
Reply With Quote
 
Lie Ryan
Guest
Posts: n/a
 
      04-07-2010
On 04/07/10 18:34, Bruno Desthuilliers wrote:
> Lie Ryan a écrit :
> (snip)
>
>> Since in function in python is a first-class object, you can instead do
>> something like:
>>
>> def process(document):
>> # note: document should encapsulate its own logic
>> document.do_one_thing()

>
> Obvious case of encapsulation abuse here. Should a file object
> encapsulate all the csv parsing logic ? (and the html parsing, xml
> parsing, image manipulation etc...) ? Should a "model" object
> encapsulate the presentation logic ? I could go on for hours here...


Yes, but no; you're taking it out of context. Is {csv|html|xml|image}
parsing logic a document's logic? Is presentation a document's logic? If
they're not, then they do not belong in document.
 
Reply With Quote
 
Bruno Desthuilliers
Guest
Posts: n/a
 
      04-08-2010
Lie Ryan a écrit :
> On 04/07/10 18:34, Bruno Desthuilliers wrote:
>> Lie Ryan a écrit :
>> (snip)
>>
>>> Since in function in python is a first-class object, you can instead do
>>> something like:
>>>
>>> def process(document):
>>> # note: document should encapsulate its own logic
>>> document.do_one_thing()

>> Obvious case of encapsulation abuse here. Should a file object
>> encapsulate all the csv parsing logic ? (and the html parsing, xml
>> parsing, image manipulation etc...) ? Should a "model" object
>> encapsulate the presentation logic ? I could go on for hours here...

>
> Yes, but no; you're taking it out of context. Is {csv|html|xml|image}
> parsing logic a document's logic? Is presentation a document's logic? If
> they're not, then they do not belong in document.


Is len() a list logic ? If yes, it should belong to list !-)

There are two points here : the first is that we (that is, at least, you
and me) just don't know enough about the OP's project to tell whether
something should belong to the document or not. period. The second point
is that objects don't live in a splendid isolation, and it's perfectly
ok to have code outside an object's method working on the object.

wrt/ these two points, your "document should encapsulate its own logic"
note seems a bit dogmatic (and not necessarily right) to me - hence my
answer.

 
Reply With Quote
 
Tim Arnold
Guest
Posts: n/a
 
      04-08-2010
On Apr 8, 4:20*am, Bruno Desthuilliers <bruno.
(E-Mail Removed)> wrote:
> Lie Ryan a écrit :
>
>
>
>
>
> > On 04/07/10 18:34, Bruno Desthuilliers wrote:
> >> Lie Ryan a écrit :
> >> (snip)

>
> >>> Since in function in python is a first-class object, you can instead do
> >>> something like:

>
> >>> def process(document):
> >>> * * # note: document should encapsulate its own logic
> >>> * * document.do_one_thing()
> >> Obvious case of encapsulation abuse here. Should a file object
> >> encapsulate all the csv parsing logic ? (and the html parsing, xml
> >> parsing, image manipulation etc...) ? Should a "model" object
> >> encapsulate the presentation logic ? I could go on for hours here...

>
> > Yes, but no; you're taking it out of context. Is {csv|html|xml|image}
> > parsing logic a document's logic? Is presentation a document's logic? If
> > they're not, then they do not belong in document.

>
> Is len() a list logic ? If yes, it should belong to list !-)
>
> There are two points here : the first is that we (that is, at least, you
> and me) just don't know enough about the OP's project to tell whether
> something should belong to the document or not. period. The second point
> is that objects don't live in a splendid isolation, and it's perfectly
> ok to have code outside an object's method working on the object.
>
> wrt/ these two points, your "document should encapsulate its own logic"
> note seems a bit dogmatic (and not necessarily right) to me - hence my
> answer.


The 'document' in this case is an lxml Elementtree, so I think it
makes sense to have code outside the object (e.g. static methods)
working on the object.
thanks,
--Tim
 
Reply With Quote
 
Lie Ryan
Guest
Posts: n/a
 
      04-09-2010
On 04/08/10 18:20, Bruno Desthuilliers wrote:
> Lie Ryan a écrit :
>> On 04/07/10 18:34, Bruno Desthuilliers wrote:
>>> Lie Ryan a écrit :
>>> (snip)
>>>
>>>> Since in function in python is a first-class object, you can instead do
>>>> something like:
>>>>
>>>> def process(document):
>>>> # note: document should encapsulate its own logic
>>>> document.do_one_thing()
>>> Obvious case of encapsulation abuse here. Should a file object
>>> encapsulate all the csv parsing logic ? (and the html parsing, xml
>>> parsing, image manipulation etc...) ? Should a "model" object
>>> encapsulate the presentation logic ? I could go on for hours here...

>>
>> Yes, but no; you're taking it out of context. Is {csv|html|xml|image}
>> parsing logic a document's logic? Is presentation a document's logic? If
>> they're not, then they do not belong in document.

>
> Is len() a list logic ? If yes, it should belong to list !-)


Yes, that's why list.__len__() belongs to list while len() is a
convenience function that doesn't carry any concrete implementation.

> There are two points here : the first is that we (that is, at least, you
> and me) just don't know enough about the OP's project to tell whether
> something should belong to the document or not. period.


I think I see your point here. I retract my suggestion that it is
suitable for OP's purpose since I just realized OP is in a better
position to make the decision.

> The second point
> is that objects don't live in a splendid isolation, and it's perfectly
> ok to have code outside an object's method working on the object.
> wrt/ these two points, your "document should encapsulate its own logic"
> note seems a bit dogmatic (and not necessarily right) to me - hence my
> answer.


I agree with you about there are certain logics that should not be
inside the object (that's why I qualify the statement with `should`).
Glue logic, by definition, cannot be inside an object. I don't think we
are actually in disagreement here. But I think the dogma, followed with
caution, is generally good.
 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Object creation - Do we really need to create a parent for a derieved object - can't the base object just point to an already created base object jon wayne C++ 9 09-22-2005 02:06 AM
Pass by reference / pass by value Jerry Java 20 09-09-2005 06:08 PM
Re: pass javascript object reference to a session object naijacoder naijacoder ASP .Net 0 09-15-2004 02:01 AM
Re: pass javascript object reference to a session object Girish bharadwaj ASP .Net 0 09-15-2004 01:55 AM
function: cannot pass a string and use as an object with netscape! Geniium Javascript 2 11-04-2003 11:51 AM



Advertisments