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Mon, 24 Feb 2014 18:24:52 -0800 (PST) In-Reply-To: References: Date: Mon, 24 Feb 2014 18:24:52 -0800 Subject: Re: [Baypiggies] Class decorator to capture the creation and deletion of objects From: Alex Martelli To: Sangeeth Saravanaraj Content-Type: multipart/alternative; boundary=e89a8ff1ce02097e0704f331cb25 Cc: python-list@python.org, BayPiggies , bangpypers@python.org, chennaipy@python.org X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 384 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1393295100 news.xs4all.nl 2870 [2001:888:2000:d::a6]:60350 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:67014 --e89a8ff1ce02097e0704f331cb25 Content-Type: text/plain; charset=UTF-8 Off the cuff, I'd make saveme into a function, not a class; the saveme function would alter the class passed in as its only argument (saving __init__ and/or __del__ methods it may have and replacing them with other -- nested -- functions that call them and do the rest of the job) and return the same class object it received. No time to actually write the code but this seems a much sounder architecture. Alex On Mon, Feb 24, 2014 at 4:49 PM, Sangeeth Saravanaraj < sangeeth.saravanaraj@gmail.com> wrote: > This question was initially asked in tutor@python.org; Now I am widening > the audience to gain attention. > > I want to create a decorator which should do the following things: > => When an object of the decorated class is created, the objects name > (say the value of the incoming "id" argument) should be stored as a record > in a table in a database. > => When an object of the decorated class is deleted, the record with this > deleted objects name (i.e. object.id) should be removed from the table. > > Now, for example - consider the following snippet: > > @saveme > class A(object): > def __init__(self, id): > self.id = id > > @saveme > class B(object): > def __init__(self, id): > self.id = id > > "saveme" should do what I have explained earlier. > > a1 = A("A1") > a2 = A("A2") > a3 = A("A3") > b1 = B("B1") > b2 = B("B2") > > At this point if I query and print all the records in a table, I should > get the following output: > ["A1", "A2", "A3", "B1", "B2"] > > del a1 > del a2 > del a3 > del b1 > del b2 > > At this point, all entries in the table should be deleted; query should > return an empty list! > > And, I want to highlight that the classes that are being decorated with > "saveme" can de derived classes too [which initialises its base classes > using super() method]! > > Now the following is what I have tried: > > class saveme(object): > def __init__(self, klass): > print "saveme::__init__()" > self._klass = klass > > def __call__(self, *args, **kwargs): > print "saveme::__call__()" > obj = self._klass(*args, **kwargs) > # creation of DB record will happen here! > # i.e. something like add_to_db(kwargs.get("id")) > return obj > > def __del__(self): > # deletion of DB record will happen here! > # i.e. something like remove_from_db(id) > # TODO: how to retrieve the "id" here?! > print "saveme::__del__()" > > > class Parent1(object): > def __init__(self): > print "Parent1:: __init__()" > super(Parent1, self).__init__() > > > class Parent2(object): > def __init__(self): > print "Parent2:: __init__()" > super(Parent2, self).__init__() > > > @saveme > class A(Parent1, Parent2): > def __init__(self, id): > print "A::__init__()" > self.id = id > #super(A, self).__init__() > > > #@saveme > #class B(object): > # def __init__(self, id): > # print "B::__init__()" > # self.id = id > > > def main(): > a1 = A(id="A1") > # b1 = B(id="B1") > > if __name__ == "__main__": > main() > > > When executed the above, I ran in to the following: > > saveme::__init__() > saveme::__call__() > A::__init__() > Traceback (most recent call last): > File "1.py", line 54, in > main() > File "1.py", line 50, in main > a1 = A(id="A1") > File "1.py", line 10, in __call__ > obj = self._klass(*args, **kwargs) > File "1.py", line 39, in __init__ > super(A, self).__init__() > TypeError: must be type, not saveme > saveme::__del__() > > > When I commented "super(A, self).__init__()" in the class A :: __init__() > method, it returned an object of type A and I was able to see the prints in > the __call__ and __del__ methods but the __init__() methods of the base > classes (Parent1 & Parent2) were not called! > > From the error message, what I could understand is - the object returned > by saveme::__call__() is not of type A but of type saveme. But when I put a > print in the saveme::__call__() I could see it returns an object of type A > and not saveme. > > Now the question is - with this approach to capture the initiation and > deletion events of an object, how do I initialise the base classes using > super()? > > Or, is there any other better way to capture the __call__ and __del__ > events for an object of a certain class - if so, how?! > > Thank you, > > Sangeeth > > > PS: > http://stackoverflow.com/questions/21826854/typeerror-when-using-super-method-with-class-decorator-for-a-derived-class > > > _______________________________________________ > Baypiggies mailing list > Baypiggies@python.org > To change your subscription options or unsubscribe: > https://mail.python.org/mailman/listinfo/baypiggies > --e89a8ff1ce02097e0704f331cb25 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Off the cuff, I'd make saveme into a function, not a c= lass; the=C2=A0saveme=C2=A0function would alter the class passed in as its = only argument (saving __init__ and/or __del__ methods it may have and repla= cing them with other -- nested -- functions that call them and do the rest = of the job) and return the same class object it received.

No time to actually write the code but this seems a much sou= nder architecture.


Alex
<= br>


On Mon, Feb 24, 2014 at 4:49 PM, Sangeeth Saravanaraj <= ;sangee= th.saravanaraj@gmail.com> wrote:
This question was ini= tially asked in tutor= @python.org; Now I am widening the audience to gain attention.

I want = to create a decorator which should do the following things:
=C2=A0=3D> When an object of the decorated class is c= reated, the objects name (say the value of the incoming "id" argu= ment) should be stored as a record in a table in a database.=C2=A0
=C2=A0=3D> When an object of the decorated = class is deleted, the record with this deleted objects name (i.e. object.id) should be removed fr= om the table.=C2=A0

Now, for example - consider the foll= owing snippet:

@saveme
class A(object):
=C2=A0 =C2=A0 def __init__(self, id):
=C2= =A0 =C2=A0 =C2=A0 =C2=A0 self.= id =3D id

@saveme
class B(object):
=C2=A0 =C2=A0 def __init__(self, id):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 self.id =3D id

"saveme" should do what I have explained earlier.

a1 =3D = A("A1")
a2 =3D A("A2"= )
a3 =3D A("A3")
b1 =3D B("B1")
b2 =3D B("B= 2")

At this point if I query and print all the records in a table, I sho= uld get the following output:=C2=A0
["A1", "A2", "A3"= ;, "B1", "B2"]

del a1
d= el a2
del a3
del b= 1
del b2

At this point, all entries in the = table should be deleted; query should return an empty list!

And, I = want to highlight that the classes that are being decorated with "save= me" can de derived classes too [which initialises its base classes usi= ng super() method]!

Now the= following is what I have tried:

class saveme(object):
=C2=A0 =C2=A0 def __init__(self, klass):
= =C2=A0 =C2=A0 =C2=A0 =C2=A0 print "saveme::__init__()"
=C2=A0 =C2=A0 =C2=A0 =C2=A0 self._klass =3D klass

=C2=A0 =C2=A0 def __call__(self, *args, **kwargs):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 print "saveme::__call__()"<= /div>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 obj =3D self.= _klass(*args, **kwargs)
=C2=A0 =C2=A0 =C2=A0 =C2=A0 # creation of DB record will happen here!
=
=C2=A0 =C2=A0 =C2=A0 =C2=A0 # i.e. something l= ike add_to_db(kwargs.get("id"))
=C2=A0 =C2=A0 =C2=A0 =C2=A0 return obj

=C2=A0 =C2=A0 def __del__(self):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 # deletion of DB= record will happen here!
=C2=A0 =C2=A0 = =C2=A0 =C2=A0 # i.e. something like remove_from_db(id)
=C2=A0 =C2=A0 =C2=A0 =C2=A0 # TODO: how to ret= rieve the "id" here?!
=C2=A0 = =C2=A0 =C2=A0 =C2=A0 print "saveme::__del__()"


class Parent1(object):
=C2=A0 =C2=A0 def __init__(self):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 print "Parent1:: __init__()&= quot;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 super(Parent1, self).__init__()


class Parent2(object):
=C2=A0 =C2=A0 def __init__(self):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 print "Parent= 2:: __init__()"
=C2=A0 =C2=A0 =C2=A0= =C2=A0 super(Parent2, self).__init__()
<= br>

@saveme
clas= s A(Parent1, Parent2):
=C2=A0 =C2=A0 def = __init__(self, id):
=C2=A0 =C2=A0 =C2=A0 = =C2=A0 print "A::__init__()"
=C2=A0 =C2=A0 =C2=A0 =C2=A0 self.id =3D id
=C2=A0 =C2=A0 =C2=A0 =C2=A0 #super(A, self).__init__()


#@saveme
#cl= ass B(object):
# =C2=A0 =C2=A0def __init_= _(self, id):
# =C2=A0 =C2=A0 =C2=A0 =C2= =A0print "B::__init__()"
# =C2=A0 =C2=A0 =C2=A0 =C2=A0self.id =3D id


def main():
=C2=A0 =C2=A0 a1 =3D A(id=3D"A1")
# =C2=A0 =C2=A0b1 =3D B(id=3D"B1")

if __name__ =3D=3D "__ma= in__":
=C2=A0 =C2=A0 main()


When executed the= above, I ran in to the following:

saveme::__init__()
saveme::__call__()
A::__init__()
Traceback (most recent call last):
= =C2=A0 File "1.py", line 54, in <module>
=C2=A0 =C2=A0 main()
=C2=A0 File "1.py", line 50, in main
=C2=A0 =C2=A0 a1 =3D A(id=3D"A1")
=C2=A0 File "1.py", line 10, in __call__
=C2=A0 =C2=A0 obj =3D self._klass(*args, **kwa= rgs)
=C2=A0 File "1.py", line 3= 9, in __init__
=C2=A0 =C2=A0 super(A, sel= f).__init__()
TypeError: must be type, not saveme
savem= e::__del__()


=
When I commented "super(A, self).__init__()" in the class A :: __= init__() method, it returned an object of type A and I was able to see the = prints in the __call__ and __del__ methods but the __init__() methods of th= e base classes (Parent1 & Parent2) were not called!=C2=A0

From the error message, what I = could understand is - the object returned by saveme::__call__() is not of t= ype A but of type saveme. But when I put a print in the saveme::__call__() = I could see it returns an object of type A and not saveme.

Now the question is - with this= approach to capture the initiation and deletion events of an object, how d= o I initialise the base classes using super()?=C2=A0

Or, is there any other better w= ay to capture the __call__ and __del__ =C2=A0events for an object of a cert= ain class - if so, how?!

Thank you,

Sangeeth


PS:=C2=A0http://stackoverflow.com/questions/21826854/typeerror-when-using-su= per-method-with-class-decorator-for-a-derived-class


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