Why the 'self' argument?

[Grzegorz Staniak]

Hello,

I'm a newbie Python user, a systems administrator - I've been trying
to switch from Perl to Python for administrative tasks - and one thing
I cannot understand so far is why I need the special 'self' (or anything
esle) argument in class method definitions. I might have missed an
explanation in the docs, a quick Google search did not help. Is there
somewhere on the web you could direct me to?

Thanks,

--
Grzegorz Staniak <gstaniak@zagie l.com.pl>

[Grant Edwards]

Re: Why the 'self' argument?

In article <vlh7otscuo8gb1 @news.supernews .com>, John Roth wrote:
[color=blue]
> Technically, it would be possible to make "self" a reserved
> word, and not have to put it in the method declaration.
> However, there are a lot of people who use something other than
> the word "self," so that would break existing code.[/color]

It would also make the language more complex and irregular.

--
Grant Edwards grante Yow! Could I have a drug
at overdose?
visi.com

[Gerhard Häring]

Re: Why the 'self' argument?

Grzegorz Staniak wrote:[color=blue]
> Hello,
>
> I'm a newbie Python user, a systems administrator - I've been trying
> to switch from Perl to Python for administrative tasks - and one thing
> I cannot understand so far is why I need the special 'self' (or anything
> esle) argument in class method definitions. I might have missed an
> explanation in the docs, a quick Google search did not help. Is there
> somewhere on the web you could direct me to?[/color]

This entry of the Python FAQ:

http://www.python.org/doc/faq/general.html#why-must-self-be-used-explicitly-in-method-definitions-and-calls

-- Gerhard

[vivek@cs.unipune.ernet.in]

Re: Why the 'self' argument?

On Fri, Sep 05, 2003 at 01:58:41PM +0000, Grzegorz Staniak wrote:[color=blue]
> Hello,
>
> I'm a newbie Python user, a systems administrator - I've been trying
> to switch from Perl to Python for administrative tasks - and one thing
> I cannot understand so far is why I need the special 'self' (or anything
> esle) argument in class method definitions. I might have missed an
> explanation in the docs, a quick Google search did not help. Is there
> somewhere on the web you could direct me to?
>
> Thanks,
>
> --
> Grzegorz Staniak <gstaniak@zagie l.com.pl>
> --
> http://mail.python.org/mailman/listinfo/python-list[/color]

The first argument in a class method is a reference to that class itself. It is
same as "this" pointer in c++ and java. Using this argument you can access the
class variables. ex:

class test:
def __init__(self,a rg1,arg2): #this acts as class constructor
self.name=arg1
self.value=arg2


def changeName(self ,newName): #here we change the name class variable
self.name=newNa me
..........and so on

regards
Vivek Kumar

[John Roth]

Re: Why the 'self' argument?


"Grant Edwards" <grante@visi.co m> wrote in message
news:3f58a2bd$0 $156$a1866201@n ewsreader.visi. com...[color=blue]
> In article <vlh7otscuo8gb1 @news.supernews .com>, John Roth wrote:
>[color=green]
> > Technically, it would be possible to make "self" a reserved
> > word, and not have to put it in the method declaration.
> > However, there are a lot of people who use something other than
> > the word "self," so that would break existing code.[/color]
>
> It would also make the language more complex and irregular.[/color]

How?

John Roth
[color=blue]
>
> --
> Grant Edwards grante Yow! Could I have a[/color]
drug[color=blue]
> at overdose?
> visi.com[/color]

[Grant Edwards]

Re: Why the 'self' argument?

In article <vlhba85atlte07 @news.supernews .com>, John Roth wrote:[color=blue]
>
> "Grant Edwards" <grante@visi.co m> wrote in message
> news:3f58a2bd$0 $156$a1866201@n ewsreader.visi. com...[color=green]
>> In article <vlh7otscuo8gb1 @news.supernews .com>, John Roth wrote:
>>[color=darkred]
>> > Technically, it would be possible to make "self" a reserved
>> > word, and not have to put it in the method declaration.
>> > However, there are a lot of people who use something other than
>> > the word "self," so that would break existing code.[/color]
>>
>> It would also make the language more complex and irregular.[/color]
>
> How?[/color]

1) It would add a reserved word.

2) It would mean that there's some sort of difference between
a function and a method.

--
Grant Edwards grante Yow! I have a TINY BOWL in
at my HEAD
visi.com

[Alex Martelli]

class methods vs instance methods (was Re: Why the 'self' argument?)

vivek@cs.unipun e.ernet.in wrote:
...[color=blue]
> The first argument in a class method is a reference to that class itself.[/color]

Yes, that's what defines CLASS methods (as opposed to ordinary, or
INSTANCE, methods). But the original poster was not asking about class
methods, and you're not giving any examples of them -- I only see
instance methods. I suspect a terminology problem -- you may be using
the term "class method" in a way that is totally inappropriate to
Python, where classes are first-level objects.


So -- in Python, you see...:

To make a class method you have to call on the classmethod built-in. E.g.:

class Example(object) :

def instmeth(self): print 'Instance method of', self

def clasmeth(cls): print 'Class method of', cls
clasmeth = classmethod(cla smeth)

inst = Example()


Now you can call:

inst.instmeth()

or:

inst.clasmeth()
Example.clasmet h()

(both of these do exactly the same thing), but NOT:

Example.instmet h()

since an INSTANCE method, differently from a CLASS method,
needs to be passed the instance as the first argument, either
implicitly (by CALLING it on the instance) or explicitly
by passing it when calling the method on the class, as in:

Example.instmet h(inst)


Alex

[John Roth]

Re: Why the 'self' argument?


"Grant Edwards" <grante@visi.co m> wrote in message
news:3f58b1f6$0 $155$a1866201@n ewsreader.visi. com...[color=blue]
> In article <vlhba85atlte07 @news.supernews .com>, John Roth wrote:[color=green]
> >
> > "Grant Edwards" <grante@visi.co m> wrote in message
> > news:3f58a2bd$0 $156$a1866201@n ewsreader.visi. com...[color=darkred]
> >> In article <vlh7otscuo8gb1 @news.supernews .com>, John Roth wrote:
> >>
> >> > Technically, it would be possible to make "self" a reserved
> >> > word, and not have to put it in the method declaration.
> >> > However, there are a lot of people who use something other than
> >> > the word "self," so that would break existing code.
> >>
> >> It would also make the language more complex and irregular.[/color]
> >
> > How?[/color]
>
> 1) It would add a reserved word.[/color]

So? For most people, self *is* a reserved word anyway. A lot of
novices think it is. Making it official simplifies things, IMO.
[color=blue]
> 2) It would mean that there's some sort of difference between
> a function and a method.[/color]

I don't understand your point. There is currently a difference
between a function and a method that could be eliminated by
making self a reserved word and removing it from the method
header. Or have you never tried to invoke a method from the
wrong context and gotten the "unbound method" error?

There's no reason why a function in the module space couldn't
use self to refer to the module. It would simplify things by
removing much of the need for the global keyword.

John Roth[color=blue]
>
> --
> Grant Edwards[/color]

[John Roth]

Re: Why the 'self' argument?


"Mel Wilson" <mwilson@the-wire.com> wrote in message
news:qpOW/ks/KnRV089yn@the-wire.com...[color=blue]
> In article <vlhidvcpq9a980 @news.supernews .com>,
> "John Roth" <newsgroups@jhr othjr.com> wrote:[color=green]
> >"Grant Edwards" <grante@visi.co m> wrote in message
> >news:3f58b1f6$ 0$155$a1866201@ newsreader.visi .com...[color=darkred]
> >> In article <vlhba85atlte07 @news.supernews .com>, John Roth wrote:
> >> >
> >> > "Grant Edwards" <grante@visi.co m> wrote in message
> >> > news:3f58a2bd$0 $156$a1866201@n ewsreader.visi. com...
> >> 2) It would mean that there's some sort of difference between
> >> a function and a method.[/color]
> >
> >I don't understand your point. There is currently a difference
> >between a function and a method that could be eliminated by
> >making self a reserved word and removing it from the method
> >header. Or have you never tried to invoke a method from the
> >wrong context and gotten the "unbound method" error?[/color]
>
> There's no difference in the sense that a method is
> simply a function whose first parameter refers to the class
> instance to be worked on. No magic words, no "undeclared "
> parameters. It means that in my demo code in limitcases.py
> (somewhere in the newsgroup lately) I can say
>
> limitcases.Lowe st.__str__ = lambda x: "-Infinity"
>
> to give the Lowest class a new method sans ennuis.[/color]

But why do it that way? Neither Java nor Ruby require
giving the instance a name. Hence my comment that requiring
it is more complex than not requiring it.

John Roth[color=blue]
>
> Regards. Mel.[/color]

[Michael Peuser]

Re: Why the 'self' argument?


"Grzegorz Staniak" <gstaniak@inka. zagiel.pl>
[color=blue]
> I'm a newbie Python user, a systems administrator - I've been trying
> to switch from Perl to Python for administrative tasks - and one thing
> I cannot understand so far is why I need the special 'self' (or anything
> esle) argument in class method definitions.[/color]

You probably do not mean "class methods"; this a technical term in OOP.

The Python class basics are *very* similar to the implementation of Perl
classes!!
You should not have much problems....

Kindly
Michael P

[Grant Edwards]

Re: Why the 'self' argument?

In article <vlhtm1bmc7m295 @news.supernews .com>, John Roth wrote:
[color=blue][color=green]
>> There's no difference in the sense that a method is simply a
>> function whose first parameter refers to the class instance to
>> be worked on. No magic words, no "undeclared " parameters. It
>> means that in my demo code in limitcases.py (somewhere in the
>> newsgroup lately) I can say
>>
>> limitcases.Lowe st.__str__ = lambda x: "-Infinity"
>>
>> to give the Lowest class a new method sans ennuis.[/color]
>
> But why do it that way?[/color]

So that there's no difference between a function and a method.

Simplicity and orthogonality are good things -- despite what
C++ proponents thing.
[color=blue]
> Neither Java nor Ruby require giving the instance a name.[/color]

So?

Personally I don't like invisible, implied, automatic stuff
like that.

Can't stand automatic transmissions either -- the damn things
have no idea what's ahead, and always seem to be in the wrong
gear at critical moments.
[color=blue]
> Hence my comment that requiring it is more complex than not
> requiring it.[/color]

No, it's more complex the Java/Ruby way, since you have to have
two sets of rules for what a name means depending on whether
you're inside a "normal" function or a method. In Python
there's just one set of rules -- mostly.

I have enough trouble remembering how the stuff I design works. ;)

I don't want to have to keep track of more scoping/name-space
rules than absolutely necessary.

--
Grant Edwards grante Yow! Intra-mural sports
at results are filtering
visi.com through th' plumbing...

[Bengt Richter]

Re: Why the 'self' argument?

On Fri, 5 Sep 2003 22:51:10 +0200, "Michael Peuser" <mpeuser@web.de > wrote:
[color=blue]
>
>"Grzegorz Staniak" <gstaniak@inka. zagiel.pl>
>[color=green]
>> I'm a newbie Python user, a systems administrator - I've been trying
>> to switch from Perl to Python for administrative tasks - and one thing
>> I cannot understand so far is why I need the special 'self' (or anything
>> esle) argument in class method definitions.[/color]
>
>You probably do not mean "class methods"; this a technical term in OOP.
>[/color]
I suspect the OP meant "class (method definitions)," not "(class method)" definitions." ;-)

Re "self," there must be a FAQ entry, but I'm too lazy to look for it.

Regards,
Bengt Richter

[John Roth]

Re: Why the 'self' argument?


"Grant Edwards" <grante@visi.co m> wrote in message
news:3f59014d$0 $152$a1866201@n ewsreader.visi. com...[color=blue]
> In article <vlhtm1bmc7m295 @news.supernews .com>, John Roth wrote:
>[color=green][color=darkred]
> >> There's no difference in the sense that a method is simply a
> >> function whose first parameter refers to the class instance to
> >> be worked on. No magic words, no "undeclared " parameters. It
> >> means that in my demo code in limitcases.py (somewhere in the
> >> newsgroup lately) I can say
> >>
> >> limitcases.Lowe st.__str__ = lambda x: "-Infinity"
> >>
> >> to give the Lowest class a new method sans ennuis.[/color]
> >
> > But why do it that way?[/color]
>
> So that there's no difference between a function and a method.
>
> Simplicity and orthogonality are good things -- despite what
> C++ proponents thing.
>[color=green]
> > Neither Java nor Ruby require giving the instance a name.[/color]
>
> So?
>
> Personally I don't like invisible, implied, automatic stuff
> like that.
>
> Can't stand automatic transmissions either -- the damn things
> have no idea what's ahead, and always seem to be in the wrong
> gear at critical moments.
>[color=green]
> > Hence my comment that requiring it is more complex than not
> > requiring it.[/color]
>
> No, it's more complex the Java/Ruby way, since you have to have
> two sets of rules for what a name means depending on whether
> you're inside a "normal" function or a method. In Python
> there's just one set of rules -- mostly.[/color]

As I said earlier, it's quite possible to define it so that there
is always an instance of some kind; whether that's an instance
a class or the module itself.
[color=blue]
> I have enough trouble remembering how the stuff I design works. ;)
>
> I don't want to have to keep track of more scoping/name-space
> rules than absolutely necessary.[/color]

I think my comments have shown that you can reduce the
amount of scoping / name space rules noticably.

John Roth[color=blue]
>
> --
> Grant Edwards grante Yow! Intra-mural[/color]
sports[color=blue]
> at results are filtering
> visi.com through th' plumbing...[/color]

[John Roth]

Re: Why the 'self' argument?


"Michael Peuser" <mpeuser@web.de > wrote in message
news:bjat11$70s $07$1@news.t-online.com...[color=blue]
>
> "Grzegorz Staniak" <gstaniak@inka. zagiel.pl>
>[color=green]
> > I'm a newbie Python user, a systems administrator - I've been trying
> > to switch from Perl to Python for administrative tasks - and one thing
> > I cannot understand so far is why I need the special 'self' (or anything
> > esle) argument in class method definitions.[/color]
>
> You probably do not mean "class methods"; this a technical term in OOP.
>
> The Python class basics are *very* similar to the implementation of Perl
> classes!!
> You should not have much problems....[/color]

Actually, I think he *does* mean "class methods." They are new in 2.2
after all, and I missed the qualification the first time around.

John Roth[color=blue]
>
> Kindly
> Michael P
>
>[/color]

[Grant Edwards]

Re: Why the 'self' argument?

In article <vli387c9mm140a @news.supernews .com>, John Roth wrote:
[color=blue][color=green]
>> So that there's no difference between a function and a method.
>>
>> Simplicity and orthogonality are good things -- despite what
>> C++ proponents thing.
>>[color=darkred]
>> > Hence my comment that requiring it is more complex than not
>> > requiring it.[/color]
>>
>> No, it's more complex the Java/Ruby way, since you have to have
>> two sets of rules for what a name means depending on whether
>> you're inside a "normal" function or a method. In Python
>> there's just one set of rules -- mostly.[/color]
>
> As I said earlier, it's quite possible to define it so that there
> is always an instance of some kind; whether that's an instance
> a class or the module itself.[/color]

I don't follow. You're talking about defining a keyword that
always refers to the first parameter passed to a function? And
the declared parameters would refer to the 2nd through Nth
parameters? What if the keyword isn't used in the function
definition, then do the delcared parameters refer to the 1st
through Nth?
[color=blue]
> I think my comments have shown that you can reduce the amount
> of scoping / name space rules noticably.[/color]

Compared to what? It sure sounds like you're introducing more
rules than there are now. How would you propose reducing the
number of rules?

--
Grant Edwards grante Yow! Somewhere in Tenafly,
at New Jersey, a chiropractor
visi.com is viewing "Leave it to
Beaver"!