Programming Forums > Script Programming > Python

Fred · May 21st, 2005, 6:00 PM

I did not touch python for qiiet a while, and now i am readin some code on the net, and i am wondering: 'Where the heck does this 'self' in the classes come from??'
Here an example:

(Toggle Plain Text)

class Bag:
    def __init__(self):
        self.data = []
    def add(self, x):
        self.data.append(x)
    def addtwice(self, x):
        self.add(x)
        self.add(x)

what exactely is the self.data doing there
Hwy not use JUST data... I read the reason to this somewhere a looong time ago, but can't remember or refind it.
Any feedback would be appriciated!!!
thx

Sane · May 21st, 2005, 8:42 PM

self automatically applies it throughout the entire class, as long as you put self in all the function arguments.

In your example, self.data can be altered in a different function from where it was called in (def add). If you were to try to do the same thing with functions rather classes, it would crash because data wouldn't be defined yet.

ZenMasterJG · May 22nd, 2005, 5:22 AM

if i remember correctly, self is the reference to the object your talking about that python sends to class method calls. since each object AND each method is its own namespace, without self python wouldn't know which data you were talking about. if you just said data, python would look for a data variable in the method's namespace, which isnt what you want. self.data tells python to look in the object's namespace.

Fred · May 22nd, 2005, 3:45 PM

I think i got it again...
Thanks a lot for the help guys!!!! Almost forgot what a great forum this was...

jonaskoelker · May 23rd, 2005, 11:32 AM

to be fair, the first argument doesn't have to be named `self'; that's just a convention.

Cerulean · Jun 23rd, 2005, 5:51 PM

Just clarifying - by default methods in the class block are bound methods, meaning calling myobject.foo() will pass "myobject" as the first parameter to the foo method. If I have the following class..

(Toggle Plain Text)

class Foo:
    def bar(self):
        print "bar"

The following two are equivalent:

(Toggle Plain Text)

f = Foo()
f.bar()

(Toggle Plain Text)

f = Foo()
Foo.bar(f)

If you don't want this behaviour, you can make your method static like so:

(Toggle Plain Text)

class Foo:
    def bar():
        print "foo"
    bar = classmethod(bar)
// You can now call the method in the class.
Foo.bar()

jonaskoelker · Aug 20th, 2005, 8:36 AM

Cerulean: class methods recieve the class as implicit first argument. static methods (`>>> help(staticmethod)') don't get an implicit argument.

Also, in 2.4+, there's decorators:

(Toggle Plain Text)

class Foo:
    @classmethod
    def bar(cls, foo): print cls, type(cls)

hth --Jonas

Cerulean · Aug 20th, 2005, 1:41 PM

Quote:

Cerulean: class methods recieve the class as implicit first argument. static methods (`>>> help(staticmethod)') don't get an implicit argument.

Indeed. I'm quite aware of that..?

May 21st, 2005, 6:00 PM	#1
Fred Programmer Join Date: Feb 2005 Posts: 67 Rep Power: 4	Classes and methods I did not touch python for qiiet a while, and now i am readin some code on the net, and i am wondering: 'Where the heck does this 'self' in the classes come from??' Here an example: (Toggle Plain Text) class Bag: def __init__(self): self.data = [] def add(self, x): self.data.append(x) def addtwice(self, x): self.add(x) self.add(x) class Bag: def __init__(self): self.data = [] def add(self, x): self.data.append(x) def addtwice(self, x): self.add(x) self.add(x) what exactely is the self.data doing there Hwy not use JUST data... I read the reason to this somewhere a looong time ago, but can't remember or refind it. Any feedback would be appriciated!!! thx

May 21st, 2005, 8:42 PM	#2
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 1,888 Rep Power: 5	self automatically applies it throughout the entire class, as long as you put self in all the function arguments. In your example, self.data can be altered in a different function from where it was called in (def add). If you were to try to do the same thing with functions rather classes, it would crash because data wouldn't be defined yet. __________________ Waterloo's Canadian Computing Competition (CCC) - Stage 2 Problems, Solutions, and Test Data

May 23rd, 2005, 11:32 AM	#5
jonaskoelker Newbie Join Date: May 2005 Location: Denmark Posts: 13 Rep Power: 0	to be fair, the first argument doesn't have to be named `self'; that's just a convention. __________________ now go away or I shall taunt you a second time :D

Jun 23rd, 2005, 5:51 PM	#6
Cerulean Professional Programmer Join Date: Apr 2005 Location: London, England Posts: 459 Rep Power: 4	Just clarifying - by default methods in the class block are bound methods, meaning calling myobject.foo() will pass "myobject" as the first parameter to the foo method. If I have the following class.. (Toggle Plain Text) class Foo: def bar(self): print "bar" class Foo: def bar(self): print "bar" The following two are equivalent: (Toggle Plain Text) f = Foo() f.bar() f = Foo() f.bar() (Toggle Plain Text) f = Foo() Foo.bar(f) f = Foo() Foo.bar(f) If you don't want this behaviour, you can make your method static like so: (Toggle Plain Text) class Foo: def bar(): print "foo" bar = classmethod(bar) // You can now call the method in the class. Foo.bar() class Foo: def bar(): print "foo" bar = classmethod(bar) // You can now call the method in the class. Foo.bar()

Aug 20th, 2005, 8:36 AM	#7
jonaskoelker Newbie Join Date: May 2005 Location: Denmark Posts: 13 Rep Power: 0	Cerulean: class methods recieve the class as implicit first argument. static methods (`>>> help(staticmethod)') don't get an implicit argument. Also, in 2.4+, there's decorators: (Toggle Plain Text) class Foo: @classmethod def bar(cls, foo): print cls, type(cls) class Foo: @classmethod def bar(cls, foo): print cls, type(cls) hth --Jonas __________________ now go away or I shall taunt you a second time :D

May 22nd, 2005, 5:22 AM	#3
ZenMasterJG Hobbyist Programmer Join Date: Nov 2004 Location: Boston, MA Posts: 148 Rep Power: 4	if i remember correctly, self is the reference to the object your talking about that python sends to class method calls. since each object AND each method is its own namespace, without self python wouldn't know which data you were talking about. if you just said data, python would look for a data variable in the method's namespace, which isnt what you want. self.data tells python to look in the object's namespace.

May 22nd, 2005, 3:45 PM	#4
Fred Programmer Join Date: Feb 2005 Posts: 67 Rep Power: 4	I think i got it again... Thanks a lot for the help guys!!!! Almost forgot what a great forum this was...

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Programming Forums > Script Programming > Python

Classes and methods