__init__ explanation please

[Erik Lind]

I'm new to Python, and OOP. I've read most of Mark Lutz's book and more
online and can write simple modules, but I still don't get when __init__
needs to be used as opposed to creating a class instance by assignment. For
some strange reason the literature seems to take this for granted. I'd
appreciate any pointers or links that can help clarify this.

Thanks

[Daniel Fetchinson]

I'm new to Python, and OOP. I've read most of Mark Lutz's book and more

online and can write simple modules, but I still don't get when __init__
needs to be used as opposed to creating a class instance by assignment. For
some strange reason the literature seems to take this for granted. I'd
appreciate any pointers or links that can help clarify this.

I'm not sure if I understand your question correctly but maybe this will help:

If you want code to be run upon creating an instance of your class you
would use __init__. Most common examples include setting attributes on
the instance and doing some checks, e.g.

class Person:
def __init__( self, first, last ):
if len( first ) 50 or len( last ) 50:
raise Exception( 'The names are too long.' )
self.first = first
self.last = last

And you would use your class like so,

p1 = Person( 'John', 'Smith' )
p2 = Person( "Some long fake name that you don't really want to
except, I don't know if it's really longer than 50 but let's assume it
is", "Smith" )
# This last one would raise an exception so you know that something is not okay

HTH,
Daniel

[Colin J. Williams]

Erik Lind wrote:

I'm new to Python, and OOP. I've read most of Mark Lutz's book and more
online and can write simple modules, but I still don't get when __init__
needs to be used as opposed to creating a class instance by assignment. For
some strange reason the literature seems to take this for granted. I'd
appreciate any pointers or links that can help clarify this.

Thanks

You don't always need __init__ if
__new__ is used with a "new" class.

Colin W.

[Daniel Fetchinson]

Please keep discussion on the list......

I'm not sure if I understand your question correctly but maybe this will
help:

If you want code to be run upon creating an instance of your class you
would use __init__. Most common examples include setting attributes on
the instance and doing some checks, e.g.

class Person:
def __init__( self, first, last ):
if len( first ) 50 or len( last ) 50:
raise Exception( 'The names are too long.' )
self.first = first
self.last = last

And you would use your class like so,

p1 = Person( 'John', 'Smith' )
p2 = Person( "Some long fake name that you don't really want to
except, I don't know if it's really longer than 50 but let's assume it
is", "Smith" )
# This last one would raise an exception so you know that something is not
okay

HTH,
Daniel

Is not the code run when I create an instance by assignement somewhere else?

I take the point that one might want to check for potential exceptions
immediately, but most examples in the literature aren't doing that and don't
seem to be doing anything that would not be done when creating an instance
by assignment later somewhere. I'm missing something basic here.

What do you mean by "create an instance by asignment somewhere else"?

[Jeroen Ruigrok van der Werven]

-On [20080113 01:41], Erik Lind (el***@spamcop. net) wrote:

>I'm new to Python, and OOP. I've read most of Mark Lutz's book and more
online and can write simple modules, but I still don't get when __init__
needs to be used as opposed to creating a class instance by assignment.

I personally tend to see __init__ or __new__ as equivalent to what other
languages call a constructor.

(And I am sure some people might disagree with that. ;))

--
Jeroen Ruigrok van der Werven <asmodai(-at-)in-nomine.org/ asmodai
イェルーン ラウフãƒ*ッ ク ヴァン デル ウェルヴェ ン
http://www.in-nomine.org/ | http://www.rangaku.org/
The riddle master himself lost the key to his own riddles one day, and
found it again at the bottom of his heart.

[Fredrik Lundh]

Jeroen Ruigrok van der Werven wrote:

I personally tend to see __init__ or __new__ as equivalent to what other
languages call a constructor.

(And I am sure some people might disagree with that. ;))

given that they do different things, I'm not sure it's that helpful to
describe them *both* as constructors.

</F>

[Fredrik Lundh]

Erik Lind wrote:

I'm new to Python, and OOP. I've read most of Mark Lutz's book and more
online and can write simple modules, but I still don't get when __init__
needs to be used as opposed to creating a class instance by assignment.

nothing is ever created by plain assignment in Python; to create a class
instance in Python, you *call* the class object. an example:

class MyClass:
pass

# create three separate instances
obj1 = MyClass()
obj2 = MyClass()
obj3 = MyClass()

(it's the () that creates the object, not the =)

if you want to initialize the method's state (that is, set some
attributes), you can do that from the outside:

obj1.attrib = "some value"

or in an "initialization " method in the class:

class MyClass:
def init(self):
self.attrib = "some value"

obj1 = MyClass()
obj1.init()

but in both cases, you'll end up with an inconsistent object state (in
this case, no attribute named "attrib") if you forget to do this.

obj1 = MyClass()
print obj1.attrib # this will fail

to avoid such mistakes, you can use __init__ instead. this is just a
initialization method that's automatically called by Python *after* the
object is created, but *before* the call to the class object returns.

class MyClass:
def __init__(self):
self.attrib = "some value"

obj1 = MyClass()
print obj1.attrib # this will succeed

also, any arguments that you pass to the class object call are passed on
to the initialization method.

class MyClass:
def __init__(self, value):
self.attrib = value

obj1 = MyClass("hello" )
print obj1.attrib # prints "hello"

as Jeroen points out, this is pretty much the same thing as a
constructor in other languages -- that is, a piece of code that's
responsible for setting up an object's state.

Python's a bit different; the object is in fact created before the
call to __init__, but this doesn't matter much in practice; if
construction fails, the assignment will fail, so the object will be
lost, and is reclaimed by the GC later on.

(unless you explicitly store a reference to the object somewhere else,
of course:

>>class MyClass:

.... def __init__(self):
.... global secret
.... secret = self
.... raise ValueError("oop s! failed!")
.... def method(self):
.... print "here I am!"
....

>>obj = MyClass()

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 5, in __init__
ValueError: oops! failed!

>>obj

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'obj' is not defined

>>secret.method ()

here I am!

)

finally, if you want full control also over the actual creation of the
object, more recent Python versions support a __new__ method that can be
used instead of __init__, or as a complement. but that's an advanced
topic, and is nothing you need to worry about while trying to the hang
of class basics.

hope this helps!

</F>

[Ben Finney]

Jeroen Ruigrok van der Werven <as*****@in-nomine.orgwrite s:

-On [20080113 01:41], Erik Lind (el***@spamcop. net) wrote:

I'm new to Python, and OOP. I've read most of Mark Lutz's book and
more online and can write simple modules, but I still don't get
when __init__ needs to be used as opposed to creating a class
instance by assignment.

I personally tend to see __init__ or __new__ as equivalent to what
other languages call a constructor.

That's getting the two of them confused. __new__ is a constructor,
__init__ is not.

(And I am sure some people might disagree with that. ;))

It isn't really a matter for much debate.

<URL:http://www.python.org/doc/ref/customization.h tml>

__new__ is the constructor: it creates the instance and returns it.

Along the way, it calls __init__ on the *already-created* instance, to
ask it to initialise itself ready for use. So, __init__ is an
"initialise r" for the instance.

Python, unlike many other OO languages, fortunately has these two
areas of functionality separate. It's far more common to need to
customise instance initialisation than to customise creation of the
instance. I override __init__ for just about every class I write; I
can count the number of times I've needed to override __new__ on the
fingers of one foot.

--
\ "Reichel's Law: A body on vacation tends to remain on vacation |
`\ unless acted upon by an outside force." -- Carol Reichel |
_o__) |
Ben Finney

[A.T.Hofkamp]

On 2008-01-13, Erik Lind <el***@spamcop. netwrote:

I'm new to Python, and OOP. I've read most of Mark Lutz's book and more
online and can write simple modules, but I still don't get when __init__
needs to be used as opposed to creating a class instance by assignment. For
some strange reason the literature seems to take this for granted. I'd
appreciate any pointers or links that can help clarify this.

I think you mean the following:
You'd like to do

p = Person('me', 'here', 31)

and you are wondering why you need the __init__() function in

class Person(object):
def __init__(self, name, addres, age):
self.name = name
self.address = address
self.age = age

right?

If so, the answer is that while you think you are doing "Person('me ', 'here', 31)",
you are in reality executing "Person.__init_ _(self, 'me', 'here', 31)", where
'self' is refers to a shiny new, empty object created for you.

(and the 'self' is obtained by the Person.__new__ function I think, but others
here have much better knowledge about this).
Sincerely,
Albert

[Ben Finney]

"A.T.Hofkam p" <ha*@se-162.se.wtb.tue. nlwrites:

while you think you are doing "Person('me ', 'here', 31)", you are in
reality executing "Person.__init_ _(self, 'me', 'here', 31)", where
'self' is refers to a shiny new, empty object created for you.

This is misleading, and founders on many discrepancies, not least of
which is that '__init__' always returns None, yet the 'Person()' call
returns the new instance. So it's quite untrue to say that one is "in
reality" calling the '__init__' method.

What one is "in reality" calling is the '__new__' method of the Person
class. That function, in turn, is creating a new Person instance, and
calling the '__init__' method of the newly-created instance. Finally,
the '__new__' method returns that instance back to the caller.

--
\ "Probably the toughest time in anyone's life is when you have |
`\ to murder a loved one because they're the devil." -- Emo |
_o__) Philips |
Ben Finney