hi,
python's staticmethod is the equivalent of java staticmethod right?
with classmethod, I can call the method without the need for creating
an instance right? since the difference between the two is that
classmethod receives the class itself as implicti first argument. From
my understanding classmethod are for dealing with class attributes?
Can somebody teach me the real use of classmethod & staticmethod?
Thanks
james
14  2721 
On Tue, 24 Jul 2007 03:19:05 +0000, james_027 wrote:
python's staticmethod is the equivalent of java staticmethod right?
Correct. `staticmethod` is essentially just a function moved into a class
and accessible at the class object and instances of that class.
As Python opposed to Java has functions, `staticmethod` isn't that useful.
with classmethod, I can call the method without the need for creating
an instance right? since the difference between the two is that
classmethod receives the class itself as implicti first argument. From
my understanding classmethod are for dealing with class attributes?
It's possible to access class attributes and, maybe more important, it's
possible to call the class to return an instance. So if you call a
`classmethod` on a subclass an instance of that subclass is returned.
Silly example:
class A(object):
def __init__(self, x, y):
print 'init A'
self.x = x
self.y = y
@classmethod
def from_str(cls, string):
return cls(*map(float, string.split(', ')))
class B(A):
def __init__(self, x, y):
print 'init B'
A.__init__(self , x, y)
def main():
B.from_str('42, 23')
Ciao,
Marc 'BlackJack' Rintsch
james_027 a écrit :
hi,
python's staticmethod is the equivalent of java staticmethod right?
IIRC, yes. A 'staticmethod' is in fact nothing more than a function
attached to a class, and which can be called on the class or an instance
of. Note that since Python supports modules and functions, staticmethods
are of little practical use.
with classmethod, I can call the method without the need for creating
an instance right? since the difference between the two is that
classmethod receives the class itself as implicti first argument.
Yes.
From
my understanding classmethod are for dealing with class attributes?
Not necessarily - you can access class attributes from within an
instance method (but obviously a classmethod cannot access instance
attributes).
Can somebody teach me the real use of classmethod & staticmethod?
The 'real' use is (are) the one(s) you'll find. FWIW, I use
staticmethods for helper functions that don't need access to the class
or instance but are too specific to a class to be of any use as plain
functions. Which is not a very frequent case. Classmethods are more
usefull - mostly as alternate constructors or utility methods for an
alternate constructor, but there are other possible uses (sorry, I have
no concrete example at hand).
hi,
The 'real' use is (are) the one(s) you'll find. FWIW, I use
staticmethods for helper functions that don't need access to the class
or instance but are too specific to a class to be of any use as plain
functions. Which is not a very frequent case. Classmethods are more
usefull - mostly as alternate constructors or utility methods for an
alternate constructor, but there are other possible uses (sorry, I have
no concrete example at hand).
You mean like the example from Marc
Thanks
james
james_027 a écrit :
hi,
>The 'real' use is (are) the one(s) you'll find. FWIW, I use
staticmethod s for helper functions that don't need access to the class
or instance but are too specific to a class to be of any use as plain
functions. Which is not a very frequent case. Classmethods are more
usefull - mostly as alternate constructors or utility methods for an
alternate constructor, but there are other possible uses (sorry, I have
no concrete example at hand).
You mean like the example from Marc
Marc's example is typically an alternate constructor. This is indeed one
of the most obvious use case of classmethod, but what I meant is that
there are others cases where classmethods can help.
On 2007-07-24, Alex Popescu <no************ *****@gmail.com wrote:
Bruno Desthuilliers <br************ ********@wtf.we bsiteburo.oops. comwrote
in news:46******** *************** @news.free.fr:
>>
[snip...]
Not necessarily - you can access class attributes from within an
instance method (but obviously a classmethod cannot access instance
attributes).
What I am doing wrong here then:
class MyClass(object) :
class_list = ['a', 'b']
def instance_method (self):
print "instance_metho d with class list %s" % class_list
There's no implicit self or class for Python identifiers.
The name class_list must be quailified: self.class_list or
MyClass.class_l ist.
--
Neil Cerutti
Neil Cerutti <ho*****@yahoo. comwrote in
news:sl******** ************@FI AD06.norwich.ed u:
On 2007-07-24, Alex Popescu <no************ *****@gmail.com wrote:
>Bruno Desthuilliers <br************ ********@wtf.we bsiteburo.oops. com>
wrote in news:46******** *************** @news.free.fr:
[snip...]
>>
class MyClass(object) :
class_list = ['a', 'b']
def instance_method (self):
print "instance_metho d with class list %s" % class_list
There's no implicit self or class for Python identifiers.
The name class_list must be quailified: self.class_list or
MyClass.class_l ist.
After more investigation I have figured this out by myself, but thanks for
the details.
Now I am wondering if in the above case there is a prefered way:
MyClass.class_l ist or self.__class__. class_list? (IMO the 2nd is more safe
in terms of refactorings).
../alex
--
..w( the_mindstorm )p.
On Tue, 24 Jul 2007 21:35:58 +0000, Alex Popescu wrote:
Neil Cerutti <ho*****@yahoo. comwrote in
news:sl******** ************@FI AD06.norwich.ed u:
>On 2007-07-24, Alex Popescu <no************ *****@gmail.com wrote:
>>Bruno Desthuilliers <br************ ********@wtf.we bsiteburo.oops. com>
wrote in news:46******** *************** @news.free.fr:
[snip...]
>>>
class MyClass(object) :
class_list = ['a', 'b']
def instance_method (self):
print "instance_metho d with class list %s" % class_list
There's no implicit self or class for Python identifiers.
The name class_list must be quailified: self.class_list or
MyClass.class_ list.
After more investigation I have figured this out by myself, but thanks for
the details.
Now I am wondering if in the above case there is a prefered way:
MyClass.class_l ist or self.__class__. class_list? (IMO the 2nd is more safe
in terms of refactorings).
Consider what happens when you sub-class:
class MyClass(object) :
class_list = [1, 2, 3]
def method(self, x):
return sum(MyClass.cla ss_list) + x
class MySubclass(MyCl ass):
class_list = ['a', 'b', 'c'] # over-ride the class attribute
expecting_a_str ing = MySubclass().me thod('x')
Use a direct reference to MyClass.class_l ist when you want a direct
reference to MyClass regardless of which instance or class you are calling
from.
Use self.class_list when you want to use inheritance.
Use self.__class__. class_list when you have an instance method and you
need the class it belongs to.
Use a classmethod and the first argument (by convention called klass
or cls) when you don't care about the instance and just want the class.
--
Steven.
"Steven D'Aprano" <st***@REMOVE.T HIS.cybersource .com.auwrote in
news:pa******** *************** *****@REMOVE.TH IS.cybersource. com.au:
On Tue, 24 Jul 2007 21:35:58 +0000, Alex Popescu wrote:
>Neil Cerutti <ho*****@yahoo. comwrote in
news:sl******* *************@F IAD06.norwich.e du:
>>On 2007-07-24, Alex Popescu <no************ *****@gmail.com wrote:
Bruno Desthuilliers
<br********* ***********@wtf .websiteburo.oo ps.comwrote in
news:46***** *************** ***@news.free.f r:
[snip...]
Use self.class_list when you want to use inheritance.
As a matter of style, how do you figure out that class_list is a class
attribute and not an instance attribute? (I don't remember seeing anything
in the PEP describing the coding style).
tia,
../alex
--
..w( the_mindstorm )p.
On 2007-07-25, Alex Popescu <no************ *****@gmail.com wrote:
As a matter of style, how do you figure out that class_list is
a class attribute and not an instance attribute? (I don't
remember seeing anything in the PEP describing the coding
style).
Check out dir(MyClass) and dir(MyClass()) for some insight, if it
turns out that it matters. Preferably, the user of a class
doesn't have to really think about it much.
--
Neil Cerutti This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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