Re: Trying to come to grips with static methods
Im my opinion, class method are used to store some "functions" related to a class in the scope of the class. For example, I often use static methods like that: class Foo: On 7/12/05, Steven D'Aprano <[EMAIL PROTECTED]> wrote: I've been doing a lot of reading about static methods in Python, and I'mnot exactly sure what they are useful for or why they were introduced.Here is a typical description of them, this one from Guido: "The new descriptor API makes it possible to add static methods and classmethods. Static methods are easy to describe: they behave pretty much likestatic methods in C++ or Java." http://www.python.org/2.2.3/descrintro.htmlGreat. So I have learn an entire new language to understand staticmethods. Perhaps not -- hence this cry for help.As near as I can see it, static methods are object methods that act just like functions. Er. I always thought that object methods *were* functions,except they had some runtime magic that passed the object itself as thefirst argument.>From Guido's example:>>> class C: ... def foo(x, y):... print "staticmethod", x, y... foo = staticmethod(foo)...>>> C.foo(1, 2)staticmethod 1 2>>> c = C()>>> c.foo(1, 2) staticmethod 1 2So I compare with an ordinary class function, er, method:>>> class D:... def foo(self, x, y):... print "method", x, y...>>> D.foo (1, 2)Traceback (most recent call last): File "", line 1, in ?TypeError: unbound method foo() must be called with D instance as firstargument (got int instance instead)Okay, that is to be expected. Actually, I expected an exception that I hadn't passed enough arguments (2 arguments when foo expects 3), but inhindsight it is obvious enough.First point of confusion. In the above exception, foo is called an unboundmethod. But type(D.foo) returns even though foo is being access through the class, not an instance. And type(D().foo) returnsthe same.Can I assume that in Python "unbound method" is just another way of saying"a method of a class that expects to be called via an instance"? I next tried this:>>> D.foo(D(), 1, 2)method 1 2>>> D().foo(1, 2)method 1 2Clear as mud. An ordinary method called from an instance is the same as astatic method called from anywhere, provided you don't -- or rather, can't -- try to access self from the static method.When would you use a static method instead of an ordinary method? It hasbeen suggested that you might use it for functions that don't need toaccess self. But that doesn't seem very convincing to me, because there is already a perfectly good idiom for that:>>> class E:... def foo(): # returns calculated value... return 1... foo = staticmethod(foo)... def bar(self):... return 1 # just ignore the value of self ...>>> E.foo()1>>> e = E()>>> e.bar()1What are some usage cases for using Class.StaticMethod() instead ofinstance.method()? Everything I've read seems to just assume that the benefits of static methods are so obvious that they don't need explaining.Unfortunately, I haven't come from a background in OO and I'm easilyconfused, hence this post.--Steven.-- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Trying to come to grips with static methods
Steven D'Aprano wrote: > I've been doing a lot of reading about static methods in Python, and I'm > not exactly sure what they are useful for or why they were introduced. > > Here is a typical description of them, this one from Guido: > > "The new descriptor API makes it possible to add static methods and class > methods. Static methods are easy to describe: they behave pretty much like > static methods in C++ or Java." > http://www.python.org/2.2.3/descrintro.html > > Great. So I have learn an entire new language to understand static > methods. Perhaps not -- hence this cry for help. > > As near as I can see it, static methods are object methods that act just > like functions. Er. I always thought that object methods *were* functions, > except they had some runtime magic that passed the object itself as the > first argument. [snip] > What are some usage cases for using Class.StaticMethod() instead of > instance.method()? Everything I've read seems to just assume that the > benefits of static methods are so obvious that they don't need explaining. > Unfortunately, I haven't come from a background in OO and I'm easily > confused, hence this post. staticmethods don't see a whole lot of use in Python. They see use in Java because everything has to be stuck in a class, even if they don't actually require an instance of the class to work. In Python, you usually just define a function. I think staticmethod was implemented along with classmethod primarily for completeness than anything else. OTOH, I do find myself using them occasionally to group such functions appropriately. I'll be writing a utility method that doesn't actually need an instance or the class to be passed in, so I make it a staticmethod. It *could* just as well be a module-level function, but sometimes it really belongs tucked in with the class. It could also be a regular method, but then I can't call it from the outside without actually having an instance which isn't actually needed. It could also be a classmethod, but since it doesn't actually need any information from the class to do its work, I find that the extra reminder of staticmethod helps my brain understand what it's doing. All told, I'd probably vote -0.5 on a builtin staticmethod if it came up today. -- Robert Kern [EMAIL PROTECTED] "In the fields of hell where the grass grows high Are the graves of dreams allowed to die." -- Richard Harter -- http://mail.python.org/mailman/listinfo/python-list
Re: Trying to come to grips with static methods
(sorry, my fingers send the mail by there own ;-) Im my opinion, class method are used to store some functionalities (function) related to a class in the scope of the class. For example, I often use static methods like that: class Point: def __init__(self, x, y): self.x, self.y = x, y def fromXML(xmlText): x, y = functionToParseXMLUsingMinidomForExample(xmlText) return Point(x, y) fromXML = staticmethod(fromXML) Here, it is used to define some kind of second constructor... Note that class decorator can simplify the notation, but break the compatility with older Python... Cyril On 7/12/05, Steven D'Aprano <[EMAIL PROTECTED] > wrote: I've been doing a lot of reading about static methods in Python, and I'mnot exactly sure what they are useful for or why they were introduced.Here is a typical description of them, this one from Guido: "The new descriptor API makes it possible to add static methods and classmethods. Static methods are easy to describe: they behave pretty much likestatic methods in C++ or Java." http://www.python.org/2.2.3/descrintro.htmlGreat. So I have learn an entire new language to understand staticmethods. Perhaps not -- hence this cry for help.As near as I can see it, static methods are object methods that act just like functions. Er. I always thought that object methods *were* functions,except they had some runtime magic that passed the object itself as thefirst argument.>From Guido's example:>>> class C: ... def foo(x, y):... print "staticmethod", x, y... foo = staticmethod(foo)...>>> C.foo(1, 2)staticmethod 1 2>>> c = C()>>> c.foo(1, 2) staticmethod 1 2So I compare with an ordinary class function, er, method:>>> class D:... def foo(self, x, y):... print "method", x, y...>>> D.foo (1, 2)Traceback (most recent call last): File "", line 1, in ?TypeError: unbound method foo() must be called with D instance as firstargument (got int instance instead)Okay, that is to be expected. Actually, I expected an exception that I hadn't passed enough arguments (2 arguments when foo expects 3), but inhindsight it is obvious enough.First point of confusion. In the above exception, foo is called an unboundmethod. But type(D.foo) returns even though foo is being access through the class, not an instance. And type(D().foo) returnsthe same.Can I assume that in Python "unbound method" is just another way of saying"a method of a class that expects to be called via an instance"? I next tried this:>>> D.foo(D(), 1, 2)method 1 2>>> D().foo(1, 2)method 1 2Clear as mud. An ordinary method called from an instance is the same as astatic method called from anywhere, provided you don't -- or rather, can't -- try to access self from the static method.When would you use a static method instead of an ordinary method? It hasbeen suggested that you might use it for functions that don't need toaccess self. But that doesn't seem very convincing to me, because there is already a perfectly good idiom for that:>>> class E:... def foo(): # returns calculated value... return 1... foo = staticmethod(foo)... def bar(self):... return 1 # just ignore the value of self ...>>> E.foo()1>>> e = E()>>> e.bar()1What are some usage cases for using Class.StaticMethod() instead ofinstance.method()? Everything I've read seems to just assume that the benefits of static methods are so obvious that they don't need explaining.Unfortunately, I haven't come from a background in OO and I'm easilyconfused, hence this post.--Steven.-- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Trying to come to grips with static methods
Cyril Bazin wrote:
> (sorry, my fingers send the mail by there own ;-)
>
> Im my opinion, class method are used to store some functionalities
> (function) related to a class in the scope of the class.
>
> For example, I often use static methods like that:
>
> class Point:
> def __init__(self, x, y):
> self.x, self.y = x, y
>
> def fromXML(xmlText):
> x, y = functionToParseXMLUsingMinidomForExample(xmlText)
> return Point(x, y)
> fromXML = staticmethod(fromXML)
>
> Here, it is used to define some kind of second constructor...
>
> Note that class decorator can simplify the notation, but break the
> compatility with older Python...
Huh? classmethod was introduced with staticmethod, and in fact, this use
case is exactly what classmethods are for, not staticmethods.
In [2]: class Point(object): # <-- note inheritance from object
...: def __init__(self, x, y):
...: self.x, self.y = x, y
...: def fromXML(cls, xmlText):
...: x, y = parseXML(xmlText)
...: return cls(x, y)
...: fromXML = classmethod(fromXML)
...:
In [3]: class NewPoint(Point):
...: pass
...:
In [4]: def parseXML(xmlText):
...: return 1, 4
...:
In [5]: p = NewPoint.fromXML('')
In [6]: isinstance(p, NewPoint)
Out[6]: True
--
Robert Kern
[EMAIL PROTECTED]
"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter
--
http://mail.python.org/mailman/listinfo/python-list
Re: Trying to come to grips with static methods
Ok, sorry, you are right Robert.
What about this one:
class Parser(object):
def toParser(p):
if type(p) == str:
if len(p) == 1:
return lit(p)
return txt(p)
return p
toParser = staticmethod(toParser)
This is meant to translate p to a parser if it's not one.
'lit' is a function that take a string of length 1 and return a parser of char.
'txt' is a function that take a string of any length and return a parser.
I hope it is a better example!
That one of the rare case I used static method in Python...
Cyril
On 7/12/05, Robert Kern <[EMAIL PROTECTED]> wrote:
Cyril Bazin wrote:> (sorry, my fingers send the mail by there own ;-)>> Im my opinion, class method are used to store some functionalities> (function) related to a class in the scope of the class.
>> For example, I often use static methods like that:>> class Point:> def __init__(self, x, y):> self.x, self.y = x, y>> def fromXML(xmlText):> x, y = functionToParseXMLUsingMinidomForExample(xmlText)
> return Point(x, y)> fromXML = staticmethod(fromXML)>> Here, it is used to define some kind of second constructor...>> Note that class decorator can simplify the notation, but break the
> compatility with older Python...Huh? classmethod was introduced with staticmethod, and in fact, this usecase is exactly what classmethods are for, not staticmethods.In [2]: class Point(object): # <-- note inheritance from object
...: def __init__(self, x, y):...: self.x, self.y = x, y...: def fromXML(cls, xmlText):...: x, y = parseXML(xmlText)...: return cls(x, y)...: fromXML = classmethod(fromXML)
...:In [3]: class NewPoint(Point):...: pass...:In [4]: def parseXML(xmlText):...: return 1, 4...:In [5]: p = NewPoint.fromXML('')
In [6]: isinstance(p, NewPoint)Out[6]: True--Robert Kern[EMAIL PROTECTED]"In the fields of hell where the grass grows high Are the graves of dreams allowed to die."
-- Richard Harter--http://mail.python.org/mailman/listinfo/python-list
--
http://mail.python.org/mailman/listinfo/python-list
Re: Trying to come to grips with static methods
| I've been doing a lot of reading about static methods in Python, and possibly getting over-confused by the minutia of the CPython implementation, as well as by the misnomer. Conceptually, a 'static method' is a function attribute of a class that is to be used as a function and not as a method (where 'methods', in Python, get semi-magic first parameters). Note that function attributes of instances are also just functions, and not methods (which sometimes fools people), as are function attributes of modules. | not exactly sure what they are useful for or why they were introduced. Completeness (as RK said), occasional real usefulness, and for C++&Java programmers. Python did fine without them. | As near as I can see it, static methods are object methods that act just | like functions. Almost: class function/method, depending on your meaning of method. See above. |Er. I always thought that object methods *were* functions, | except they had some runtime magic that passed the object itself as the | first argument. Substitute class for object and class or instance for object itself and you have it. The runtime magic is a minor abbreviation but its major purpose is inheritance. Now you can more on to something more useful like metaclasses or decorators ;-). Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
