Re: [Tutor] intefaces in python
Am 28.06.2009 um 17:00 schrieb Amit Sethi: Hi , I don't suppose python has a concept of interfaces. But can somebody tell me if their is a way i can implement something like a java interface in python. The Web Framework Zope has a notion of interfaces. http://pypi.python.org/pypi/zope.interface/3.5.1 There are some quite interesting things you can make with interfaces. Eg. the Component Architecture is build on interfaces and adapters. http://wiki.zope.org/zope3/WhatAreInterfaces More on this here: http://docs.zope.org/ juh smime.p7s Description: S/MIME cryptographic signature ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
"Amit Sethi" wrote I think ideally i want a compile Error just like java .. I think you are trying to make Python act like Java which is always a really bad mistake when using a programming language. As Bjarne Stroustrup used to say (repeatedly) "C++ is not Smalltalk". And Python is not Java. If you stop trying to design like Java and start using the extra power of Python you will find your solutions are both more flexible and more simple. Python does not compile so trying to build in "compile time" checking makes no sense. You could do some checks immediately after class definition or more likely class assignment but how would it really benefit you? Get used to how interpreted dynamic languages work and use their features to your benefit instead of trying to force them to act like statically typed compiled languages. It will give you a great sense of freedom! -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
well dir(object) , how would that help . All the functions in base class would automatically be inherited by the objects of plug-in class ... so they would come in the list even if it was not implemented... On Mon, Jun 29, 2009 at 5:58 PM, Luke Paireepinart wrote: > Amit Sethi wrote: >> >> I think ideally i want a compile Error just like java .. > > Why? For developers, so they'll know if their plugin meets the interface > requirements? > Have you considered just making a unit test that will call all interface > functions with appropriate parameters? Then they can just run the test on > their code and if it passes they will know they implemented the minimum > required functions (whether they do what they're supposed to do is a > different story...) It would provide the same level of interface security > as Java would, except it would allow them to decide how much of the contract > they need to enforce for their specific application. That's one of the > things I like about python... for example, if I want to override stdout so > all my print statements go to a file, I just have to create a new class that > has a write() method that writes anything it's passed to a file... I don't > have to implement all of the other functionality that whatever object > resides there before has. So in that case, if I ran your unit test on my > replacement stdout, it may say "Hey, you failed the writeline() test, your > object doesn't have this!" but since I know I'm not going to use writeline, > I can just ignore the warning. Or if I meant for it to have writeline, I > can say "oh snap I need to go implement writeline!" But as I said, that > wouldn't enforce the interface, just inform them of it if they desired that, > which I would personally prefer but you may not. > That's really the idea behind duck typing. Assume that they wrote > sufficient code to handle whatever you use it for and if they didn't it'll > blow up in their face. >> >> @Dave Angel >> >> You have said >> "you could arrange that when the plugin is first encountered, you >> validate that it has all the required methods and data members. Not >> by calling them, but by scanning the object for their existence." >> >> that would be ideal ... can you enlighten me on this how may one do that. >> >> > > Not sure if it's what he meant, but you can just do a dir(object) and it > will return a list with all method / member names. then you could just > confirm that the names are there. > -- A-M-I-T S|S ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
Amit Sethi wrote: I think ideally i want a compile Error just like java .. Why? For developers, so they'll know if their plugin meets the interface requirements? Have you considered just making a unit test that will call all interface functions with appropriate parameters? Then they can just run the test on their code and if it passes they will know they implemented the minimum required functions (whether they do what they're supposed to do is a different story...) It would provide the same level of interface security as Java would, except it would allow them to decide how much of the contract they need to enforce for their specific application. That's one of the things I like about python... for example, if I want to override stdout so all my print statements go to a file, I just have to create a new class that has a write() method that writes anything it's passed to a file... I don't have to implement all of the other functionality that whatever object resides there before has. So in that case, if I ran your unit test on my replacement stdout, it may say "Hey, you failed the writeline() test, your object doesn't have this!" but since I know I'm not going to use writeline, I can just ignore the warning. Or if I meant for it to have writeline, I can say "oh snap I need to go implement writeline!" But as I said, that wouldn't enforce the interface, just inform them of it if they desired that, which I would personally prefer but you may not. That's really the idea behind duck typing. Assume that they wrote sufficient code to handle whatever you use it for and if they didn't it'll blow up in their face. @Dave Angel You have said "you could arrange that when the plugin is first encountered, you validate that it has all the required methods and data members. Not by calling them, but by scanning the object for their existence." that would be ideal ... can you enlighten me on this how may one do that. Not sure if it's what he meant, but you can just do a dir(object) and it will return a list with all method / member names. then you could just confirm that the names are there. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
wait even in the above example i would have to run all the functions in __init__ that is plain stupid ... i was just brain storming On Mon, Jun 29, 2009 at 5:34 PM, Amit Sethi wrote: > I think ideally i want a compile Error just like java .. but from the > discussion here ... i wrote this little example: > > class a(object): > def __init__(self): > self.query() > try: > if self.query_not_implemented==True: > raise NotImplementedError > except AttributeError: > pass > def query(self): > self.query_not_implemented=True > > At least by this when ever the derived class object is created without > implementing a particular function ,it raises a NotImplementedError > which i can later use. > > > @Dave Angel > > You have said > "you could arrange that when the plugin is first encountered, you > validate that it has all the required methods and data members. Not > by calling them, but by scanning the object for their existence." > > that would be ideal ... can you enlighten me on this how may one do that. > > > On Mon, Jun 29, 2009 at 5:01 PM, Dave Angel wrote: >> >> Amit Sethi wrote: >> >>> Well I want to implement plug-in like mechanism for an application . I want >>> to define some minimum functions that any body writing a plugin has to >>> implement. For that i thought an interface would be best because in a >>> scenario where the function is not implemented some kind of error would >>> occur. I would love to hear if you think their is a better way to achieve >>> this >> >> In Java, deriving a class from such an interface, but neglecting to >> implement those methods will cause a compile error (if I recall correctly, >> it's been several years). In Python, the error will happen at run time. >> But an existing runtime error will occur even without such an interface, so >> it wouldn't seem you gain much. In Python, the interface does two things: >> >> 1) it's a comment, a common place to look for certain behavior. >> 2) it's potentially a source for an IDE to provide tool-tips or code >> completion >> 3) it can generate a different error, which is perhaps more useful to the >> developer unsure of how the method is spelled or used. This way he/she >> knows whether to fix the caller or the implementation. >> >> #3 seems valid to me. >> >> >> >> However, for the particular use-case, you might want to stretch a bit >> further. Since you've got one "user" (your code), and many "providers" (the >> plug-in writers) perhaps you could arrange that when the plugin is first >> encountered, you validate that it has all the required methods and data >> members. Not by calling them, but by scanning the object for their >> existence. >> >> >> ___ >> Tutor maillist - tu...@python.org >> http://mail.python.org/mailman/listinfo/tutor > > > > -- > A-M-I-T S|S > -- A-M-I-T S|S ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
I think ideally i want a compile Error just like java .. but from the discussion here ... i wrote this little example: class a(object): def __init__(self): self.query() try: if self.query_not_implemented==True: raise NotImplementedError except AttributeError: pass def query(self): self.query_not_implemented=True At least by this when ever the derived class object is created without implementing a particular function ,it raises a NotImplementedError which i can later use. @Dave Angel You have said "you could arrange that when the plugin is first encountered, you validate that it has all the required methods and data members. Not by calling them, but by scanning the object for their existence." that would be ideal ... can you enlighten me on this how may one do that. On Mon, Jun 29, 2009 at 5:01 PM, Dave Angel wrote: > > Amit Sethi wrote: > >> Well I want to implement plug-in like mechanism for an application . I want >> to define some minimum functions that any body writing a plugin has to >> implement. For that i thought an interface would be best because in a >> scenario where the function is not implemented some kind of error would >> occur. I would love to hear if you think their is a better way to achieve >> this > > In Java, deriving a class from such an interface, but neglecting to implement > those methods will cause a compile error (if I recall correctly, it's been > several years). In Python, the error will happen at run time. But an > existing runtime error will occur even without such an interface, so it > wouldn't seem you gain much. In Python, the interface does two things: > > 1) it's a comment, a common place to look for certain behavior. > 2) it's potentially a source for an IDE to provide tool-tips or code > completion > 3) it can generate a different error, which is perhaps more useful to the > developer unsure of how the method is spelled or used. This way he/she knows > whether to fix the caller or the implementation. > > #3 seems valid to me. > > > > However, for the particular use-case, you might want to stretch a bit > further. Since you've got one "user" (your code), and many "providers" (the > plug-in writers) perhaps you could arrange that when the plugin is first > encountered, you validate that it has all the required methods and data > members. Not by calling them, but by scanning the object for their existence. > > > ___ > Tutor maillist - tu...@python.org > http://mail.python.org/mailman/listinfo/tutor -- A-M-I-T S|S ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
Amit Sethi wrote: Well I want to implement plug-in like mechanism for an application . I want to define some minimum functions that any body writing a plugin has to implement. For that i thought an interface would be best because in a scenario where the function is not implemented some kind of error would occur. I would love to hear if you think their is a better way to achieve this In Java, deriving a class from such an interface, but neglecting to implement those methods will cause a compile error (if I recall correctly, it's been several years). In Python, the error will happen at run time. But an existing runtime error will occur even without such an interface, so it wouldn't seem you gain much. In Python, the interface does two things: 1) it's a comment, a common place to look for certain behavior. 2) it's potentially a source for an IDE to provide tool-tips or code completion 3) it can generate a different error, which is perhaps more useful to the developer unsure of how the method is spelled or used. This way he/she knows whether to fix the caller or the implementation. #3 seems valid to me. However, for the particular use-case, you might want to stretch a bit further. Since you've got one "user" (your code), and many "providers" (the plug-in writers) perhaps you could arrange that when the plugin is first encountered, you validate that it has all the required methods and data members. Not by calling them, but by scanning the object for their existence. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
"Amit Sethi" wrote Well I want to implement plug-in like mechanism for an application . I want to define some minimum functions that any body writing a plugin has to implement. For that i thought an interface would be best because in a scenario where the function is not implemented some kind of error would occur. I would love to hear if you think their is a better way to achieve this Well you could define a real default plugin class that actually does something. Then anyone who wants to create a plugin can inherit from that and either extend the default functions or override them with something different. That will reduce the code that plug-in writers need to create and prevent any error messages appearing for the user. It also means the default plug in acts as sample code for the interface too. HTH, Alan G. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
Well I want to implement plug-in like mechanism for an application . I want to define some minimum functions that any body writing a plugin has to implement. For that i thought an interface would be best because in a scenario where the function is not implemented some kind of error would occur. I would love to hear if you think their is a better way to achieve this On Mon, Jun 29, 2009 at 2:49 PM, Alan Gauld wrote: > "Amit Sethi" wrote > >> class MyInterface(object): >> doSomething(line): >> raise NotImplementedError >> doSomethingElse(line): >> raise NotImplementedError >> >> I think that is exactly the kind of structure i was looking for ... >> > > As a matter of interest, why? What do you anticipate using this for? > I have found a few cases where abstract interfaces are useful but they are > very few and far between. > > -- > Alan Gauld > Author of the Learn to Program web site > http://www.alan-g.me.uk/ > > > ___ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > -- A-M-I-T S|S ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
"Amit Sethi" wrote class MyInterface(object): doSomething(line): raise NotImplementedError doSomethingElse(line): raise NotImplementedError I think that is exactly the kind of structure i was looking for ... As a matter of interest, why? What do you anticipate using this for? I have found a few cases where abstract interfaces are useful but they are very few and far between. -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
class MyInterface(object):
doSomething(line):
raise NotImplementedError
doSomethingElse(line):
raise NotImplementedError
I think that is exactly the kind of structure i was looking for ...
On Mon, Jun 29, 2009 at 12:58 PM, Andre Engels wrote:
> On Sun, Jun 28, 2009 at 5:00 PM, Amit Sethi
> wrote:
> > Hi , I don't suppose python has a concept of interfaces. But can somebody
> > tell me if their is a way i can implement something like a java
> interface
> > in python.
>
> Sure. Interfaces are just Java's compensation for not having multiple
> inheritance. Python does have multiple inheritance, so that's what one
> would use. Although one could also use duck typing, and then use
> 'nothing' as an implementation...
>
> More specific:
>
>
> Java Interface:
> public interface MyInterface {
>string doSomething(string line);
>string doSomethingElse(string line);
> }
>
> Java Implementation:
> public class MyImplementation {
> string doSomething(string line) {
> return "I did something with" + line;
> }
> string doSomethingElse(string line) {
> return "I did something else."
> }
> }
>
> ==
> Python Interface:
>
> class MyInterface(object):
>doSomething(line):
>raise NotImplementedError
>doSomethingElse(line):
>raise NotImplementedError
>
> Python Implementation:
> class MyImplementation(MyInterface):
>doSomething(line):
>return "I did something with "+line
>doSomethingElse(line):
>return "I did something else."
>
> ==
> Python interface using duck typing:
>
> # Hey guys, when you call something a 'MyInterface', it needs methods
> doSomething and doSomethingElse
>
> Python Implementation using duck typing:
>
> class MyImplementation(object):
># These things implement MyInterface
>doSomething(line):
>return "I did something with "+line
>doSomethingElse(line):
>return "I did something else."
>
>
> --
> André Engels, andreeng...@gmail.com
>
--
A-M-I-T S|S
___
Tutor maillist - Tutor@python.org
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Re: [Tutor] intefaces in python
On Sun, Jun 28, 2009 at 5:00 PM, Amit Sethi wrote:
> Hi , I don't suppose python has a concept of interfaces. But can somebody
> tell me if their is a way i can implement something like a java interface
> in python.
Sure. Interfaces are just Java's compensation for not having multiple
inheritance. Python does have multiple inheritance, so that's what one
would use. Although one could also use duck typing, and then use
'nothing' as an implementation...
More specific:
Java Interface:
public interface MyInterface {
string doSomething(string line);
string doSomethingElse(string line);
}
Java Implementation:
public class MyImplementation {
string doSomething(string line) {
return "I did something with" + line;
}
string doSomethingElse(string line) {
return "I did something else."
}
}
==
Python Interface:
class MyInterface(object):
doSomething(line):
raise NotImplementedError
doSomethingElse(line):
raise NotImplementedError
Python Implementation:
class MyImplementation(MyInterface):
doSomething(line):
return "I did something with "+line
doSomethingElse(line):
return "I did something else."
==
Python interface using duck typing:
# Hey guys, when you call something a 'MyInterface', it needs methods
doSomething and doSomethingElse
Python Implementation using duck typing:
class MyImplementation(object):
# These things implement MyInterface
doSomething(line):
return "I did something with "+line
doSomethingElse(line):
return "I did something else."
--
André Engels, andreeng...@gmail.com
___
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
Bob Rea wrote: > On Sun June 28 2009 1:48 pm, Alan Gauld wrote: >> advantages to them over defining, say, a mixin > > Noob, here, wanted to know what a mixin is > eh, just getting back into learning python > googled it, still not sure what it is, but > wikipedia says the name come from ice cream mixins at > Steve's Ice Cream Parlor > > Ah yes, i was a frequenter there > > Will watch out for what it is when I get farther into python > may have to name them things like jimmies and > heath_bar_crunch > In object-oriented programming languages, a mixin is a class that provides a certain functionality to be inherited by a subclass, while not meant for instantiation (the generating of objects of that class). Inheriting from a mixin is not a form of specialization but is rather a means of collecting functionality. A class may inherit most or all of its functionality from one or more mixins through multiple inheritance. -- http://en.wikipedia.org/wiki/Mixin In regular inheritance, you inherit from another class to inherit its interfaces. In mixin inheritance, you inherit for its implementations. The mixin concept takes it a bit further; the mixin class (parent class) may not be usable by itself (i.e. instantiating a mixin class may not make any sense) and must be inherited . Example: # Flavours class Vanilla(object): pass class Chocolate(object): pass # Additions class Nuts(object): pass class Cookies(object): pass # Items class IceCream(): pass # Usable classes class ChocolateIceCream(Chocolate, IceCream): pass class VanillaAndNutsIceCream(Vanilla, Nuts, IceCream): pass class ChocolateAndCookiesIceCream(Chocolate, Cookies, IceCream): pass Vanilla(), Chocolate(), Nuts(), and Cookies() are not meant to be instantiated directly; they are meant to be inherited. These classes are called mixin classes. In python's standard lib, an example of mixin class is threading.Thread() ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
On Sun June 28 2009 1:48 pm, Alan Gauld wrote: > advantages to them over defining, say, a mixin Noob, here, wanted to know what a mixin is eh, just getting back into learning python googled it, still not sure what it is, but wikipedia says the name come from ice cream mixins at Steve's Ice Cream Parlor Ah yes, i was a frequenter there Will watch out for what it is when I get farther into python may have to name them things like jimmies and heath_bar_crunch -- Bob Rea mailto:gapet...@stsams.org http://www.petard.us http://www.petard.us/blog http://www.petard.us/gallery Where is Bill Stringfellow now that we really need him? ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] intefaces in python
"Amit Sethi" wrote Hi , I don't suppose python has a concept of interfaces. Yes and No, I'll come back to that... But can somebody tell me if their is a way i can implement something like a java interface in python. First, can you tell me why you would want to? Java style interfaces tend to make your code much less reusable and much less flexible if you are using a dynamically typed language. There are very few real advantages to them over defining, say, a mixin or using multiple inheritance (which of course Java can't do) The normal way of defining a Java style Python interface class is simply to define a class that has a set of methods thaty either do or return nothing or raise some kind of NotImplementedError exception. But usually providing a mixin is a much more powerful style of programming in Python since you can provide partial implementations of the methods or generic methods that are not dependant on the types of the parameters. Coming back to the original question, Python has a very strong concept of an interface, that is how it checks types, if an interface does not exist it will raise a TypeError. But it does it at runtime and it does it at the method level not the class level. Very different to Microsoft's concept which was designed to meet the needs of COM and was subsequently adopted by Java. There has also been talk of introducing syntax to create interfaces into Python which I personally think is a very, very poor idea! But quite a lot of what I think are poor ideas get intro Python so that doesn't mean much! :-) Alan G. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
[Tutor] intefaces in python
Hi , I don't suppose python has a concept of interfaces. But can somebody tell me if their is a way i can implement something like a java interface in python. -- A-M-I-T S|S ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
