Re: Does Class implements Interface?
Jonathan, Stephen and Max, thank you all for the tips and tricks. Much appreciated. Manu -- http://mail.python.org/mailman/listinfo/python-list
Re: Does Class implements Interface?
On Aug 27, 2009, at 9:16 AM, Emanuele D'Arrigo wrote: Are there resources such as tools, recipes, documents or strategies that could help me deal with these issues? I've already looked into the ABC module and the zope.interface. I'm just fishing for more things to look at. You say above that you looked at zope.interface. Did you look at zope.interface.verify? Specifically, verify.txt doctests state that: An object provides an interface if - either its class declares that it implements the interfaces, or the object declares that it directly provides the interface - the object defines all the methods required by the interface - all the methods have the correct signature - the object defines all non-method attributes required by the interface zope.interface.verify.verifyObject(I, obj) will check all of the above. Similarly, zope.interface.verify.verifyClass(I, C) will check the class. It is a good practice to call these functions in the regression tests. If this is not what you are looking for, sorry for the noise. Zvezdan -- http://mail.python.org/mailman/listinfo/python-list
Re: Does Class implements Interface?
On Thu, Aug 27, 2009 at 9:16 AM, Emanuele D'Arrigo man...@gmail.com wrote: Greetings everybody, let's say I have a Class C and I'd like to verify if it implements Interface I. If I is available to me as a class object I can use issubclass(C, I) and I can at least verify that I is a superclass of C. There are a couple of issues with this approach however: 1) C might override not just I's methods code but also I's methods signatures, effectively changing the interface. 2) What if I is -not- available to me as the superclass of C but as a description of the interface, i.e. as an IDL(*) file? Are there resources such as tools, recipes, documents or strategies that could help me deal with these issues? I've already looked into the ABC module and the zope.interface. I'm just fishing for more things to look at. Manu (*) http://en.wikipedia.org/wiki/Interface_description_language Python's general philosophy is that it's Easier to Ask Forgiveness than Permission. Don't check the interface, just assume it's true and wrap the code block in a try/except in case it isn't. -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Does Class implements Interface?
On Aug 27, 6:16 am, Emanuele D'Arrigo man...@gmail.com wrote: Greetings everybody, let's say I have a Class C and I'd like to verify if it implements Interface I. If I is available to me as a class object I can use issubclass(C, I) and I can at least verify that I is a superclass of C. There are a couple of issues with this approach however: 1) C might override not just I's methods code but also I's methods signatures, effectively changing the interface. 2) What if I is -not- available to me as the superclass of C but as a description of the interface, i.e. as an IDL(*) file? Are there resources such as tools, recipes, documents or strategies that could help me deal with these issues? I've already looked into the ABC module and the zope.interface. I'm just fishing for more things to look at. Have you heard of duck typing? Ignore all those things and rely on human (aka natural language) documentation. That is, if you want to see if a class will work for an interface, go read the docs on the interface (or rather, what the function expects the interface to be) and what the class provides and see if they fit. -- http://mail.python.org/mailman/listinfo/python-list
Re: Does Class implements Interface?
On Aug 27, 9:42 pm, Jonathan Gardner jgard...@jonathangardner.net wrote: Have you heard of duck typing? Yes. Ignore all those things and rely on human (aka natural language) documentation. That is, if you want to see if a class will work for an interface, go read the docs on the interface (or rather, what the function expects the interface to be) and what the class provides and see if they fit. Apologies, my fault, I didn't explain that humans are out of the loop entirely. It's only at runtime that the program obtains the class object that might or might not conform to an expected interface. In fact the program might obtain multiple objects and have to decide which one conforms best to the expected interface. Of course I could use hasattr() and the inspect module (anything else?) to find out if existing attributes and method signatures are those expected. Or I could just deal with the exceptions as they are raised. However, if I could somehow rely that the object I'm getting has the right methods and signatures I could avoid peppering the code with try/except constructs. I was just wondering then if this has been somewhat dealt with and has been wrapped in a neat package, set of functions, recipe or pattern. Manu -- http://mail.python.org/mailman/listinfo/python-list
Re: Does Class implements Interface?
On Aug 27, 3:09 pm, Emanuele D'Arrigo man...@gmail.com wrote: Apologies, my fault, No apology is necessary. I didn't explain that humans are out of the loop entirely. It's only at runtime that the program obtains the class object that might or might not conform to an expected interface. In fact the program might obtain multiple objects and have to decide which one conforms best to the expected interface. Of course I could use hasattr() and the inspect module (anything else?) to find out if existing attributes and method signatures are those expected. Or I could just deal with the exceptions as they are raised. However, if I could somehow rely that the object I'm getting has the right methods and signatures I could avoid peppering the code with try/except constructs. I was just wondering then if this has been somewhat dealt with and has been wrapped in a neat package, set of functions, recipe or pattern. Don't bother with introspection using hasattr() and such. Use exceptions. If the object that is instantiated at run-time doesn't provide the right interface, throw an exception and direct it to the responsible party. This isn't much different than what you'd do if you're writing an app that expects a string of digits but is instead given a string of non- digits. Rather than inspecting the string before converting it (introspection), just try to convert it and catch the exception. Then handle the exception such that you tell the person who entered the data, I wanted numbers, not letters and allow them to try again. When you stop asking Did I get something I expect? except only when it is absolutely necessary, then you can get back to writing your program. -- http://mail.python.org/mailman/listinfo/python-list
Re: Does Class implements Interface?
On Aug 27, 3:09 pm, Emanuele D'Arrigo man...@gmail.com wrote: On Aug 27, 9:42 pm, Jonathan Gardner jgard...@jonathangardner.net wrote: Have you heard of duck typing? Yes. I was just wondering then if this has been somewhat dealt with and has been wrapped in a neat package, set of functions, recipe or pattern. As a joke (but only partly), here's the python code you'd use to check to see if a value conforms with expectations: code /code As you can see, the above code is not only easy to write, but easy to write unit tests for, compile, run, and debug. It simply doesn't get better than this. -- http://mail.python.org/mailman/listinfo/python-list
Re: Does Class implements Interface?
Emanuele D'Arrigo wrote: On Aug 27, 9:42 pm, Jonathan Gardner jgard...@jonathangardner.net wrote: Have you heard of duck typing? Yes. Ignore all those things and rely on human (aka natural language) documentation. That is, if you want to see if a class will work for an interface, go read the docs on the interface (or rather, what the function expects the interface to be) and what the class provides and see if they fit. Apologies, my fault, I didn't explain that humans are out of the loop entirely. It's only at runtime that the program obtains the class object that might or might not conform to an expected interface. In fact the program might obtain multiple objects and have to decide which one conforms best to the expected interface. Of course I could use hasattr() and the inspect module (anything else?) to find out if existing attributes and method signatures are those expected. Or I could just deal with the exceptions as they are raised. However, if I could somehow rely that the object I'm getting has the right methods and signatures I could avoid peppering the code with try/except constructs. I was just wondering then if this has been somewhat dealt with and has been wrapped in a neat package, set of functions, recipe or pattern. Manu Check out Alex Martelli's recipe 'Checking wheter an object has necessary attributes' in the Python Cookbook: http://books.google.co.uk/books?id=Q0s6Vgb98CQCprintsec=frontcoverdq=python+cookbook#v=onepageq=f=false http://books.google.co.uk/books?id=Q0s6Vgb98CQCprintsec=frontcoverdq=python+cookbook#v=onepageq=f=false Max -- http://mail.python.org/mailman/listinfo/python-list
