[Tutor] intefaces in python

[Dave Angel]

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.