Tkinter callback arguments

[Diez B. Roggisch]

Alf P. Steinbach schrieb:
> * Diez B. Roggisch:
>>> Your comment about "computed" makes it more clear what that's all about.
>>> Also Bertrand Meyer (Eiffel language creator) had idea like that, he
>>> called it "referential transparency". But I think when Python has this
>>> nice property mechanism, why do people change direct data attributes 
>>> into
>>> properties and not the other way around or not at all, I mean using only
>>> properties for logical
>>> data attributes  --  e.g. assuring correctness first via read-only
>>> property?
>>
>> I fail to see where read-only-ness of an attribute is a priori more 
>> correct
>> than having modifyable attributes.
> 
> No, I didn't mean that it is more correct to have an attribute as 
> read-only. I meant that letting a logical data attribute start out as a 
> read only property can help to ensure correctness.

Which is the same thing said with other words.


> For example, consider 
> two rectangle classes R1 and R2, where R2 might be a successor to R1, at 
> some point in system evolution replacing R1. R1 has logical data members 
> left, top, width and height, and R2 has logical data members left, top, 
> right and bottom. With R1 direct changes of left and top keeps the 
> rectangle's size (since that size is specified by width and height), 
> while with R2 it changes the rectangle's size. R1 is implemented with 
> logical data members as directly exposed data attributes. Some code in 
> the system deals only with R1 objects and for convenience or efficiency 
> or whatever uses direct modification instead of set_position method. Due 
> to new requirements it instead has to deal with R2 objects, with same 
> methods. But due to the direct modification of object state it now 
> changes the rectangle sizes, but at first it's not noticed since the 
> attempted rectangle position changes are very small. People get upset. 
> The bug is fixed. Time has been wasted.

If there is need for mutable rectangles, there is need for mutable 
rectangles. Using properties instead of attributes doesn't help - if you 
change semantics of something, code might break. In Python, attributes 
are part of the public interface as long as you don't explicitly define 
them otherwise.

But preliminary assumptions about what could be in some yet unseen 
future is introducing more code with more chances of errors, reducing 
the flexibility at the same time. I still fail to see where that's good.

Code for what you need code for. It works - in all languages, but 
especially in Python.

Diez