[Tutor] Python Assignment Expression and Callable Expression

[Steven D'Aprano]

On Fri, Sep 19, 2014 at 02:41:31AM +0000, Wang Lei (ERIAN) wrote:
> Hi, everyone:
> 
> I really believe that python is a great language but assignment and callable are not flexible:
> 
> I wish that I can do this:
> 
> class matrixArray(list):
> bla bla bla
>     def __call__(self, rid, cid):
>         return self.head[rid][cid]

You can. __call__ is used for making objects a callable, function-like 
object. C++ calls them "functors". (Not the same thing as what Haskell 
calls functors!)


> Once I call mat(1,1), I can get a result. but I want it more a value not a reference.

You want to change the entire execution model of Python? Why?

Like most modern languages, such as Ruby, Java (mostly), and Javascript, 
Python values are references, not unboxed low-level primitive values.


> Considering "mat(1,1) = 5" expression, I wish the parser can 
> dynamically map "mat(1,1)" to reference of value of that "or anonymous 
> reference" or reference defined in class.

I'm afraid I don't understand what you are trying to say, but if you 
want to assign to individual items in a matrix, use indexing, not 
function call:

mat[1, 1] = 5

will work perfectly once you add a __setitem__ method to your matrix 
class.


> I don't want to modify the 
> lexical parsing in C++ but failed after trying different method in 
> pythonic ways:

What does C++ have to do with this?

> decorator: fun -> object mapping, because it just substitute function 
> name with new function and cannot read "self" object.
> 
> I wish python developers could think of an idea to update the lexical 
> parsing method or simply provide us a tool to modify it in python 
> context.

I think that their answer will be:

"We will not complicate the language, making our job enormously harder, 
and Python much harder to learn and use, just because a beginner to 
Python who doesn't understand what the language can do or how to use it, 
wants to program in C++ instead of Python. If you want to program in 
C++, use C++. If you want to program in Python, learn Python."

What do you expect to do with:

mat(1, 1) = 5


that cannot be done with this instead?

mat[1, 1] = 5



-- 
Steven