Modifying Class Object

John Posner jjposner at optimum.net
Tue Feb 16 01:33:19 CET 2010


On 2/15/2010 6:09 PM, Steven D'Aprano wrote:
> On Mon, 15 Feb 2010 21:25:23 +0000, Arnaud Delobelle wrote:
>
>> John Posner<jjposner at optimum.net>  writes: [...]
>>>>    x = s[0]
>> [...]
>>>    assigns the name *x* to the object that *s[0]* refers to
>>
>> s[0] does not refer to an object, it *is* an object (once evaluated of
>> course, otherwise it's just a Python expression).
>
> Precisely. Treated as an expression, that is, as a string being evaluated
> by the compiler, we would say that it *refers to* an object (unless
> evaluation fails, in which case it refers to nothing at all). But treated
> as whatever you get after the compiler is done with it, that is, post-
> evaluation, we would say that it *is* an object.
>
> This subtle distinction is essentially the difference between a label and
> the thing that is labeled.

Is this your only quibble with my writeup? If, so, I'm gratified. And 
your objections make perfect sense. Still, I'll attempt to justify my 
phrasing. I was originally going to write:

   assigns the name *x* to the object that THE NAME *s[0]* refers to

... but I didn't want to start a distracting argument on the use of the 
phrase *the name* to describe the 4-char string *s[0]*. So now I'll try 
to (briefly) make my case.

Yes, it might be more correct to say that *s[0]* is an expression 
(equivalent to the more obvious expression *s.__getitem__(0)*). But in 
common usage, the 4-char string *s[0]* _behaves_ like a name. If you 
accept this viewpoint, the story on Python assignment statements becomes 
quite simple ...

Syntactic sugar aside, there are only two kinds of assignment statements:

1. NAME = EXPRESSION

The EXPRESSION creates a new object, and the NAME is assigned to that 
object. Examples:

  x = x + 1
  obj = MyClass(1, 2, "red")
  mywordlist = mysentence.split()

2. NAME2 = NAME1

No new object is created. NAME2 becomes another name (an "alias") for 
the existing object that currently has NAME1 assigned to it. Examples:

  y = x
  s[0] = s[42]
  mydict["spamwich"] == this_sandwich
  obj.color = MYCOLORS.LTGREEN

This viewpoint might fail in advanced areas of Python programming: 
properties/descriptors, double-underscore methods, etc. But in my own 
day-to-day usage (admittedly, I'm a hobbyist Python programmer, not a 
professional), it's never failed me to think this way:

  * A dict is a collection of user-devised names, each of which
    is assigned to an object.
  * A list/tuple is an interpreter-maintained collection of integer
    names (0, 1, 2, ...), each of which is assigned to an object.
  * A class instance is very much like a dict.

Tx,
John



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