[Python-Dev] Re: closure semantics

[Zack Weinberg]

Alex Martelli <aleaxit at yahoo.com> writes:

> On Friday 24 October 2003 12:27 am, Zack Weinberg wrote:
>    ...
>> Frankly, I wish Python required one to write explicit declarations for
>> all variables in the program:
>>
>> var x, y, z # module scope
>>
>> class bar:
>>    classvar I, J, K # class variables
>
> Seems like a great way to get uninitialized variables to me.

No, they get a magic cookie value that triggers an exception on use.
Which, incidentally, disambiguates the present UnboundLocalError - is
that a typo, or is that failure to initialize the variable on this
code path?  Consider, eg.


def foo(x):
   s = 2
   if x:
      a = 1
   return a

...
> But then what added value is that 'classvar' boilerplate dirtying
> things up?  Might as well take it off and get
>     I = 2.3
>     J = (2, 3)
>     K = 23
>
> which is just what we have now.
...
>
> There is absolutely no help (not one minute later, not six months later)
> "comprehending" the program just because some silly language mandates
> redundancy, such as a noiseword 'classvar' in front of the assignments.

Understand that I do almost all my programming in typed languages,
where that keyword isn't noise, it's a critical part of the
declaration.

I see where you're coming from with regard to noisewords.  There are
plausible alternatives, although they're all more complicated to
implement and explain, compared to

   var a, b = 2, c = foo()  # a throws UninitializedLocalError if used
                            # before set
   ...
   d     # throws UnboundLocalError
   e = 1 # ALSO throws UnboundLocalError

But in this domain, I am mostly content with the language as is.

I think there really *is* a language deficiency with regard to
declaring class versus instance variables.

class foo:
   A = 1  # these are class variables
   B = 2
   C = 3   

   def __init__(self):
      self.a = 4   # these are instance variables
      self.b = 5
      self.c = 6

I find this imperative syntax for declaring instance variables
profoundly unintuitive.  Further, on my first exposure to Python, I
thought A, B, C were instance variables, although it wasn't hard to
understand why they aren't.

People like to rag on the popularity of __slots__ (for reasons which
are never clearly spelled out, but never mind) -- has anyone
considered that it's popular because it's a way of declaring the set
of instance variables, and there is no other way in the language?

zw