The del statement

[George Sakkis]

One of the few Python constructs that feels less elegant than
necessary to me is the del statement. For one thing, it is overloaded
to mean three different things:
(1) del x: Remove x from the current namespace
(2) del x[i]: Equivalent to x.__delitem__(i )
(3) del x.a: Equivalent to x.__delattr__(' a') (or delattr(x,'a'))

(1) is useful, or even necessary, at least as long as there are only
two namespaces (local and global) instead of a separate namespace per
block, so that's ok for now. The other two though, don't justify IMO a
separate statement.

I would strongly prefer (2) to be implemented by a normal method
instead of a special syntax and method. See the inconsistency:
s = ['a', 'b']
s.pop(0)
del s[0]

Likewise for dicts. Why not s.del(0) ? And just in case someone argues
"for the same reason we have __getitem__ and __setitem__", it is not
the same; the syntax for get/set (s[0], s[0] = 'a') doesn't introduce
a new keyword (or overload an existing one), it is pretty intuitive
and used across many languages. As for (3), it is pretty uncommon to
deserve its own syntax; delattr() or directly modifying self.__dict__
are good enough.

I understand that no more proposals are accepted for Python 3 but it
looks like a missed opportunity to make the language a bit simpler and
more consistent. Anyone else have an opinion on this?

George

[Arnaud Delobelle]

Re: The del statement



George Sakkis wrote:

One of the few Python constructs that feels less elegant than
necessary to me is the del statement. For one thing, it is overloaded
to mean three different things:
(1) del x: Remove x from the current namespace
(2) del x[i]: Equivalent to x.__delitem__(i )
(3) del x.a: Equivalent to x.__delattr__(' a') (or delattr(x,'a'))

Note that the 'X = Y' construct has the corresponding three meanings:

(1) x = 4 # Bind x to 4 in the 'current namespace'
(2) x[i] = 4 # equivalent to x.__setitem__(i , 4)
(3) x.a = 4 # Equivalent to x.__setattr__(' a', 4)

What conclusion should we draw from that?

--
Arnaud

[Duncan Booth]

Re: The del statement

Arnaud Delobelle <arnodel@google mail.comwrote:

>
>
George Sakkis wrote:

>One of the few Python constructs that feels less elegant than
>necessary to me is the del statement. For one thing, it is overloaded
>to mean three different things:
>(1) del x: Remove x from the current namespace
>(2) del x[i]: Equivalent to x.__delitem__(i )
>(3) del x.a: Equivalent to x.__delattr__(' a') (or delattr(x,'a'))

>
Note that the 'X = Y' construct has the corresponding three meanings:
>
(1) x = 4 # Bind x to 4 in the 'current namespace'
(2) x[i] = 4 # equivalent to x.__setitem__(i , 4)
(3) x.a = 4 # Equivalent to x.__setattr__(' a', 4)

I think you both missed a case:

(1b) global x; del x # Remove x from global namespace
(1b) global x; x = 4 # Bind x to 4 in the global namespace

What conclusion should we draw from that?

That Python is simple and consistent.

[Arnaud Delobelle]

Re: The del statement



Duncan Booth wrote:

Arnaud Delobelle <arnodel@google mail.comwrote:
>

George Sakkis wrote:

One of the few Python constructs that feels less elegant than
necessary to me is the del statement. For one thing, it is overloaded
to mean three different things:
(1) del x: Remove x from the current namespace
(2) del x[i]: Equivalent to x.__delitem__(i )
(3) del x.a: Equivalent to x.__delattr__(' a') (or delattr(x,'a'))

Note that the 'X = Y' construct has the corresponding three meanings:

(1) x = 4 # Bind x to 4 in the 'current namespace'
(2) x[i] = 4 # equivalent to x.__setitem__(i , 4)
(3) x.a = 4 # Equivalent to x.__setattr__(' a', 4)

>
I think you both missed a case:
>
(1b) global x; del x # Remove x from global namespace
(1b) global x; x = 4 # Bind x to 4 in the global namespace

This is why you put 'current namespace' in quotes! But all three of
us missed the case:

(1-3000) What about nonlocal?

That Python is simple and consistent.

Seems reasonable to me.

--
Arnaud

[Duncan Booth]

Re: The del statement

Arnaud Delobelle <arnodel@google mail.comwrote:

>
>
Duncan Booth wrote:

>Arnaud Delobelle <arnodel@google mail.comwrote:
>>

>
>
George Sakkis wrote:
>One of the few Python constructs that feels less elegant than
>necessary to me is the del statement. For one thing, it is

overloaded

>to mean three different things:
>(1) del x: Remove x from the current namespace
>(2) del x[i]: Equivalent to x.__delitem__(i )
>(3) del x.a: Equivalent to x.__delattr__(' a') (or delattr(x,'a'))
>
Note that the 'X = Y' construct has the corresponding three

meanings:

>
(1) x = 4 # Bind x to 4 in the 'current namespace'
(2) x[i] = 4 # equivalent to x.__setitem__(i , 4)
(3) x.a = 4 # Equivalent to x.__setattr__(' a', 4)

>>
>I think you both missed a case:
>>
> (1b) global x; del x # Remove x from global namespace
> (1b) global x; x = 4 # Bind x to 4 in the global namespace

>
This is why you put 'current namespace' in quotes! But all three of
us missed the case:

I'd assumed you were just trying to say that the current namespace might
be different things: locals or a class namespace. I would say that the
global statement makes the assign/del refer to a non-current namespace.

>
(1-3000) What about nonlocal?

What about nonlocal? You can't (in Python 2.x) assign or del a name from
an enclosing scope if that's what you mean. If that isn't what you mean
then you'll have to explain it to me in simpler terms.

[George Sakkis]

Re: The del statement

On May 8, 2:58 am, Arnaud Delobelle <arno...@google mail.comwrote:

George Sakkis wrote:

One of the few Python constructs that feels less elegant than
necessary to me is the del statement. For one thing, it is overloaded
to mean three different things:
(1) del x: Remove x from the current namespace
(2) del x[i]: Equivalent to x.__delitem__(i )
(3) del x.a: Equivalent to x.__delattr__(' a') (or delattr(x,'a'))

>
Note that the 'X = Y' construct has the corresponding three meanings:
>
(1) x = 4 # Bind x to 4 in the 'current namespace'
(2) x[i] = 4 # equivalent to x.__setitem__(i , 4)
(3) x.a = 4 # Equivalent to x.__setattr__(' a', 4)
>
What conclusion should we draw from that?

I think you're trying to imply that it is consistent with setting a
value (same with getting). I guess what bugs me about "del" is that
it's a keyword and not some universally well-known punctuation. Do you
you feel that Python misses a "pop" keyword and respective
expressions ?

(1) pop x: Remove x from the current namespace and return it.
(2) pop x[i]: Instead of x.pop(i)
(3) pop x.a: Equivalent to "_y=x.a; del x.a; return y"

George

[Terry Reedy]

Re: The del statement


"George Sakkis" <george.sakkis@ gmail.comwrote in message
news:02860f2a-cd1d-4b1d-ad49-08b13032ba21@2g 2000hsn.googleg roups.com...
| One of the few Python constructs that feels less elegant than
| necessary to me is the del statement. For one thing, it is overloaded
| to mean three different things:
| (1) del x: Remove x from the current namespace
| (2) del x[i]: Equivalent to x.__delitem__(i )
| (3) del x.a: Equivalent to x.__delattr__(' a') (or delattr(x,'a'))

Since I see del x.a as deleting a from the attribute namespace of x, I see
this as the same meaning. A namespace is a specialized association (keys
are identifier strings). A dict is more generalized (keys merely
hashable). So ditto for del dic[key].

The only different meaning is del somelist[i]. The implicit association
between counts in range(n=len(som elist)) *is* broken, but unless i == n-1,
items are 'shifted down' so that some other item become associated with i,
and j's for i < j < n-1 get new associations and n-1 is left with none.
One could imagine del somelist[i] as simple leaving a void in the list.
(Which is not to say that that would be terribly useful.)

tjr

[Michael Torrie]

Re: The del statement

George Sakkis wrote:

I think you're trying to imply that it is consistent with setting a
value (same with getting). I guess what bugs me about "del" is that
it's a keyword and not some universally well-known punctuation. Do you
you feel that Python misses a "pop" keyword and respective
expressions ?

Typically the word "pop" has a different meaning than you describe.
Most programmers think of pop in conjunction with push, for working with
FIFOs. Normally pop would not be expected to take any arguments, or
maybe an argument of a number describing how many items to pop off of a
FIFO (stack). Thus I don't think a pop method would be the right way to
go here.

I don't feel python misses a pop keyword because pop is already used as
methods of classes implementing stacks and FIFOs where appropriate.

>
(1) pop x: Remove x from the current namespace and return it.
(2) pop x[i]: Instead of x.pop(i)
(3) pop x.a: Equivalent to "_y=x.a; del x.a; return y"

The confusion over del does stem from it's behavior, so I can see where
you're coming from. del doesn't delete anything; it just removes a
name. However pop would be just as confusing in my opinion, given that
it's already associated with data structures.