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Addressing the last element of a list

Isn't there an easier way than

lst[len(lst) - 1] = ...

?

Nov 8 '05
57 41432
Op 2005-11-15, Mike Meyer schreef <mw*@mired.org>:
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
Like having an assignment operator (let use @= for it) next to a
(re)bind operator.
We could then have something like the following.
a = 5
b = a
a @= 7
b ==> would result in 7.
You've just overwritten the object referred to by the token "5" in the
source code with the value 7, so you get:
print 5
7

You have a valid point, but I think a different approach is possible.
Don't make the distinction between mutable and immutable types but
between mutable and immutable objects. So an int wouldn't be an
immutable type, but 5 would be an immutable object.
So the code above I gave would throw an exception, but the following
might work.
a @= 5
b = a
b @= 7
a ==> would result in 7.


Which solves that issue but brings up - well, more issues. What
happens if the third line is 'b @= "seven"'? Does this require an
extra level of indirection in the implementation? Avoiding that kind
of thing was the reason for suggesting this:


It depends on how far you want to go.

I think one can argue that in case of an inplace replacement, this
only makes sense if the two objects belong to the same class. So
no extra level of indirection is then required.

Otherwise it really depends on how the builtin objects are implemented.
For user classes, a somewhat simplistic implementation could be
something like:

def '@=' (one, two):
one.__dict__.clear()
one.__dict__.update(two.__dict__)

But if such an approach is usable with the builtin classes, I don't
know. I'm sure this will have it's own issues though.
Another approach is to have a special object type if you want to allow
assignment to the object it refers to. That's what Python does now, it
just doesn't have a syntax just to support that. If it did, your
example might look like:
a := 5
b = @a
a := 7
@b ==> would result in 7

Could it also work the other way? By somehow manipulating b change a?


It's intended to work the other way as well. But the second line is
wrong: it should have been "b = a". "b = @a" assigns b the value
referenced by a, and @b would then be an error. "b = a" assigns b to
the reference that is a, so that @b returns it's value.

The critical thing is that this doesn't introduce any new facilities
into the language, just some new syntax, so there's no implementation
impact. In fact, if you're willing to put up with some notational
abuse, we can do this now:

class Ref(object):
_unbound = object()
def __new__(cls, value = _unbound):
"""We're an object, but need to ignore the optional argument."""

return object.__new__(cls)

def __init__(self, value = _unbound):
"""Bind the optional value, if provided."""
if value is not self._unbound:
self._value = value

def __pos__(self):
"""Return value, if bound."""
try:
return self._value
except AttributeError:
raise ValueError, "%s object does not have a value stored." % \
self.__class__.__name__

def __iadd__(self, value):
self._value = value
return self

Usage:
x = Ref.Ref()
x += 23
+x 23 a = x
x += 25
+a 25 a += "this is a test"
+x

'this is a test'

Since it doesn't have real lannguage support, things like +x += 25
don't work. That's the only obvious gotcha. Maybe ~ would be a better
prefix.


I'm a bit puzzled on how you would implement this as real language
support. As far as I understand this only works with Ref objects.

You can't do something like the following.

l = [3, 7]
a = Ref(l[0])

Instead you would have to do something like
l = [Ref(3), Ref(7)]
a = l[0]

So it seems that if you want to use this idea for implementing langauge
support you will have to wrap all objects in a Ref internally and this
seems some kind of extra indirection too.

--
Antoon Pardon
Nov 22 '05 #51
Op 2005-11-15, Mike Meyer schreef <mw*@mired.org>:
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
Like having an assignment operator (let use @= for it) next to a
(re)bind operator.
We could then have something like the following.
a = 5
b = a
a @= 7
b ==> would result in 7.
You've just overwritten the object referred to by the token "5" in the
source code with the value 7, so you get:
print 5
7

You have a valid point, but I think a different approach is possible.
Don't make the distinction between mutable and immutable types but
between mutable and immutable objects. So an int wouldn't be an
immutable type, but 5 would be an immutable object.
So the code above I gave would throw an exception, but the following
might work.
a @= 5
b = a
b @= 7
a ==> would result in 7.


Which solves that issue but brings up - well, more issues. What
happens if the third line is 'b @= "seven"'? Does this require an
extra level of indirection in the implementation? Avoiding that kind
of thing was the reason for suggesting this:


It depends on how far you want to go.

I think one can argue that in case of an inplace replacement, this
only makes sense if the two objects belong to the same class. So
no extra level of indirection is then required.

Otherwise it really depends on how the builtin objects are implemented.
For user classes, a somewhat simplistic implementation could be
something like:

def '@=' (one, two):
one.__dict__.clear()
one.__dict__.update(two.__dict__)

But if such an approach is usable with the builtin classes, I don't
know. I'm sure this will have it's own issues though.
Another approach is to have a special object type if you want to allow
assignment to the object it refers to. That's what Python does now, it
just doesn't have a syntax just to support that. If it did, your
example might look like:
a := 5
b = @a
a := 7
@b ==> would result in 7

Could it also work the other way? By somehow manipulating b change a?


It's intended to work the other way as well. But the second line is
wrong: it should have been "b = a". "b = @a" assigns b the value
referenced by a, and @b would then be an error. "b = a" assigns b to
the reference that is a, so that @b returns it's value.

The critical thing is that this doesn't introduce any new facilities
into the language, just some new syntax, so there's no implementation
impact. In fact, if you're willing to put up with some notational
abuse, we can do this now:

class Ref(object):
_unbound = object()
def __new__(cls, value = _unbound):
"""We're an object, but need to ignore the optional argument."""

return object.__new__(cls)

def __init__(self, value = _unbound):
"""Bind the optional value, if provided."""
if value is not self._unbound:
self._value = value

def __pos__(self):
"""Return value, if bound."""
try:
return self._value
except AttributeError:
raise ValueError, "%s object does not have a value stored." % \
self.__class__.__name__

def __iadd__(self, value):
self._value = value
return self

Usage:
x = Ref.Ref()
x += 23
+x 23 a = x
x += 25
+a 25 a += "this is a test"
+x

'this is a test'

Since it doesn't have real lannguage support, things like +x += 25
don't work. That's the only obvious gotcha. Maybe ~ would be a better
prefix.


I'm a bit puzzled on how you would implement this as real language
support. As far as I understand this only works with Ref objects.

You can't do something like the following.

l = [3, 7]
a = Ref(l[0])

Instead you would have to do something like
l = [Ref(3), Ref(7)]
a = l[0]

So it seems that if you want to use this idea for implementing langauge
support you will have to wrap all objects in a Ref internally and this
seems some kind of extra indirection too.

--
Antoon Pardon
Nov 22 '05 #52
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
Op 2005-11-15, Mike Meyer schreef <mw*@mired.org>:
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
> Like having an assignment operator (let use @= for it) next to a
> (re)bind operator.
> We could then have something like the following.
> a = 5
> b = a
> a @= 7
> b ==> would result in 7.
You've just overwritten the object referred to by the token "5" in the
source code with the value 7, so you get:
print 5
7
You have a valid point, but I think a different approach is possible.
Don't make the distinction between mutable and immutable types but
between mutable and immutable objects. So an int wouldn't be an
immutable type, but 5 would be an immutable object.
So the code above I gave would throw an exception, but the following
might work.
a @= 5
b = a
b @= 7
a ==> would result in 7.
Which solves that issue but brings up - well, more issues. What
happens if the third line is 'b @= "seven"'? Does this require an
extra level of indirection in the implementation? Avoiding that kind
of thing was the reason for suggesting this:


It depends on how far you want to go.

I think one can argue that in case of an inplace replacement, this
only makes sense if the two objects belong to the same class. So
no extra level of indirection is then required.


Now factor in inheritance. Do users of this facility have to worry
about static OO typing? That is, two objects that are otherwise
completely interchangeable will raise an exception if you try to
assign one to a variable holding the other, because they have the
wrong type. That seems unpythonic.
Otherwise it really depends on how the builtin objects are implemented.
For user classes, a somewhat simplistic implementation could be
something like:

def '@=' (one, two):
one.__dict__.clear()
one.__dict__.update(two.__dict__)
Wouldn't one.__dict__ = dict(**two.__dict__) be a bit better?

Anyway, this won't' work if the class of one of the objects has
slots.
The critical thing is that this doesn't introduce any new facilities
into the language, just some new syntax, so there's no implementation
impact. In fact, if you're willing to put up with some notational
abuse, we can do this now:

class Ref(object):
_unbound = object()
def __new__(cls, value = _unbound):
"""We're an object, but need to ignore the optional argument."""

return object.__new__(cls)

def __init__(self, value = _unbound):
"""Bind the optional value, if provided."""
if value is not self._unbound:
self._value = value

def __pos__(self):
"""Return value, if bound."""
try:
return self._value
except AttributeError:
raise ValueError, "%s object does not have a value stored." % \
self.__class__.__name__

def __iadd__(self, value):
self._value = value
return self

Usage:
> x = Ref.Ref()
> x += 23
> +x

23
> a = x
> x += 25
> +a

25
> a += "this is a test"
> +x

'this is a test'

Since it doesn't have real lannguage support, things like +x += 25
don't work. That's the only obvious gotcha. Maybe ~ would be a better
prefix.


I'm a bit puzzled on how you would implement this as real language
support. As far as I understand this only works with Ref objects.


Correct. That's the point.
You can't do something like the following.

l = [3, 7]
a = Ref(l[0])

Instead you would have to do something like
l = [Ref(3), Ref(7)]
a = l[0]

So it seems that if you want to use this idea for implementing langauge
support you will have to wrap all objects in a Ref internally and this
seems some kind of extra indirection too.


No, the idea is to expose this class to the user. They would have to
explicitly wrap objects that they want to be able to be get a
reference to. Other object types can be implemented without having to
worry about assigning to them. If you want to assign to an object, you
cdreate it as a Ref. You still have to have a special syntax so you
can distinguish assigning to a Ref from binding a name, and getting
the value of the Ref vs. getting the Ref.

<mike
--
Mike Meyer <mw*@mired.org> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
Nov 22 '05 #53
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
Op 2005-11-15, Mike Meyer schreef <mw*@mired.org>:
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
> Like having an assignment operator (let use @= for it) next to a
> (re)bind operator.
> We could then have something like the following.
> a = 5
> b = a
> a @= 7
> b ==> would result in 7.
You've just overwritten the object referred to by the token "5" in the
source code with the value 7, so you get:
print 5
7
You have a valid point, but I think a different approach is possible.
Don't make the distinction between mutable and immutable types but
between mutable and immutable objects. So an int wouldn't be an
immutable type, but 5 would be an immutable object.
So the code above I gave would throw an exception, but the following
might work.
a @= 5
b = a
b @= 7
a ==> would result in 7.
Which solves that issue but brings up - well, more issues. What
happens if the third line is 'b @= "seven"'? Does this require an
extra level of indirection in the implementation? Avoiding that kind
of thing was the reason for suggesting this:


It depends on how far you want to go.

I think one can argue that in case of an inplace replacement, this
only makes sense if the two objects belong to the same class. So
no extra level of indirection is then required.


Now factor in inheritance. Do users of this facility have to worry
about static OO typing? That is, two objects that are otherwise
completely interchangeable will raise an exception if you try to
assign one to a variable holding the other, because they have the
wrong type. That seems unpythonic.
Otherwise it really depends on how the builtin objects are implemented.
For user classes, a somewhat simplistic implementation could be
something like:

def '@=' (one, two):
one.__dict__.clear()
one.__dict__.update(two.__dict__)
Wouldn't one.__dict__ = dict(**two.__dict__) be a bit better?

Anyway, this won't' work if the class of one of the objects has
slots.
The critical thing is that this doesn't introduce any new facilities
into the language, just some new syntax, so there's no implementation
impact. In fact, if you're willing to put up with some notational
abuse, we can do this now:

class Ref(object):
_unbound = object()
def __new__(cls, value = _unbound):
"""We're an object, but need to ignore the optional argument."""

return object.__new__(cls)

def __init__(self, value = _unbound):
"""Bind the optional value, if provided."""
if value is not self._unbound:
self._value = value

def __pos__(self):
"""Return value, if bound."""
try:
return self._value
except AttributeError:
raise ValueError, "%s object does not have a value stored." % \
self.__class__.__name__

def __iadd__(self, value):
self._value = value
return self

Usage:
> x = Ref.Ref()
> x += 23
> +x

23
> a = x
> x += 25
> +a

25
> a += "this is a test"
> +x

'this is a test'

Since it doesn't have real lannguage support, things like +x += 25
don't work. That's the only obvious gotcha. Maybe ~ would be a better
prefix.


I'm a bit puzzled on how you would implement this as real language
support. As far as I understand this only works with Ref objects.


Correct. That's the point.
You can't do something like the following.

l = [3, 7]
a = Ref(l[0])

Instead you would have to do something like
l = [Ref(3), Ref(7)]
a = l[0]

So it seems that if you want to use this idea for implementing langauge
support you will have to wrap all objects in a Ref internally and this
seems some kind of extra indirection too.


No, the idea is to expose this class to the user. They would have to
explicitly wrap objects that they want to be able to be get a
reference to. Other object types can be implemented without having to
worry about assigning to them. If you want to assign to an object, you
cdreate it as a Ref. You still have to have a special syntax so you
can distinguish assigning to a Ref from binding a name, and getting
the value of the Ref vs. getting the Ref.

<mike
--
Mike Meyer <mw*@mired.org> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
Nov 22 '05 #54
Mike Meyer wrote:
Wouldn't one.__dict__ = dict(**two.__dict__) be a bit better?


Those stars are redundant here. It would be cleaner to write:

one.__dict__ = dict(two.__dict__)
Nov 22 '05 #55
Mike Meyer wrote:
Wouldn't one.__dict__ = dict(**two.__dict__) be a bit better?


Those stars are redundant here. It would be cleaner to write:

one.__dict__ = dict(two.__dict__)
Nov 22 '05 #56
Op 2005-11-16, Mike Meyer schreef <mw*@mired.org>:
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
Op 2005-11-15, Mike Meyer schreef <mw*@mired.org>:
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
>> Like having an assignment operator (let use @= for it) next to a
>> (re)bind operator.
>> We could then have something like the following.
>> a = 5
>> b = a
>> a @= 7
>> b ==> would result in 7.
> You've just overwritten the object referred to by the token "5" in the
> source code with the value 7, so you get:
> print 5
> 7
You have a valid point, but I think a different approach is possible.
Don't make the distinction between mutable and immutable types but
between mutable and immutable objects. So an int wouldn't be an
immutable type, but 5 would be an immutable object.
So the code above I gave would throw an exception, but the following
might work.
a @= 5
b = a
b @= 7
a ==> would result in 7.

Which solves that issue but brings up - well, more issues. What
happens if the third line is 'b @= "seven"'? Does this require an
extra level of indirection in the implementation? Avoiding that kind
of thing was the reason for suggesting this:


It depends on how far you want to go.

I think one can argue that in case of an inplace replacement, this
only makes sense if the two objects belong to the same class. So
no extra level of indirection is then required.


Now factor in inheritance. Do users of this facility have to worry
about static OO typing? That is, two objects that are otherwise
completely interchangeable will raise an exception if you try to
assign one to a variable holding the other, because they have the
wrong type. That seems unpythonic.


I don't care much about pythonic or not. It seems a very volatile
concept, that adapts as the language evolves. I wouldn't be
surprised if augmented arithmetic operations would have been
considered unpythonic at some point. You could see this
limitation as a wart, but IMO the idea would still be usefull
with that limitation.

The limitation would ensure for example that although the value
of an argument could change, it's class couldn't. I think that
is worthwhile.
Otherwise it really depends on how the builtin objects are implemented.
For user classes, a somewhat simplistic implementation could be
something like:

def '@=' (one, two):
one.__dict__.clear()
one.__dict__.update(two.__dict__)


Wouldn't one.__dict__ = dict(**two.__dict__) be a bit better?

Anyway, this won't' work if the class of one of the objects has
slots.


So? I wrote it was a simplistic idea. but things like setattr and
getattr, still work with objects that have slots. So which attributes
are present for a particulare object has to be available in the
python interpreter. So my guess is that slots wont be the big stumbling
block, should one consider implementing something like this.
The critical thing is that this doesn't introduce any new facilities
into the language, just some new syntax, so there's no implementation
impact. In fact, if you're willing to put up with some notational
abuse, we can do this now:

class Ref(object):
_unbound = object()
def __new__(cls, value = _unbound):
"""We're an object, but need to ignore the optional argument."""

return object.__new__(cls)

def __init__(self, value = _unbound):
"""Bind the optional value, if provided."""
if value is not self._unbound:
self._value = value

def __pos__(self):
"""Return value, if bound."""
try:
return self._value
except AttributeError:
raise ValueError, "%s object does not have a value stored." % \
self.__class__.__name__

def __iadd__(self, value):
self._value = value
return self

Usage:

>> x = Ref.Ref()
>> x += 23
>> +x
23
>> a = x
>> x += 25
>> +a
25
>> a += "this is a test"
>> +x
'this is a test'

Since it doesn't have real lannguage support, things like +x += 25
don't work. That's the only obvious gotcha. Maybe ~ would be a better
prefix.


I'm a bit puzzled on how you would implement this as real language
support. As far as I understand this only works with Ref objects.


Correct. That's the point.
You can't do something like the following.

l = [3, 7]
a = Ref(l[0])

Instead you would have to do something like
l = [Ref(3), Ref(7)]
a = l[0]

So it seems that if you want to use this idea for implementing langauge
support you will have to wrap all objects in a Ref internally and this
seems some kind of extra indirection too.


No, the idea is to expose this class to the user. They would have to
explicitly wrap objects that they want to be able to be get a
reference to.


Well that seems less usefull to me.

Let me explain, what I would like it for.

Sometime I'm programming with tree like structures. Occasionaly such
a tree has to be restructured. Now of course I could write it like
this:

def restruct(tr):

if ...:
tr.left = restruct(tr.left)
tr.right = restruct(tr.right)
tmp = tr.left
tr.left = tmp.right
tmp.right = tr
return tmp.

...
tree = restruct(tree)

But this feels wrong to me. It gives the impression that the following
would give you a new restructured tree while the old one is still
available, which isn't so:

newtree = restruct(tree)

IMO being able to write:

restruct(tree)

With as a result that tree would now be the new root of the restructered
tree, would better convey what is going on. Now having to make my tree
object into a ref, which would mean all nodes in the tree, seems
overkill. Maybe we can consider in-out parameters in python? It is
not as powerfull as reference variables or in place replacements
but from what I have seen here in the newsgroup would be sufficient
for most users that ask for something like this.

--
Antoon Pardon
Nov 22 '05 #57
Op 2005-11-16, Mike Meyer schreef <mw*@mired.org>:
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
Op 2005-11-15, Mike Meyer schreef <mw*@mired.org>:
Antoon Pardon <ap*****@forel.vub.ac.be> writes:
>> Like having an assignment operator (let use @= for it) next to a
>> (re)bind operator.
>> We could then have something like the following.
>> a = 5
>> b = a
>> a @= 7
>> b ==> would result in 7.
> You've just overwritten the object referred to by the token "5" in the
> source code with the value 7, so you get:
> print 5
> 7
You have a valid point, but I think a different approach is possible.
Don't make the distinction between mutable and immutable types but
between mutable and immutable objects. So an int wouldn't be an
immutable type, but 5 would be an immutable object.
So the code above I gave would throw an exception, but the following
might work.
a @= 5
b = a
b @= 7
a ==> would result in 7.

Which solves that issue but brings up - well, more issues. What
happens if the third line is 'b @= "seven"'? Does this require an
extra level of indirection in the implementation? Avoiding that kind
of thing was the reason for suggesting this:


It depends on how far you want to go.

I think one can argue that in case of an inplace replacement, this
only makes sense if the two objects belong to the same class. So
no extra level of indirection is then required.


Now factor in inheritance. Do users of this facility have to worry
about static OO typing? That is, two objects that are otherwise
completely interchangeable will raise an exception if you try to
assign one to a variable holding the other, because they have the
wrong type. That seems unpythonic.


I don't care much about pythonic or not. It seems a very volatile
concept, that adapts as the language evolves. I wouldn't be
surprised if augmented arithmetic operations would have been
considered unpythonic at some point. You could see this
limitation as a wart, but IMO the idea would still be usefull
with that limitation.

The limitation would ensure for example that although the value
of an argument could change, it's class couldn't. I think that
is worthwhile.
Otherwise it really depends on how the builtin objects are implemented.
For user classes, a somewhat simplistic implementation could be
something like:

def '@=' (one, two):
one.__dict__.clear()
one.__dict__.update(two.__dict__)


Wouldn't one.__dict__ = dict(**two.__dict__) be a bit better?

Anyway, this won't' work if the class of one of the objects has
slots.


So? I wrote it was a simplistic idea. but things like setattr and
getattr, still work with objects that have slots. So which attributes
are present for a particulare object has to be available in the
python interpreter. So my guess is that slots wont be the big stumbling
block, should one consider implementing something like this.
The critical thing is that this doesn't introduce any new facilities
into the language, just some new syntax, so there's no implementation
impact. In fact, if you're willing to put up with some notational
abuse, we can do this now:

class Ref(object):
_unbound = object()
def __new__(cls, value = _unbound):
"""We're an object, but need to ignore the optional argument."""

return object.__new__(cls)

def __init__(self, value = _unbound):
"""Bind the optional value, if provided."""
if value is not self._unbound:
self._value = value

def __pos__(self):
"""Return value, if bound."""
try:
return self._value
except AttributeError:
raise ValueError, "%s object does not have a value stored." % \
self.__class__.__name__

def __iadd__(self, value):
self._value = value
return self

Usage:

>> x = Ref.Ref()
>> x += 23
>> +x
23
>> a = x
>> x += 25
>> +a
25
>> a += "this is a test"
>> +x
'this is a test'

Since it doesn't have real lannguage support, things like +x += 25
don't work. That's the only obvious gotcha. Maybe ~ would be a better
prefix.


I'm a bit puzzled on how you would implement this as real language
support. As far as I understand this only works with Ref objects.


Correct. That's the point.
You can't do something like the following.

l = [3, 7]
a = Ref(l[0])

Instead you would have to do something like
l = [Ref(3), Ref(7)]
a = l[0]

So it seems that if you want to use this idea for implementing langauge
support you will have to wrap all objects in a Ref internally and this
seems some kind of extra indirection too.


No, the idea is to expose this class to the user. They would have to
explicitly wrap objects that they want to be able to be get a
reference to.


Well that seems less usefull to me.

Let me explain, what I would like it for.

Sometime I'm programming with tree like structures. Occasionaly such
a tree has to be restructured. Now of course I could write it like
this:

def restruct(tr):

if ...:
tr.left = restruct(tr.left)
tr.right = restruct(tr.right)
tmp = tr.left
tr.left = tmp.right
tmp.right = tr
return tmp.

...
tree = restruct(tree)

But this feels wrong to me. It gives the impression that the following
would give you a new restructured tree while the old one is still
available, which isn't so:

newtree = restruct(tree)

IMO being able to write:

restruct(tree)

With as a result that tree would now be the new root of the restructered
tree, would better convey what is going on. Now having to make my tree
object into a ref, which would mean all nodes in the tree, seems
overkill. Maybe we can consider in-out parameters in python? It is
not as powerfull as reference variables or in place replacements
but from what I have seen here in the newsgroup would be sufficient
for most users that ask for something like this.

--
Antoon Pardon
Nov 22 '05 #58

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