append on lists

[Armin]

Re: append on lists

John Machin wrote:

On Sep 16, 6:45 am, Armin <a...@nospam.or gwrote:
>

>Yes, but this is very unconvenient.
>If d should reference the list a extended with a single list element
>you need at least two lines
>>
>a.append(7)
>d=a
>>
>and not more intuitive d = a.append(7)

>
Methods/functions which return a value other than the formal None and
also mutate their environment are "a snare and a delusion". Don't wish
for them.

c = [9,10]
[1,2,3,4,7].append(c) -Is this a valid expression?

The 'value' of that expression is None.

However ... that's the way of the implementation of the append method.
It's a little bit confusing to me ...

--Armin

Thanks to all !

>
Inconvenient? How often do you want to mutate a list and then set up
another reference to it?
>

[Chris Rebert]

Re: append on lists

On Tue, Sep 16, 2008 at 1:20 AM, Armin <a@nospam.orgwr ote:

John Machin wrote:

>>
>On Sep 16, 6:45 am, Armin <a...@nospam.or gwrote:
>>

>>Yes, but this is very unconvenient.
>>If d should reference the list a extended with a single list element
>>you need at least two lines
>>>
>>a.append(7)
>>d=a
>>>
>>and not more intuitive d = a.append(7)

>>
>Methods/functions which return a value other than the formal None and
>also mutate their environment are "a snare and a delusion". Don't wish
>for them.

>
>
>
c = [9,10]
[1,2,3,4,7].append(c) -Is this a valid expression?

Literally, no, because you can't call methods on literals.
However, the sentiment is valid, though probably not what you want:

>>c = [9,10]
>>a = [1,2,3,4,7]
>>b = a[:]
>>a.append(c)
>>a #note the nested list

[1, 2, 3, 4, 7, [9, 10]]

>>b

[1, 2, 3, 4, 7]

>>b.extend(c)
>>b

[1, 2, 3, 4, 7, 9, 10]

Regards,
Chris

>
The 'value' of that expression is None.
>
However ... that's the way of the implementation of the append method.
It's a little bit confusing to me ...
>
--Armin
>
Thanks to all !
>
>

>>
>Inconvenient ? How often do you want to mutate a list and then set up
>another reference to it?
>>

--

http://mail.python.org/mailman/listinfo/python-list

>

--
Follow the path of the Iguana...

http://rebertia.com

[Hrvoje Niksic]

Re: append on lists

"Chris Rebert" <clp@rebertia.c omwrites:

> c = [9,10]
> [1,2,3,4,7].append(c) -Is this a valid expression?

>
Literally, no, because you can't call methods on literals.

This is in fact not true. [1,2,3,4,7].append([9, 10]) is a perfectly
valid expression, only it doesn't do much (that you can observe).

The canonical response (no doubt already quoted in this thread) is
that returning the list from append would confuse the hell out of
people who expect a copy of the list, such as:

a = [1, 2, 3]
b = a.append(4)
# if the above "worked" in the sense that b == [1, 2, 3, 4], I for one
# would expect a to remain unchanged

[Duncan Booth]

Re: append on lists

"Chris Rebert" <clp@rebertia.c omwrote:

On Tue, Sep 16, 2008 at 1:20 AM, Armin <a@nospam.orgwr ote:

> [1,2,3,4,7].append(c) -Is this a valid expression?

>
Literally, no, because you can't call methods on literals.

Rubbish. There is no restriction about calling methods on literals. That
expression is perfectly valid but has no practical use that I can see.

There is a syntax gotcha which you may have been thinking of: to call a
method on an integer literal (or indeed to access any attribute) you have
to use whitespace between the literal and the dot otherwise you have a
float literal and a syntax error.

>>5 .__hex__()

'0x5'

The only relatively common use I can think of where you might want to call
a method directly on a literal is to produce a list of strings while being
lazy about the typing:

COLOURS = "red green blue pink yellow".split()

versus

COLOURS = ["red", "green", "blue", "pink", "yellow"]


--
Duncan Booth http://kupuguy.blogspot.com

[Hrvoje Niksic]

Re: append on lists

Duncan Booth <duncan.booth@i nvalid.invalidw rites:

The only relatively common use I can think of where you might want to call
a method directly on a literal is to produce a list of strings while being
lazy about the typing:

By far the most common is probably 'sep'.join(iter able).

[Alex Marandon]

Re: append on lists

Armin wrote:

Chris Rebert wrote:

>On Mon, Sep 15, 2008 at 1:24 PM, Armin <a@nospam.orgwr ote:

>>>
>>Hi,
>>>
>>just a dumb question.
>>>
>>Let a = [1,2,3,4,5]
>>>
>>Why is the value of a.append(7) equal None and not [1,2,3,4,5,6,7] ??

>>
>Because .append() mutates 'a' and appends the item in-place rather
>than creating and returning a new list with the item appended, and
>it's good Python style for mutating methods to have no return value
>(since all functions must have some return value, Python uses None
>when the function doesn't explicitly return anything).

>
Yes, but this is very unconvenient.

Hi,

You might be interested in using the + operator instead of append. You
could also define your own list type, based on the UserList included in
the standard library.

>>from UserList import UserList
>>class MyList(UserList ):

.... def my_append(self, value):
.... return self + [value]
....

>>l = MyList([1,2,3,4])
>>l

[1, 2, 3, 4]

>>l.my_append(5 )

[1, 2, 3, 4, 5]

>>l

[1, 2, 3, 4]

[Armin]

Re: append on lists

Duncan Booth wrote:

"Chris Rebert" <clp@rebertia.c omwrote:

>On Tue, Sep 16, 2008 at 1:20 AM, Armin <a@nospam.orgwr ote:

>> [1,2,3,4,7].append(c) -Is this a valid expression?

>Literally, no, because you can't call methods on literals.

>
Rubbish. There is no restriction about calling methods on literals. That
expression is perfectly valid but has no practical use that I can see.

The semantic of [1,2,3,4,7].append(c) and [1,2,3,4,7] + c
(with c = [8,9]) is identical, but the first expression doesn't provide
a value. Strange by design ...

--Armin

>
There is a syntax gotcha which you may have been thinking of: to call a
method on an integer literal (or indeed to access any attribute) you have
to use whitespace between the literal and the dot otherwise you have a
float literal and a syntax error.
>

>>>5 .__hex__()

'0x5'
>
The only relatively common use I can think of where you might want to call
a method directly on a literal is to produce a list of strings while being
lazy about the typing:
>
COLOURS = "red green blue pink yellow".split()
>
versus
>
COLOURS = ["red", "green", "blue", "pink", "yellow"]
>
>

[Alex Marandon]

Re: append on lists

Armin wrote:

Duncan Booth wrote:
>
The semantic of [1,2,3,4,7].append(c) and [1,2,3,4,7] + c
(with c = [8,9]) is identical,

No it's not, + doesn't alter its operands.

>>a = 1
>>b = 2
>>a + b

3

>>a

1

Were you expecting a == 3?

[Armin]

Re: append on lists

Alex Marandon wrote:

Armin wrote:

>Duncan Booth wrote:
>>
>The semantic of [1,2,3,4,7].append(c) and [1,2,3,4,7] + c
>(with c = [8,9]) is identical,

>
No it's not, + doesn't alter its operands.
>

>>a = 1
>>b = 2
>>a + b

3

That's not the point :)

What's the value of 1.add(b)? None? Or 3 ??
(if add works in the same way as append)
a + b doesn't change a,b ... but a.add(b) -a=3

[Hrvoje Niksic]

Re: append on lists

Armin <a@nospam.orgwr ites:

What's the value of 1.add(b)? None? Or 3 ??
(if add works in the same way as append)

That's exactly the point -- __add__ doesn't work as append. append is
a "destructiv e" operation, that mutates an existing object, whereas
__add__ returns a different object to be used as the "result" of the
addition. This is why append doesn't need to return anything, while
__add__ must return the new object.

The distinction is even more apparent with sort and sorted, which are
destructive and non-destructive aspects of the same operation:

>>a = [3, 2, 1]
>>a.sort() # destructive (mutates object), so no return value
>>a

[1, 2, 3]

>>a = [3, 2, 1]
>>b = sorted(a) # non-destructive (sorts a copy), returns the sorted copy
>>a

[3, 2, 1]

>>b

[1, 2, 3]

[Maric Michaud]

Re: append on lists

Le Tuesday 16 September 2008 14:23:25 Armin, vous avez écrit :

Alex Marandon wrote:

Armin wrote:

Duncan Booth wrote:
>
The semantic of [1,2,3,4,7].append(c) and [1,2,3,4,7] + c
(with c = [8,9]) is identical,

No it's not, + doesn't alter its operands.

>>a = 1
>>b = 2
>>a + b

3

>
That's not the point :)

It is, please try to understand it, in python all expressions that mutate an
object should return None, it's the case for

l.append(x)
l.sort()
l.reverse()

...

all expressions that return something, return a new object, it's the case for

1+2
1.__add__(2) (which is the same)
sorted(l)
l[i:j]
etc...


there are some noticeable exceptions :

For coding facilities, some APIs could return the modified part of the object,
ex : it = c.pop()

Returning the modifyied object itself is mostly considered bad style, because
it doesn't make clear if this the object or a copy. There is OTHO a use case
for this, for APIs that allow chaining of operations (don't find by memory
any example in stdlib, though), alike the ">>" operator in C++, but pyparsing
is a good example if I remember well. BTW, Pythoneers are not very fond of
that.

Finally, the very special case of augmented assignment operators :
x.__iadd__(y) which actually return the modified object, but this expression
isn't intended to be use by callers, refer to the manual to see a good
example of this, how one should implement __add__ and __iadd__ methods.

a += b
a = a + b

both are statments (doesn't evaluate to any object), but are not equivalentif
a is mutable.

>
What's the value of 1.add(b)? None? Or 3 ??
(if add works in the same way as append)
a + b doesn't change a,b ... but a.add(b) -a=3
--
http://mail.python.org/mailman/listinfo/python-list

--
_____________

Maric Michaud

[Armin]

Re: append on lists

Maric Michaud wrote:

Le Tuesday 16 September 2008 14:23:25 Armin, vous avez écrit :

>Alex Marandon wrote:

>>Armin wrote:
>>>Duncan Booth wrote:
>>>>
>>>The semantic of [1,2,3,4,7].append(c) and [1,2,3,4,7] + c
>>>(with c = [8,9]) is identical,
>>No it's not, + doesn't alter its operands.
>>>
>> >>a = 1
>> >>b = 2
>> >>a + b
>>>
>>3

>That's not the point :)

>
It is, please try to understand it, in python all expressions that mutate an
object should return None, it's the case for
>
l.append(x)
l.sort()
l.reverse()
>
all expressions that return something, return a new object, it's the case for
>
1+2
1.__add__(2) (which is the same)
sorted(l)
l[i:j]
etc...
>
there are some noticeable exceptions :
>
For coding facilities, some APIs could return the modified part of the object,
ex : it = c.pop()
>
Returning the modifyied object itself is mostly considered bad style, because
it doesn't make clear if this the object or a copy.

OK ... That's a good point. Thanks !

--Armin

[Grant Edwards]

Re: append on lists

On 2008-09-16, Armin <a@nospam.orgwr ote:

John Machin wrote:

>On Sep 16, 6:45 am, Armin <a...@nospam.or gwrote:
>>

>>Yes, but this is very unconvenient.
>>If d should reference the list a extended with a single list element
>>you need at least two lines
>>>
>>a.append(7)
>>d=a
>>>
>>and not more intuitive d = a.append(7)

>>
>Methods/functions which return a value other than the formal None and
>also mutate their environment are "a snare and a delusion". Don't wish
>for them.

>
>
>
c = [9,10]
[1,2,3,4,7].append(c) -Is this a valid expression?

Yes.

The 'value' of that expression is None.

Correct.

However ... that's the way of the implementation of the
append method. It's a little bit confusing to me ...

No, that's a way of calling the append method of a list. The
fact that you don't have a name bound to that list doesn't
change the fact that the list is mutable and you appended
[9,10] to it.

--
Grant Edwards grante Yow! Are the STEWED PRUNES
at still in the HAIR DRYER?
visi.com

[Grant Edwards]

Re: append on lists

On 2008-09-16, Armin <a@nospam.orgwr ote:

Duncan Booth wrote:

>"Chris Rebert" <clp@rebertia.c omwrote:

>>On Tue, Sep 16, 2008 at 1:20 AM, Armin <a@nospam.orgwr ote:
>>> [1,2,3,4,7].append(c) -Is this a valid expression?
>>Literally, no, because you can't call methods on literals.

>>
>Rubbish. There is no restriction about calling methods on literals. That
>expression is perfectly valid but has no practical use that I can see.

>
The semantic of [1,2,3,4,7].append(c) and [1,2,3,4,7] + c
(with c = [8,9]) is identical,

No, they're not:

>>a=[1,2,3,4,7]
>>c=[9,10]
>>a.append(c)
>>a

[1, 2, 3, 4, 7, [9, 10]]

>>a=[1,2,3,4,7]
>>c=[9,10]
>>a+c

[1, 2, 3, 4, 7, 9, 10]

You really ought to install a copy of Python so you can try out
things and see how they really work.

but the first expression doesn't provide a value. Strange by
design ...

Not really. You just need to make a little effort to
understand the reasoning (which has been explained to you)
behind Python's design decision.

--
Grant Edwards grante Yow! Somewhere in DOWNTOWN
at BURBANK a prostitute is
visi.com OVERCOOKING a LAMB CHOP!!

[Grant Edwards]

Re: append on lists

On 2008-09-16, Armin <a@nospam.orgwr ote:

Alex Marandon wrote:

>Armin wrote:

>>Duncan Booth wrote:
>>>
>>The semantic of [1,2,3,4,7].append(c) and [1,2,3,4,7] + c
>>(with c = [8,9]) is identical,

>>
>No it's not, + doesn't alter its operands.
>>

> >>a = 1
> >>b = 2
> >>a + b

>3

>
That's not the point :)
>
What's the value of 1.add(b)? None? Or 3 ??

>>a = 1
>>b = 2
>>1.add(b)

File "<stdin>", line 1
1.add(b)
^
SyntaxError: invalid syntax

(if add works in the same way as append)

It doesn't.

a + b doesn't change a,b ... but a.add(b) -a=3

WTH are you talking about?

--
Grant Edwards grante Yow! I just got my PRINCE
at bumper sticker ... But now
visi.com I can't remember WHO he
is ...