Two aces up Python's sleeve

Then please explain why I have to write:

i += 1

Instead of the shorter:

i ++

My short-term memory is really stressed.

Post by Stefan Ram

last_item = my_list[ -1 ]
Way cleaner than my_list[ len( my_list )- 1 ], don't you think?

In "The Mental Game of Python," Raymond Hettinger spills the
beans about our noggins only being able to juggle 7 +/- 2
things in our short-term memory.
So, "last_item = my_list[ -1 ]" might still make the cut,
while "my_list[ len( my_list)- 1 ]" could be biting off
more than we can chew.
|The problem is, the number of brain registers this uses is
|10. This is no longer a decryption effort. This is a puzzle.
|At the moment you put it together, you fully understand it.
|But if this is embedded in bigger code, every time you hit
|this line, you're going to have to pick apart "what does this
|thing do?".
Raymond Hettinger

Annada Behera

2024-11-07 07:25:53 UTC

i += 1
i ++
My short-term memory is really stressed.

I heard this behavior is because python's integers are immutable.

x,y = 5,5
id(x) == id(y)

True

5 is a object that x and y points to. ++x or x++ will redefine 5 to
6, which the interpreter forbids to keep it's state mathematically
consistent. Also, by not supporting x++ and ++x, it avoids the pre-
and post-increment (substitute-increment v. increment-substitute) bugs
that plagues C and it's children.

Stefan Ram

2024-11-07 11:03:29 UTC

Post by Annada Behera
5 is a object that x and y points to. ++x or x++ will redefine 5 to

This doesn't add up for me. Using the same "logic,"
you might as well say "x = x + 1" is off limits.

Mild Shock

2024-11-07 14:04:53 UTC

This only works for small integers. I guess
this is because tagged pointers are used
nowadays ? For large integers, also known

as bigint, it doesn't work:

Python 3.13.0a1 (tags/v3.13.0a1:ad056f0, Oct 13 2023, 09:51:17)

x, y = 5, 4+1
id(x) == id(y)

True

x, y = 10**200, 10**199*10
x == y

True

id(x) == id(y)

False

In tagged pointers a small integer is
directly inlined into the pointer. The
pointer has usually some higher bits,

that identify the type and when masking
to see the lower bits, one gets the
integer value.

But I don't know for sure whats going on,
would need to find a CPython documentation.

P.S.: I also tested PyPy it doesn't show
the same behaviour, because it computes
an exaberated id():

Python 3.10.14 (39dc8d3c85a7, Aug 27 2024, 14:33:33)
[PyPy 7.3.17 with MSC v.1929 64 bit (AMD64)]

x, y = 5, 4+1
id(x) == id(y)

True

x, y = 10**200, 10**199*10
id(x) == id(y)

True

id(x)

1600000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000
000000000000000001

Quite funny!

i += 1
i ++
My short-term memory is really stressed.

I heard this behavior is because python's integers are immutable.

x,y = 5,5
id(x) == id(y)

True
5 is a object that x and y points to. ++x or x++ will redefine 5 to
6, which the interpreter forbids to keep it's state mathematically
consistent. Also, by not supporting x++ and ++x, it avoids the pre-
and post-increment (substitute-increment v. increment-substitute) bugs
that plagues C and it's children.

Greg Ewing

2024-11-07 22:15:38 UTC

Post by Mild Shock
This only works for small integers. I guess
this is because tagged pointers are used
nowadays ?

No, it's because integers in a certain small range are cached. Not sure
what the actual range is nowadays, it used to be something like -5 to
256 I think.

BTW you have to be careful testing this, because the compiler sometimes
does constant folding, so you need to be sure it's actually computing
the numbers at run time.

--
Greg

2024-11-08 00:07:48 UTC

Post by Mild Shock
This only works for small integers. I guess
this is because tagged pointers are used
nowadays ?

No, it's because integers in a certain small range are cached. Not sure
what the actual range is nowadays, it used to be something like -5 to
256 I think.
BTW you have to be careful testing this, because the compiler sometimes
does constant folding, so you need to be sure it's actually computing
the numbers at run time.

Haven't seen the OP. Is the Newsgroup link forwarding to the email-list
correctly?

Integer interning is indeed valid for -5 <= i <= 256
("it works on my machine"! see below)

a = 0; b = 0; c = 0; d = 0

... print( a, b, end=" ", )
... print( c, d, ) if c is d else print()
... a += 1; b += 1; c -= 1; d -= 1
...
0 0 0 0
1 1 -1 -1
2 2 -2 -2
3 3 -3 -3
4 4 -4 -4
5 5 -5 -5
6 6
7 7
8 8
9 9
...
254 254
255 255
256 256
Be aware that this is implementation-dependent and not guaranteed to
hold forever.

dn  ~  python
Python 3.12.7 (main, Oct 1 2024, 00:00:00) [GCC 13.3.1 20240913 (Red
Hat 13.3.1-3)] on linux
Type "help", "copyright", "credits" or "license" for more information.

See also https://docs.python.org/3/library/sys.html#sys.intern

Thus could decide what is interned for yourself:

a_string = sys.intern( str( 1000 ) )

--
Regards,
=dn

Mild Shock

2024-11-08 00:25:52 UTC

Hi,

In Java its possible to work this way
with the Integer datatype, just call
Integer.valueOf().

I am not sure whether CPython does the
same. Because it shows me the same behaviour
for small integers that are more than

only in the range -128 to 128. You can try yourself:

Python 3.14.0a1 (tags/v3.14.0a1:8cdaca8, Oct 15 2024, 20:08:21)

x,y = 10**10, 10**9*10
id(x) == id(y)

True

Maybe the idea that objects have an address
that can be accessed via id() has been abandoned.
This is already seen in PyPy. So maybe we

are falsly assuming that id() gives na object address.

Post by Mild Shock
This only works for small integers. I guess
this is because tagged pointers are used
nowadays ?

No, it's because integers in a certain small range are cached. Not sure
what the actual range is nowadays, it used to be something like -5 to
256 I think.
BTW you have to be careful testing this, because the compiler sometimes
does constant folding, so you need to be sure it's actually computing
the numbers at run time.

Mild Shock

2024-11-08 00:29:48 UTC

x, y = 257, 257
id(x) == id(y)

False

But on Windows my recent CPython doesn't do that:

Python 3.14.0a1 (tags/v3.14.0a1:8cdaca8, Oct 15 2024, 20:08:21)

x, y = 257, 257
id(x) == id(y)

True

x,y = 10**10, 10**9*10
id(x) == id(y)

True
Maybe the idea that objects have an address
that can be accessed via id() has been abandoned.
This is already seen in PyPy. So maybe we
are falsly assuming that id() gives na object address.

This only works for small integers. I guess
this is because tagged pointers are used
nowadays ?

No, it's because integers in a certain small range are cached. Not
sure what the actual range is nowadays, it used to be something like
-5 to 256 I think.
BTW you have to be careful testing this, because the compiler
sometimes does constant folding, so you need to be sure it's actually
computing the numbers at run time.

Mild Shock

2024-11-08 00:47:14 UTC

The wiked brain of ChatGPT gives me a lead:

PEP 659
Storing data caches before the bytecode.

Maybe its an effect of constant folding
and constant pooling by the compiler?

x, y = 257, 257
id(x) == id(y)

False
Python 3.14.0a1 (tags/v3.14.0a1:8cdaca8, Oct 15 2024, 20:08:21)

x, y = 257, 257
id(x) == id(y)

True

Post by Mild Shock
Hi,
In Java its possible to work this way
with the Integer datatype, just call
Integer.valueOf().
I am not sure whether CPython does the
same. Because it shows me the same behaviour
for small integers that are more than
Python 3.14.0a1 (tags/v3.14.0a1:8cdaca8, Oct 15 2024, 20:08:21)
>>> x,y = 10**10, 10**9*10
>>> id(x) == id(y)
True
Maybe the idea that objects have an address
that can be accessed via id() has been abandoned.
This is already seen in PyPy. So maybe we
are falsly assuming that id() gives na object address.

This only works for small integers. I guess
this is because tagged pointers are used
nowadays ?

No, it's because integers in a certain small range are cached. Not
sure what the actual range is nowadays, it used to be something like
-5 to 256 I think.
BTW you have to be careful testing this, because the compiler
sometimes does constant folding, so you need to be sure it's actually
computing the numbers at run time.

Lawrence D'Oliveiro

2024-11-08 01:10:34 UTC

Post by Annada Behera
I heard this behavior is because python's integers are immutable.

Nothing to do with that.

Post by Annada Behera
++x or x++ will redefine 5 to 6, which the interpreter forbids ...

One of those is actually syntactically valid.

It just won’t do what you expect it to do.

Mild Shock

2024-11-08 01:40:56 UTC

Well you can use your Browser, since

x = 5

x ++

x = 5

++ x

6

So we have x ++ equals in Python:

x + = 1
x - 1

And ++ x equals in Python:

x += 1
x

But I don't know how to combine an
assignment and an expression into one
expession. In JavaScript one can use

x = 5

y = (x += 1, x - 1)

x = 5

y = (x += 1, x)

6

But in Python the comma would create a tuple.

Post by Annada Behera
I heard this behavior is because python's integers are immutable.

Nothing to do with that.

Post by Annada Behera
++x or x++ will redefine 5 to 6, which the interpreter forbids ...

One of those is actually syntactically valid.
It just won’t do what you expect it to do.

2024-11-08 19:09:49 UTC

Post by Mild Shock
Well you can use your Browser, since

Question: are we talking Python or JavaScript?
Trying to find a word-for-word translation serves as badly in
computer-programming languages as it does in human spoken-languages.
Learn how to adapt and embrace the differences...

Post by Mild Shock
x + = 1
x - 1

The above probably only 'works' (the way you expect) in the REPL.

Post by Mild Shock
But I don't know how to combine an
assignment and an expression into one
expession. In JavaScript one can use

Again!

"Everything should be made as simple as possible, but no simpler."

Check out "The Zen of Python" and PEP-0008 for Python idioms.

x = 5

y = (x += 1, x - 1)

x = 5

y = (x += 1, x)

6
But in Python the comma would create a tuple.

Exactly, just as driving on the left side of the road will be fine in
some countries but cause a crash in others. Learn the local rules FIRST!

x = 5
y = (x := x + 1); x

x, y

(6, 6)

However, if such were submitted for Code Review, unhappiness would result.

Was the question re-phrased to: how to ... in Python, we'd end-up with

x = 5 # define
x += 1 # increment
y = x # alias
x, y

(6, 6)

--
Regards,
=dn

Mild Shock

2024-11-08 19:49:55 UTC

Ok here you go, the "walrus operator" is
actually a good lead, we have that this
here from JavaScript:

x++

respectively

++x

Can be replaced by the Python expression:

(x := x + 1) - 1

respectively

(x := x + 1)

Here is a test only testing x++:

Python 3.14.0a1 (tags/v3.14.0a1:8cdaca8, Oct 15 2024, 20:08:21)

x = 5
(x := x + 1) - 1

x = 5
y = (x := x + 1); x

x, y

(6, 6)... irrational drivel removed ..

Thomas Passin

2024-11-08 22:00:31 UTC

Post by dn

Post by Mild Shock
Well you can use your Browser, since

Question: are we talking Python or JavaScript?
Trying to find a word-for-word translation serves as badly in computer-
programming languages as it does in human spoken-languages. Learn how to
adapt and embrace the differences...

Post by Mild Shock
x + = 1
x - 1

The above probably only 'works' (the way you expect) in the REPL.

Post by Mild Shock
But I don't know how to combine an
assignment and an expression into one
expession. In JavaScript one can use

Again!
"Everything should be made as simple as possible, but no simpler."
Check out "The Zen of Python" and PEP-0008 for Python idioms.

Post by Mild Shock
> x = 5
5
> y = (x += 1, x - 1)
5
> x = 5
5
> y = (x += 1, x)
6
But in Python the comma would create a tuple.

Exactly, just as driving on the left side of the road will be fine in
some countries but cause a crash in others. Learn the local rules FIRST!

x = 5
y = (x := x + 1); x

x, y

(6, 6)
However, if such were submitted for Code Review, unhappiness would result.
Was the question re-phrased to: how to ... in Python, we'd end-up with

x = 5 # define
x += 1 # increment
y = x # alias
x, y

(6, 6)