From Newsgroup: comp.lang.python

ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:

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


--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python

Then please explain why I have to write:

i += 1

Instead of the shorter:

i ++

My short-term memory is really stressed.

Stefan Ram schrieb:

ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:

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

--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python

Then please explain why I have to write:

    i += 1

Instead of the shorter:

    i ++

My short-term memory is really stressed.

I heard this behavior is because python's integers are immutable.
For example:
>>> 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.
--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python

Annada Behera <annada@tilde.green> wrote or quoted:

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.


--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python

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!

Annada Behera schrieb:

Then please explain why I have to write:

    i += 1

Instead of the shorter:

    i ++

My short-term memory is really stressed.

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

>>> 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.

--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python

On 8/11/24 3:04 am, Mild Shock wrote:

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
--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python

On 8/11/24 11:15, Greg Ewing via Python-list wrote:

On 8/11/24 3:04 am, Mild Shock wrote:

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
while a is b:

... 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
--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python

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.

Greg Ewing schrieb:

On 8/11/24 3:04 am, Mild Shock wrote:

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.

--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python


For example this article:

https://www.codementor.io/@arpitbhayani/python-caches-integers-16jih595jk

about the integer singletons claims:

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

Mild Shock schrieb:

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.

Greg Ewing schrieb:

On 8/11/24 3:04 am, Mild Shock wrote:

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.

--- Synchronet 3.20a-Linux NewsLink 1.114

From Newsgroup: comp.lang.python

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?

Mild Shock schrieb:

For example this article:

https://www.codementor.io/@arpitbhayani/python-caches-integers-16jih595jk

about the integer singletons claims:

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

Mild Shock schrieb:

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.

Greg Ewing schrieb:

On 8/11/24 3:04 am, Mild Shock wrote:

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.

--- Synchronet 3.20a-Linux NewsLink 1.114