msg176671 - (view) |
Author: Paddy McCarthy (paddy3118) |
Date: 2012-11-29 19:25 |
http://docs.python.org/3.3/library/stdtypes.html?highlight=to_bytes#int.to_bytes and http://docs.python.org/3.3/library/stdtypes.html?highlight=to_bytes#int.to_bytes would benefit from an example showing what they do based on simpler coding.
I have such an example that I wrote here: http://paddy3118.blogspot.co.uk/2012/11/some-identities-for-python-inttobytes.html that you can use.
I.e.
>>> n = 2491969579123783355964723219455906992268673266682165637887
>>> length = 25
>>> n2bytesbig = n.to_bytes(length, 'big')
>>> n2byteslittle = n.to_bytes(length, 'little')
>>> assert n2bytesbig == bytes( (n >> i*8) & 0xff for i in reversed(range(length)))
>>> assert n2byteslittle == bytes( (n >> i*8) & 0xff for i in range(length))
>>> assert n == sum( n2bytesbig[::-1][i] << i*8 for i in range(length) )
>>> assert n == sum( n2byteslittle[i] << i*8 for i in range(length) )
>>> assert n == int.from_bytes(n2bytesbig, byteorder='big')
>>> assert n == int.from_bytes(n2byteslittle, byteorder='little')
>>>
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msg177042 - (view) |
Author: Andrew Svetlov (asvetlov) * |
Date: 2012-12-06 14:26 |
Your example is comprehensive but not simple and obvious.
I think better to keep it out of doc.
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msg177043 - (view) |
Author: Ezio Melotti (ezio.melotti) * |
Date: 2012-12-06 14:31 |
I agree. The examples in the doc seem clear to me, whereas the ones you proposed are not as clear. Do you think there's something that they don't currently cover that should be added?
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msg177207 - (view) |
Author: Paddy McCarthy (paddy3118) |
Date: 2012-12-09 10:34 |
On 06/12/2012 14:31, Ezio Melotti wrote:
> Ezio Melotti added the comment:
>
> I agree. The examples in the doc seem clear to me, whereas the ones you proposed are not as clear. Do you think there's something that they don't currently cover that should be added?
>
> ----------
> nosy: +ezio.melotti
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue16580>
> _______________________________________
>
First, Thanks Ezio and Andrew for your replies.
My problem was that when working on bitcoin address validation I saw
code that was shifting and &'ing with 0xFF to convert to multiple bytes
and half remembered that there might be a Python function to do that. On
finding the .to_bytes method and its parameter "big" or "little", the
only way I had of working out which to use was to try each until I found
out which worked.
I therefore thought that what would have helped me was code that showed
the equivalent "expanded Python" for the method in a similar way to what
is done for some of the itertools functions etc.
If we split my request into two:
1. Is such extra explanation necessary.
2. Is my specific code that extra explanation.
I can work on the code a bit more.
Have I persuaded you that an extra explanation is necessary?
Thanks, Paddy.
P.S. I guess what is currently present shows the result of the methods
but nothing on how it could be generated. I am stating that the
generation can aid comprehension.
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msg177208 - (view) |
Author: Ezio Melotti (ezio.melotti) * |
Date: 2012-12-09 10:55 |
Usually we add plain Python equivalents when they are simple enough that the code equivalent is as understandable as the prose or more (see for example http://docs.python.org/3/library/functions.html#all, or the itertools functions you mentioned).
For this case I think it would help if you presented an equivalent function, e.g.:
def to_bytes(n, length, order):
if order == 'little':
return bytes((n >> i*8) & 0xff for i in range(length))
elif order == 'big':
return bytes((n >> i*8) & 0xff for i in reversed(range(length)))
or even:
def to_bytes(n, length, order):
indexes = range(length) if order == 'little' else reversed(range(length))
return bytes((n >> i*8) & 0xff for i in indexes)
This is also done for http://docs.python.org/3.3/library/stdtypes.html#int.bit_length just above to/from_bytes, so it might be a good addition.
If this is done, the equivalent function can also be added to the test suite, so we can verify that it's indeed equivalent.
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msg177262 - (view) |
Author: Paddy McCarthy (paddy3118) |
Date: 2012-12-10 05:57 |
On 09/12/2012 10:55, Ezio Melotti wrote:
> Ezio Melotti added the comment:
>
> Usually we add plain Python equivalents when they are simple enough that the code equivalent is as understandable as the prose or more (see for example http://docs.python.org/3/library/functions.html#all, or the itertools functions you mentioned).
> For this case I think it would help if you presented an equivalent function, e.g.:
>
> def to_bytes(n, length, order):
> if order == 'little':
> return bytes((n >> i*8) & 0xff for i in range(length))
> elif order == 'big':
> return bytes((n >> i*8) & 0xff for i in reversed(range(length)))
>
> or even:
>
> def to_bytes(n, length, order):
> indexes = range(length) if order == 'little' else reversed(range(length))
> return bytes((n >> i*8) & 0xff for i in indexes)
>
> This is also done for http://docs.python.org/3.3/library/stdtypes.html#int.bit_length just above to/from_bytes, so it might be a good addition.
> If this is done, the equivalent function can also be added to the test suite, so we can verify that it's indeed equivalent.
>
> ----------
> keywords: +easy
> stage: -> needs patch
> versions: +Python 2.7, Python 3.2, Python 3.4
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue16580>
> _______________________________________
>
The second example looks great. I like the dual use for testing too and
will try and remember both the next time I find I have ireas about the
documentation.
Thanks guys. It's appreciated!
|
msg190004 - (view) |
Author: W. Owen Parry (woparry) |
Date: 2013-05-25 21:05 |
Patch adding examples + tests for equivalence. Comments appreciated.
In particular, I'm not sure that the from_bytes example is simple enough to be useful:
def from_bytes(bytes, byteorder, signed=False):
if byteorder == 'little':
little_ordered = list(bytes)
elif byteorder == 'big':
little_ordered = list(reversed(bytes))
n = sum(little_ordered[i] << i*8 for i in range(len(little_ordered)))
if signed and little_ordered and (little_ordered[-1] & 0x80):
n -= 1 << 8*len(little_ordered)
return n
|
msg190013 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) * |
Date: 2013-05-25 21:39 |
I don't call it the examples. Here is the examples:
>>> (1234).to_bytes(2, 'big')
b'\x04\xd2'
>>> (1234).to_bytes(2, 'little')
b'\xd2\x04'
>>> (-1234).to_bytes(2, 'big', signed=True)
b'\xfb.'
>>> int.from_bytes(b'\xde\xad\xbe\xef', 'big')
3735928559
>>> int.from_bytes(b'\xde\xad\xbe\xef', 'little')
4022250974
>>> int.from_bytes(b'\xde\xad\xbe\xef', 'big', signed=True)
-559038737
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msg190071 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2013-05-26 10:35 |
The `from_bytes` example code looks fine to me, except that I'd probably use `enumerate` in the iteration; i.e.,
sum(b << 8*i for i, b in enumerate(little_ordered))
instead of
sum(little_ordered[i] << i*8 for i in range(len(little_ordered)))
Also, in:
if signed and little_ordered and (little_ordered[-1] & 0x80):
I wondered why you needed the `little_ordered` check. But I see that `int.from_bytes(b'', 'little', signed=True)` produces `0`, which is a little bit disappointing: I was expecting an exception. (A signed format should have a sign bit, which is impossible if the length of the byte string is 0.)
The `to_bytes` example code is missing range checks for the input. It may be clearer to simply state that in the docs, instead of modifying the example code to add the range checks.
|
msg399389 - (view) |
Author: Gautam Chaudhuri (quantum) * |
Date: 2021-08-11 12:46 |
I've rewritten woparry's patch so that it targets the current codebase as well as fixing the issues that Mark mentioned. It appears that the range checks on `to_bytes` have already been added to the documentation so I haven't added those.
Should a separate issue be raised for the behaviour of `int.from_bytes(b'', <byteorder>, signed=True')`? I think it's out of scope for this issue.
|
msg399607 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2021-08-15 08:27 |
@Gautam
> I think it's out of scope for this issue.
Agreed. It was just a note in passing. I don't think it's worth opening a new issue, though, unless this is genuinely causing problems for someone. Just let it lie.
|
msg399612 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2021-08-15 11:29 |
New changeset ad0a8a9c629a7a0fa306fbdf019be63c701a8028 by Gautam Chaudhuri in branch 'main':
bpo-16580: [doc] Add examples to int.to_bytes and int.from_bytes (GH-27760)
https://github.com/python/cpython/commit/ad0a8a9c629a7a0fa306fbdf019be63c701a8028
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msg399613 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2021-08-15 11:30 |
Merged. Thank you for the contribution!
|
|
Date |
User |
Action |
Args |
2022-04-11 14:57:38 | admin | set | github: 60784 |
2021-08-15 11:30:16 | mark.dickinson | set | status: open -> closed resolution: fixed stage: patch review -> resolved |
2021-08-15 11:30:03 | mark.dickinson | set | messages:
+ msg399613 |
2021-08-15 11:29:40 | mark.dickinson | set | messages:
+ msg399612 |
2021-08-15 08:27:23 | mark.dickinson | set | messages:
+ msg399607 |
2021-08-13 11:52:30 | quantum | set | stage: needs patch -> patch review pull_requests:
+ pull_request26236 |
2021-08-11 12:46:20 | quantum | set | files:
+ issue16580.patch nosy:
+ quantum messages:
+ msg399389
|
2021-08-02 22:10:17 | iritkatriel | set | title: Add examples to int.to_bytes and int.from_bytes -> [doc] Add examples to int.to_bytes and int.from_bytes versions:
+ Python 3.9, Python 3.10, Python 3.11, - Python 2.7, Python 3.2, Python 3.3, Python 3.4 |
2013-05-26 10:35:19 | mark.dickinson | set | nosy:
+ mark.dickinson messages:
+ msg190071
|
2013-05-25 21:39:18 | serhiy.storchaka | set | nosy:
+ serhiy.storchaka messages:
+ msg190013
|
2013-05-25 21:05:07 | woparry | set | files:
+ issue16580.patch
nosy:
+ woparry messages:
+ msg190004
keywords:
+ patch |
2013-01-13 16:10:59 | Ramchandra Apte | set | title: Add examples to int.to_bytres and int.from_bytes -> Add examples to int.to_bytes and int.from_bytes |
2012-12-10 05:57:39 | paddy3118 | set | messages:
+ msg177262 |
2012-12-09 10:55:36 | ezio.melotti | set | keywords:
+ easy
stage: needs patch messages:
+ msg177208 versions:
+ Python 2.7, Python 3.2, Python 3.4 |
2012-12-09 10:34:49 | paddy3118 | set | messages:
+ msg177207 |
2012-12-06 14:31:38 | ezio.melotti | set | nosy:
+ ezio.melotti messages:
+ msg177043
|
2012-12-06 14:26:29 | asvetlov | set | nosy:
+ asvetlov messages:
+ msg177042
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2012-11-29 19:25:09 | paddy3118 | create | |