[issue16580] Add examples to int.to_bytres and int.from_bytes
New submission from Paddy McCarthy: http://docs.python.org/3.3/library/stdtypes.html?highlight=to_bytes#int.to_b... and http://docs.python.org/3.3/library/stdtypes.html?highlight=to_bytes#int.to_b... 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-inttobyte... 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')
---------- assignee: docs@python components: Documentation messages: 176671 nosy: docs@python, paddy3118 priority: normal severity: normal status: open title: Add examples to int.to_bytres and int.from_bytes type: enhancement versions: Python 3.3 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16580> _______________________________________
Andrew Svetlov added the comment: Your example is comprehensive but not simple and obvious. I think better to keep it out of doc. ---------- nosy: +asvetlov _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16580> _______________________________________
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> _______________________________________
Paddy McCarthy added the comment: 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. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16580> _______________________________________
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> _______________________________________
Paddy McCarthy added the comment: 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! ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16580> _______________________________________
Changes by Ramchandra Apte <maniandram01@gmail.com>: ---------- title: Add examples to int.to_bytres and int.from_bytes -> Add examples to int.to_bytes and int.from_bytes _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16580> _______________________________________
W. Owen Parry added the comment: 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 ---------- keywords: +patch nosy: +woparry Added file: http://bugs.python.org/file30372/issue16580.patch _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16580> _______________________________________
Serhiy Storchaka added the comment: 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
---------- nosy: +serhiy.storchaka _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16580> _______________________________________
Mark Dickinson added the comment: 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. ---------- nosy: +mark.dickinson _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16580> _______________________________________
participants (7)
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Andrew Svetlov -
Ezio Melotti
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Mark Dickinson
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Paddy McCarthy
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Ramchandra Apte
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Serhiy Storchaka
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W. Owen Parry