What does this piece of python indexing code do

Question

Please explain me what does this piece of code do.

h should be 32Byte result from sha256 calculation.

I am rewriting parts of this code for my project in C++ and I'm not sure if this switches byte order per 4byte chunk or change byte order on whole 32byte number.

def reverse_hash(h):
    return struct.pack('>IIIIIIII', *struct.unpack('>IIIIIIII', h)[::-1])[::-1]

And, how does this array index work ?

   [::-1]

Thanks for any and all info

volcano · Answer 1 · 2014-01-02T12:42:58.660

3

[::-1] creates a new list with reversed order of elements

edited Jan 02 '14 at 12:42

answered Jan 02 '14 at 12:15

volcano

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alko · Accepted Answer · 2014-01-02T12:24:45.273

0

[::-1] reverse order of elements in a list, so this script change order of each 4-bytes subsequence (in order to change endiannes, I suppose):

>>> h = ''.join(map(str, range(0,21)))
>>> h
'01234567891011121314151617181920'
>>> struct.pack('>IIIIIIII', *struct.unpack('>IIIIIIII', h)[::-1])[::-1]
'32107654019821114131615181710291'

Equivalent expression:

>>> struct.pack('<IIIIIIII', *struct.unpack('>IIIIIIII', h))
'32107654019821114131615181710291'

edited Jan 02 '14 at 12:24

answered Jan 02 '14 at 12:19

alko

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Thank you, totally answered my question. – semtexzv Jan 02 '14 at 13:10
@user3153405 You're welcome. Feel free to [accept](http://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work) appropriate answer if you feel your question is resolved. – alko Jan 02 '14 at 13:13

What does this piece of python indexing code do

2 Answers2