Issue 29903: struct.Struct Addition - Python tracker (original) (raw)

Created on 2017-03-25 15:44 by palaviv, last changed 2022-04-11 14:58 by admin. This issue is now closed.

Messages (6)
msg290486 - (view)	Author: Aviv Palivoda (palaviv) *	Date: 2017-03-25 15:44
I would like to suggest that the struct.Struct class will support addition. For example you will be able to do: >>> s1 = Struct(">L") >>> s2 = Struct(">B") >>> s3 = s1 + s2 >>> s3.format b">LB"
msg290488 - (view)	Author: Serhiy Storchaka (serhiy.storchaka) *	Date: 2017-03-25 16:45
What is the purpose of this feature? Note that the layout of the structure consisting of two structures is not the same as the layout of the structure consisting of the same fields as separate structures. struct { struct { char a; short b; } s1; struct { char c; int d; } s2; } and struct { char a; short b; char c; int d; } can have different sizes and offsets of fields. As for the concrete implementation, it looks to me that Struct('2L') + Struct('25B') results to Struct('2L5B').
msg290490 - (view)	Author: Raymond Hettinger (rhettinger) *	Date: 2017-03-25 17:25
I concur with Serhiy that this proposal is likely to cause more problems than it solves.
msg290495 - (view)	Author: Aviv Palivoda (palaviv) *	Date: 2017-03-25 18:12
I have two use cases for this feature: 1. struct a { int a; #ifdef VER2 unsigned int b; #endif } Now I may do: >>> ver1 = Struct("i") >>> ver2 = ver1 + Struct("I") 2. struct a { int a; union inner { int b; unsigned int c; } u; } As you can see with this feature I may do: >>> start = Struct("i") >>> union_b = Struct("i") >>> union_c = Struct("I") >>> version_a = start + union_b >>> version_b = start + union_c If you have a big struct with many options in the union this save's copying the initial format. > As for the concrete implementation, it looks to me that Struct('2L') + Struct('25B') results to Struct('2L5B'). I will fix the case when there is no format provided.
msg290501 - (view)	Author: Martin Panter (martin.panter) *	Date: 2017-03-25 21:32
For the native alignment case (prefix code @), perhaps you can already use the “ctypes” module, which supports structures with more complicated embedded fields. For the the unaligned modes (prefixes =, <, > and !), I am a little sympathetic. In the past, I wanted to make various structures that extended from a common base structure: HSF_VOL_DESC = Struct("< B 5s B") # Python 3's "Struct.format" is a byte string! NSR_DESC = Struct(HSF_VOL_DESC.format.decode() + "B") But I think if Issue 21071 was fixed (change Struct.format to text string, or perhaps add a new attribute), I would be happy enough writing NSR_DESC = Struct(HSF_VOL_DESC.format_str + "B") Transforming Aviv’s examples: s3 = Struct(s1.format_str + s2.format_str[1:]) s3 = Struct(s1.format_str + "B") # if s2 is not needed on its own ver2 = Struct(ver1.format_str + "I") version_a = Struct(start.format_str + union_b.format_str[1:])
msg292082 - (view)	Author: STINNER Victor (vstinner) *	Date: 2017-04-21 23:37
I dislike this feature. I reject it: Serhiy, Raymond and me dislike it. I never needed this feature and a correct implementation can be very complex if you have to take care of endian, alignement, etc. It's trivial to implement you own add using the format attribute if you want the simple implementation format1+format2.