[Scons-users] SCons 3.0, sconsign files and Py2 vs Py3.. how to handle sconsign imcompatabilities

[Tim Jenness]

> On Sep 9, 2017, at 19:13, Bill Deegan <bill at baddogconsulting.com> wrote:
> 
> Greetings,
> 
> This is (I hope) the last issue gating 3.0 release.

Fantastic. I guess the question is how many people are going to be switching their scons python versions regularly and whether the .sconsign file is part of a public interface outside of scons itself.

> 
> If we set the default pickle version to 2. (Currently set to HIGHEST_AVAILABLE, which means 4 for py3.5+ and 2 for py2.7)
> 

This is the safest option for portability.

> If you run scons with py2.7 and then run again with py3.x, you'll get the following:
> ...
> scons: *** [$DISTDIR/scons-local-3.0.0.alpha.20170821.tar.gz] UnicodeDecodeError : 'ascii' codec can't decode byte 0x80 in position 0: ordinal not in range(128)
> 

is this error from SCons or from pickle? If it’s from SCons, why is py2.7 writing non-ascii and why is py3 using ascii to read it?

> If you run scons with py3 and then rerun with py2.7, you'll get the following:
>  * It will rebuild everything you just built with py3
> 

Not a complete disaster but a bit weird, it’s almost like it doesn’t really understand what it read from the file.

> If we set the default pickle version  to HIGHEST_AVAILABLE, which means 4 for py3.5+ and 2 for py2.7
> 

what happens?

> 
> Here's the options I see to deal with this:
> 1. Always append .py3 to PY3 scons runs' sconsign files

This is the safe option unless .sconsign is assumed to be a public interface.

If people really are going to switch py2/py3 regularly then this sounds like it might get really confusing.

> 2. Always append .py# to sconsign files based on what version of python was used, ignore pre-existing .sconsigns from older versions

This will lead to the same state confusion as for #1 if people are switching all the time.


> 3. Do something complicated:
>    a. Leave pickle version as HIGHEST_AVAILABLE, detect version of pickle by reading first two bytes. If incompatible version detected, change file name to .py# and try reading that one.  Always write to .sconsign? Or write to file read.
>    b. ?
> 

The best option is to work out why pickle version 2 is not compatible.

Can you get text dumps of the contents from py2 and py3 to see what differs? It must be some unicode weirdness.

— 
Tim Jenness


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