Gracefully handle readonly entries in PdoMap.save() (fixes #541)

[acolomb]

For PDO communication or mapping parameters which are read-only on a
node, the PdoMap.save() method currently aborts with an SDO exception.
However, individual parameters may simply not support configuration
changes. Rely on the Object Dictionary to tell which ones are
expected to fail and skip them. For communication record entries, log
what is not being saved.

[acolomb]

This is an alternative approach to #542, with improvements based on the discussion there.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
canopen/pdo/base.py	94.44%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[acolomb]

The one missing line reported by Codecov is related to the hack for supporting Curtis' mixed up mapping entries. I don't feel like putting effort into that part.

[sveinse]

I'd like to add something to consider.

I know that this PR is not considering the upcoming asyncio PR yet, but I'd like to inform that the PdoMap.save() and PdoMap.read() will need to be refactored for async. Today the logic and the IO is done together, which doesn't play nice with asyncio. To prevent repeating this logic twice, the asyncio PR refeactors the code following the design principles of sansio, where the IO is done outside of the logic function.

I would have love if we were able to take this into consideration when refactoring PdoMap.save() in thie PR. It would gain us all to think ahead.

[acolomb]

I'm not at all opposed to refactoring this as needed. However, to move forward quickly, I'd rather limit this PR's scope to fixing the actual unwanted / faulty behavior.

[sveinse]

No problem. As a consequence I need to update the asyncio PR to reflect the result of this PR.

[sveinse]

Are you formatting with black on the new stuff? Because its going a bit back an forth between splitting on the lines and joining them. E.g. base.py L439 is split up, while L450 is collected together.

It's cosmetics. The code changes looks good.

[acolomb]

Yes, I take black as a reference for how to format new / changed code. In this case, it's simply based on the line length limit of 96 I have configured. The old style was non-conformant in both cases, and the new loop variable allowed some shortening.