Provide a sourcemap

[denis-migdal]

Hi,

I think this could be a great thing if, in a debug mode, Brython provides a source map that'd help debugging in the browser:

• https://developer.mozilla.org/en-US/docs/Glossary/Source_map
• https://tc39.es/ecma426/2024/#mappings-structure

This seems to be working quite well for languages other than JS:

• https://kybernetikos.github.io/jsSandbox/srcmaps/dynamic.html

It seems that if we use createObjectURL, the source map may be lazy evaluated.
Hence, we might be able to create the source map only when asked.

https://forweb.dev/en/blog/2020-05-05-object-url/

I really do not have the time to test it now, but that'd be nice if somebody could take a look at it.

Note: related to #2476

Note: https://tc39.es/ecma426/2024/#base64-vlq

EDIT: This seems quite easy to implement. Understanding the specs is a little harder ^^.

Note : breakpoint to test
breakpoint()
https://www.digitalocean.com/community/tutorials/python-breakpoint

I may implement it in the train in a few days ^^.

Cordially,

[denis-migdal]

It works !

a = 2
b = 3
c = 4
b = 3
c = 4
raise 'nok'

a = 2
b = 3
c = 4
b = 3
c = 4
throw 'nok';

The sourcemap format is quite simple and easy to implement ~60 lines of code.
We just have to "map" positions from the generated code to the source code, and voilà.

@PierreQuentel if you want to add source map into Brython, we can plan a visio next week.

Note: finally blob URL doesn't work for sourceMap, we need to use data URL.

[PierreQuentel]

From the user point of view, what would be the additional benefit of a source map, compared to the current situation where the error trace already gives the lines in the original Python code ?

[denis-migdal]

You can use the browser debugger:

• set/remove breakpoints in the source code (without modifying the source code) ;
• other types of breakpoints (XHR break points, DOM break points, listener break points) ;
• see the (JS) value of variables (we could use spy expressions to get the Python value) ;
• supports for python's breakpoint().

Also when an error isn't caught by Brython (that happened at some occasions), could still be able to print a correct stacktrace.

Personally I'm more used to console.log()/print() for debugging, but others like to use debuggers.

(Also, I think I played enough with SBrython, I think we should discuss what we do from here now that I better understand Python and its constraints).