py/builtinimport: support relative import in custom __import__ callbacks - #6665

+1

@zsquareplusc sorry no one took a look at this when it was opened.

It actually looks really good, it fixes a missing piece of the import machinery. Would you be able to rebase this on the latest master? If not, I can do that.

I have rebased this on latest master. It was a bit tricky due to the many merge commits, but I managed to retain the original patch.

Code size report:

Reference:  docs/mimxrt/pinout: Use Dxx pin identifiers for Teensy boards. [c07fda7]
Comparison: extmod/asyncio: Pass globals in __import__ call. [merge of 4593843]
  mpy-cross:    +0 +0.000% 
   bare-arm:   +12 +0.021% 
minimal x86:   +47 +0.025% 
   unix x64:   +72 +0.008% standard
      stm32:   +36 +0.009% PYBV10
     mimxrt:   +40 +0.011% TEENSY40
        rp2:   +48 +0.005% RPI_PICO_W
       samd:   +44 +0.016% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:   +48 +0.011% VIRT_RV32

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.38%. Comparing base (c07fda7) to head (4593843).
⚠️ Report is 2 commits behind head on master.

@@           Coverage Diff           @@
##           master    #6665   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files         171      171           
  Lines       22294    22298    +4     
=======================================
+ Hits        21933    21937    +4     
  Misses        361      361           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

I adjusted the code here so it better matches CPython (only check the globals argument if level>0). And removed the error message to reduce code size (this error message telling that globals must be a dict would rarely be hit in the real world, so not worth spending bytes on it).

Also adjusted the tests:

• removed the .py.exp file for the test, now that the output matches CPython
• moved parts of the test to an existing test file, they have a better home there

Only 2 weeks ago I prepared a rebased branch of this PR. However I encountered open questions. And my goal was just making custom __import__ work despite relative imports, not CPython compatibility.

Now I compared my branch with this PR. This PR has:

• less code for setting globals in mp_builtin___import___default()
• TypeError check moved from tests/import/import_override2.py to builtin_import.py
• no irrelevant renaming of test symbols like on my branch

My branch is well tested since years. And the differences of this PR look good to me.
I am happy if this PR gets merged!

Thanks @oliver-joos for the feedback.