unittest: optimise for code size and memory use

[dpgeorge]

Summary

Based on discussion and testing in micropython/micropython#16311, this PR applies some optimisations to unittest to reduce its bytecode size, and reduce the amount of RAM it uses at runtime.

• reuse AssertRaisesContext for assertWarns
• remove support for running it with CPython
• optimise calling of _handle_test_exception
• only format msg if needed

The use of raise AssertionError instead of assert also means the module can now be compiled with -O3 and still function correctly (-O3 disables assert expressions!).

Testing

Tested on ADAFRUIT_ITSYBITSY_M0_EXPRESS which has about 20k heap memory.

Prior to this change:

• __init__.mpy has size 5795 bytes
• import unittest costs 9648 bytes of heap RAM (measured via g.collect(); micropython.mem_info() before and after import)

With this change:

• __init__.pym has size 5516 bytes (-279 byte change)
• import unittest costs 9440 bytes (-208 byte change)

Also, the current version of unittest unconditionally does string formatting for the error message for each call to self.assert<Condition>(...), which is really expensive in terms of RAM use and computation. The changes here improve the situation so the string formatting of msg is only done if the assert fails. That should free up a lot of RAM during the execution.

With these changes, ADAFRUIT_ITSYBITSY_M0_EXPRESS can successfully run the tests modified/added in micropython/micropython#16311, they now all pass or skip correctly.

Trade-offs and Alternatives

This module is no longer runnable under CPython due to removal of traceback module support. But IMO it's too restrictive to get this working under CPython, we need to be able to optimise it for running under MicroPython. The unittest test suite which runs under CI here anyway tests the behaviour, CPython is not needed for that.

Generative AI

Not used.

[dpgeorge]

Note that this PR is at odds with #1105 which adds a lot more features to unittest, making it a lot bigger.

[projectgus]

Changes look simple to me, that's great it reduces footprint significantly! Should churn the heap a lot less with the string formatting moved into the failure path, too.

[dpgeorge]

Thanks for the quick review.

I'll let it sit a little in case @andrewleech or someone else has any comments.

[andrewleech]

Thanks for the heads up, in agree these look like really good improvements!

I haven't run the tests, but yes seeing as the module is used in CI here I'm comfortable with that covering most of not all the changes.

Most of these changes are really clear and obvious in hindsight, and generally improve readability as well as optimising the size and runtime performance so it's a real win-win for new!