Build fails on Python 3.10 due to the collections module

[ocervell]

gcloud fails to load on Python 3.10:

ERROR: gcloud failed to load: module 'collections' has no attribute 'Mapping'

Also, lint and unit tests fail to run on Python 3.10, example run here:
https://github.com/google/slo-generator/runs/4247056898?check_suite_focus=true

[lvaylet]

Looks like it is a known issue which got fixed a couple of days ago ((Mar 17, 2022) in release 377.0.0: https://issuetracker.google.com/issues/202172882?pli=1

Let me install Python 3.10.2 and confirm make lint and make unit complete successfully.

[lvaylet]

make lint still fails but I am not surprised as the issue with gcloud is completely independent:

(venv10.3) $ make lint
flake8 --ignore=E302,E203,E261 "slo_generator"/ --max-line-length=80
flake8 --ignore=E302,E203,E261,E402 tests/ --max-line-length=80
find ./"slo_generator" ./tests -name \*.py | xargs pylint --rcfile .pylintrc --ignore-patterns=test_.*?py
************* Module slo_generator.utils
slo_generator/utils.py:422:24: E1101: Module 'collections' has no 'Mapping' member (no-member)

------------------------------------------------------------------
Your code has been rated at 9.97/10 (previous run: 9.97/10, +0.00)

make: *** [Makefile:81: pylint] Error 123

make unit fails too. Again, this is expected:

(venv10.3) $ make unit
[...]
  File "/home/laurent/workspace/github/google/slo-generator/venv3.10/lib/python3.10/site-packages/nose/suite.py", line 106, in _set_tests
    if isinstance(tests, collections.Callable) and not is_suite:
AttributeError: module 'collections' has no attribute 'Callable'
make: *** [Makefile:69: unit] Error 1

[lvaylet]

Upon investigation, there is an easy fix for make lint. We just need to replace import collections with import collections.abc and collections.Mapping with collections.abc.Mapping at call site in slo_generator/utils.py.

Fixing make unit in the same way would require updating the nose library itself with similar changes in venv/lib/python3.10/site-packages/nose/suite.py and venv/lib/python3.10/site-packages/nose/suite.py. Unfortunately, nose is no longer being maintained. The latest version (1.3.7) is explicitly not compatible with Python >= 3.10:

• Nose is incompatible with Python >= 3.10 #95
• nose is unmaintained and will stop working under future versions of Python #942
  

and the official website starts with:

Nose has been in maintenance mode for the past several years and will likely cease without a new person/team to take over maintainership. New projects should consider using Nose2, py.test, or just plain unittest/unittest2.

@ocervell WDYT? Is this a good opportunity to move to py.test?

[ocervell]

@lvaylet Sounds good to me. Had no idea nose was going to be discontinued...

[lvaylet]

Fixed by:

• ci: run linting and unit tests on multiple python versions #256
• tests: replace nose (in maintenance mode) with pytest #254