Currently, we have autouse fixtures in tests/series/test_arithmetic.py, tests/frame/test_arithmetic.py & tests/arithmetic/conftest.py, named switch_numexpr_min_elements that control if arithmetic computations in those locations use numexpr or python/numpy for calculations.
The fixtures have some issues:
1. they are run even if numexpr isn't installed
2. they only check if expression._MIN_ELEMENTS is zero, it would be better if they also checked expression.USE_NUMEXPR
3. they expose magic numbers (0, 1000000), used to check if numexpr is being used
4. They are used both as autouse and directly, i.e. called two times in the same test in some circumstances
I propose replacing these with a use_numexpr fixture that skips if numexpr is not installed and otherwise sets everything correctly up + add a fixture that only checks if numexpr is being used but doesn't change anything.