MAINT: Strip internals from TestCase class

[gfyoung]

TestCase is a vestige from the nosetest framework. We want to discourage usage of self.* method calls under our new pytest framework, so this PR strips as much as possible from the class.

Partially addresses #15990.

[codecov]

Codecov Report

Merging #16016 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #16016      +/-   ##
==========================================
- Coverage      91%   90.99%   -0.01%     
==========================================
  Files         153      153              
  Lines       50481    50469      -12     
==========================================
- Hits        45938    45926      -12     
  Misses       4543     4543

Flag	Coverage Δ
#multiple	88.76% <100%> (-0.01%)	⬇️
#single	40.44% <66.66%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/util/decorators.py	65.3% <ø> (ø)	⬆️
pandas/util/testing.py	80.8% <100%> (-0.21%)	⬇️

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[codecov]

Codecov Report

Merging #16016 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #16016      +/-   ##
==========================================
- Coverage   90.99%   90.99%   -0.01%     
==========================================
  Files         154      154              
  Lines       50484    50471      -13     
==========================================
- Hits        45940    45928      -12     
+ Misses       4544     4543       -1

Flag	Coverage Δ
#multiple	88.76% <100%> (-0.01%)	⬇️
#single	40.44% <66.66%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/util/decorators.py	65.3% <ø> (ø)	⬆️
pandas/util/testing.py	80.78% <100%> (-0.23%)	⬇️
pandas/core/common.py	91.03% <0%> (+0.34%)	⬆️

---

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[jorisvandenbossche]

An alternative would be to keep it as class methods, but remove the unittest.TestCase inheritance (just pointing out, I am personally OK with moving from self. to tm.)

[jreback]

can you rebase.

lgtm otherwise.

[jreback]

An alternative would be to keep it as class methods, but remove the unittest.TestCase inheritance (just pointing out, I am personally OK with moving from self. to tm.)

this is a precursor step to this. That's the goal.

[jorisvandenbossche]

this is a precursor step to this. That's the goal.

No, I mean that, if TestCase no longer inherits from unittest.TestCase, you can keep using self., so what I mean is that this is not a necessary precursor towards that goal

[gfyoung]

@jorisvandenbossche : Perhaps, but why should we though? These methods were never really implemented as instance methods in the first place.

[jreback]

yeah I think we should move aways from using self entirely for actually testing things (keep state is fine).

[gfyoung]

@jreback , @jorisvandenbossche : Rebased, all green, and ready to merge.

[jreback]

thanks!