Warn if torchvision imported from repo root

[jamt9000]

Suggested fix for #2239

(Should it be an error or are there cases where it would be desired during development?)

[codecov]

Codecov Report

Merging #2759 into master will decrease coverage by 0.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2759      +/-   ##
==========================================
- Coverage   73.22%   72.97%   -0.26%     
==========================================
  Files          96       96              
  Lines        8431     8325     -106     
  Branches     1317     1296      -21     
==========================================
- Hits         6174     6075      -99     
+ Misses       1857     1850       -7     
  Partials      400      400              

Impacted Files	Coverage Δ
torchvision/__init__.py	80.55% <100.00%> (+11.80%)	⬆️
torchvision/extension.py	48.64% <0.00%> (-19.54%)	⬇️
torchvision/io/video.py	69.82% <0.00%> (-10.66%)	⬇️
torchvision/io/image.py	80.00% <0.00%> (-9.14%)	⬇️
torchvision/ops/deform_conv.py	66.66% <0.00%> (-4.31%)	⬇️
torchvision/ops/boxes.py	93.25% <0.00%> (-2.15%)	⬇️
torchvision/ops/roi_align.py	68.96% <0.00%> (-2.01%)	⬇️
torchvision/ops/roi_pool.py	73.07% <0.00%> (-1.93%)	⬇️
torchvision/ops/ps_roi_pool.py	73.07% <0.00%> (-1.93%)	⬇️
torchvision/ops/ps_roi_align.py	71.42% <0.00%> (-1.91%)	⬇️
... and 8 more

---

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[andfoy]

Hi @jamt9000, thanks for this contribution! My only comment here is that we should rather check for the existence of the compiled files _C.so in both Mac and Linux and _C.pyd on Windows, since that shared library is the one that might cause failures when is not found.

[andfoy]

Maybe you can use this call to importlib to find it:

vision/torchvision/io/image.py

Line 17 in 217e26f

extfinder = importlib.machinery.FileFinder(lib_dir, loader_details) # type: ignore[arg-type]

[jamt9000]

@andfoy It looks like torchvision checks for _C.so already

vision/torchvision/extension.py

Lines 16 to 19 in 7a36388

	extfinder = importlib.machinery.FileFinder(lib_dir, loader_details)
	ext_specs = extfinder.find_spec("_C")
	if ext_specs is None:
	raise ImportError

vision/torchvision/extension.py

Lines 23 to 27 in 7a36388

	try:
	_register_extensions()
	_HAS_OPS = True
	except (ImportError, OSError):
	pass

and can continue with reduced functionality if it is missing.

Maybe it should always show a warning when _HAS_OPS = False (which could be for several reasons) and then also
show an extra message if the reason for this seems to be because of being in the root dir.

[fmassa]

@jamt9000 yes, adding checks for the existence of _HAS_OPS is something that we are considering doing, see for example #2467 (which makes me think that I should revamp that PR, I completely forgot about it)

[fmassa]

and I agree with @andfoy , we should instead check for the existence of the _C library.
I for example always run torchvision from the root folder, but I install it via python setup.py build develop, so it works fine. Also a few other systems might package things weirdly, so it's better to error on the safe side.

[fmassa]

LGTM, thanks a lot!

@mthrok @cpuhrsch this is something that could be interesting to be done to the other libraries as well. Also, I'm not sure if the current implementation works as expected in fbcode or if we need to do something else, but let's see.