Replace get_func_args

[serhii73]

Close #5872

[codecov]

Codecov Report

Merging #5885 (6a7edd2) into master (8045d7e) will decrease coverage by 0.05%.
The diff coverage is 92.59%.

❗ Current head 6a7edd2 differs from pull request most recent head e532f7e. Consider uploading reports for the commit e532f7e to get more accurate results

@@            Coverage Diff             @@
##           master    #5885      +/-   ##
==========================================
- Coverage   88.85%   88.81%   -0.05%     
==========================================
  Files         162      162              
  Lines       11057    11114      +57     
  Branches     1801     1802       +1     
==========================================
+ Hits         9825     9871      +46     
- Misses        954      961       +7     
- Partials      278      282       +4     

Impacted Files	Coverage Δ
scrapy/mail.py	77.77% <0.00%> (-0.88%)	⬇️
scrapy/pipelines/files.py	72.00% <50.00%> (+0.09%)	⬆️
scrapy/utils/misc.py	96.52% <60.00%> (-1.35%)	⬇️
scrapy/utils/python.py	89.67% <80.95%> (-1.30%)	⬇️
scrapy/signalmanager.py	81.81% <83.33%> (+1.81%)	⬆️
scrapy/core/engine.py	83.68% <90.90%> (-0.51%)	⬇️
scrapy/statscollectors.py	89.28% <92.85%> (-2.88%)	⬇️
scrapy/core/scheduler.py	93.23% <93.75%> (-0.52%)	⬇️
scrapy/core/downloader/__init__.py	92.80% <100.00%> (+0.05%)	⬆️
scrapy/core/scraper.py	84.94% <100.00%> (+0.33%)	⬆️
... and 11 more

[wRAR]

@elacuesta we may want to copy this to https://github.com/scrapy/itemloaders/blob/master/itemloaders/utils.py

[wRAR]

We had some discussions regarding the operator.itemgetter case and so far it's not clear to me how to handle it in the best way. On CPython inspect.signature(operator.itemgetter(2)) raises ValueError: callable operator.itemgetter(2) is not supported by signature and my initial suggestion was to catch this exception and return []. I've just checked this on PyPy and it returns <Signature (obj)>, so it works there. So it seems that we still need separate checks in the test for CPython and PyPy, where PyPy will assert ["obj"] like it does now, and CPython will expect []?

Specifically omitting operator.itemgetter like in this PR is not a good idea because get_func_args() previously worked for it on PyPy and because there should be other cases that doesn't work on CPython. Not handling the exception is also worse than returning []. So keeping separate test conditions seems the best approach to me.