bpo-39549: reprlib.Repr uses a “fillvalue” attribute

[fish2000]

Whereas the code for reprlib.Repr had previously used a hardcoded
string value of '...', this PR updates it to use of a “fillvalue”
attribute, whose value defaults to '...' and can be reset in either
individual reprlib.Repr instances or in subclasses thereof.

A new assert has been added to test_reprlib.py to verify the new
attributes’ functionality.

https://bugs.python.org/issue39549

[codecov]

Codecov Report

Merging #18343 into master will decrease coverage by 1.06%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #18343       +/-   ##
===========================================
- Coverage   83.18%   82.11%    -1.07%     
===========================================
  Files        1570     1954      +384     
  Lines      414127   583278   +169151     
  Branches    44406    44409        +3     
===========================================
+ Hits       344505   478983   +134478     
- Misses      59981    94653    +34672     
- Partials     9641     9642        +1     

Impacted Files	Coverage Δ
Lib/test/test_asyncore.py	93.62% <0.00%> (-0.18%)	⬇️
Lib/idlelib/squeezer.py	93.23% <0.00%> (ø)	⬆️
Modules/expat/xmltok_impl.c	62.10% <0.00%> (ø)
Modules/_decimal/libmpdec/difradix2.c	100.00% <0.00%> (ø)
Modules/md5module.c	94.01% <0.00%> (ø)
Modules/_tracemalloc.c	84.59% <0.00%> (ø)
Modules/unicodedata.c	89.34% <0.00%> (ø)
Parser/token.c	94.28% <0.00%> (ø)
Modules/_blake2/blake2module.c	90.90% <0.00%> (ø)
Modules/resource.c	77.51% <0.00%> (ø)
... and 380 more

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[eamanu]

Hi,

I think setting a variable could be confused in the sense that is more quickly see
the '...' than "self.fillvalue". But is a personal point of view.

Despite of this could be a good improve to if we want to avoid 'hardcoded' things.

[rhettinger]

Why not pass the fillvalue into _init_?

[fish2000]

@rhettinger I would be happy to do that – I only set it up this way to match the existing manner of setting attributes on reprlib.Repr instances. Shall I go ahead and change it to use an argument?

[rhettinger]

Please update the main documentation as well: Doc/library/reprlib.rst

Include a versionchanged directive.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[rhettinger]

Thanks Zach.

[fish2000]

🎉🎉🎉