REFACTOR-#4796: Introduce constant for __reduced__ column name

[noloerino]

Signed-off-by: Jonathan Shi jhshi@ponder.io

What do these changes do?

Adds the MODIN_UNNAMED_SERIES_LABEL to replace hardcoded instances of `"reduced" found throughout the codebase.

• commit message follows format outlined here
• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves REFACTOR: Introduce constant for __reduced__ column names #4796
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date
• added (Issue Number: PR title (PR Number)) and github username to release notes for next major release

[codecov]

Codecov Report

Merging #4799 (a46400a) into master (3e981ba) will increase coverage by 4.58%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4799      +/-   ##
==========================================
+ Coverage   85.28%   89.87%   +4.58%     
==========================================
  Files         259      260       +1     
  Lines       19381    19666     +285     
==========================================
+ Hits        16530    17674    +1144     
+ Misses       2851     1992     -859     

Impacted Files	Coverage Δ
...frame/pandas/exchange/dataframe_protocol/column.py	76.36% <ø> (ø)
modin/pandas/groupby.py	93.71% <ø> (ø)
...n/core/dataframe/algebra/default2pandas/default.py	96.55% <100.00%> (ø)
...n/core/dataframe/algebra/default2pandas/groupby.py	96.50% <100.00%> (+0.02%)	⬆️
modin/core/dataframe/algebra/groupby.py	95.14% <100.00%> (ø)
modin/core/dataframe/pandas/dataframe/dataframe.py	94.33% <100.00%> (+<0.01%)	⬆️
modin/core/storage_formats/base/query_compiler.py	99.22% <100.00%> (+<0.01%)	⬆️
...odin/core/storage_formats/pandas/query_compiler.py	96.09% <100.00%> (ø)
...entations/omnisci_on_native/dataframe/dataframe.py	92.14% <100.00%> (+<0.01%)	⬆️
...tal/core/storage_formats/omnisci/query_compiler.py	94.49% <100.00%> (ø)
... and 46 more

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[pyrito]

Thank you, this seems more readable. What does __reduced__ even mean in this context?

[noloerino]

@pyrito The __reduced__ label is used for the internal query compiler representation of unnamed Series objects in order to distinguish between an N-element Series and a 1xN Dataframe (see relevant query compiler docs).

[mvashishtha]

LGTM @noloerino , thank you!

[mvashishtha]

@noloerino I have a minor comment.

[mvashishtha]

@prutskov @modin-project/modin-omnisci we need an omnisci approval.

[prutskov]

Overall, LGTM. But I found additional places in different parts of modin with direct using of __reduced__ string. We should change __reduced__->MODIN_UNNAMED_SERIES_LABEL everywhere (code/comments, docs, tests) to not worry about this places in the future in case we will want to change string value of MODIN_UNNAMED_SERIES_LABEL

[noloerino]

@prutskov makes sense, I've replaced it in comments/docs as well.

[prutskov]

Additional one minor suggestion

[prutskov]

Thanks, @noloerino! LGTM

[YarShev]

LGTM, left a comment.