background: use __len__ for chunk length, minor style fixes

[pkulev]

• Use len for providing length of inner list This is common python approach. len calls len of countable object.
• Use valid variable names in test instead of P and S, remove unused import.

Closes issue(s): [ ]

Description of the changes:

• What's new:
• Fixes:
• Refactoring:

Reviewers: [ @taptap ]

Task list:

• [x] submit pull request
• [x] review
• [x] testing
• [x] fixes after review, squashing, rebasing to master

[codecov-io]

Current coverage is 55.29% (diff: 100%)

Merging #43 into master will decrease coverage by 0.05%

@@             master        #43   diff @@
==========================================
  Files            38         38          
  Lines          1729       1727     -2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits            957        955     -2   
  Misses          772        772          
  Partials          0          0          

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