Add comments to io/file.py

[rly]

Motivation

io/file.py and object mapping is a confusing concept. This came up in discussion today. This PR adds comments to the methods in io/file.py.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.98%. Comparing base (3c5fa48) to head (a82fae8).
Report is 19 commits behind head on dev.

❗ There is a different number of reports uploaded between BASE (3c5fa48) and HEAD (a82fae8). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (3c5fa48)	HEAD (a82fae8)
integration	4	0

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1925      +/-   ##
==========================================
- Coverage   91.88%   83.98%   -7.91%     
==========================================
  Files          27       27              
  Lines        2651     2648       -3     
  Branches      692      691       -1     
==========================================
- Hits         2436     2224     -212     
- Misses        142      329     +187     
- Partials       73       95      +22     

Flag	Coverage Δ
integration	?
unit	83.98% <ø> (+0.01%)	⬆️

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[stephprince]

very helpful, thanks!

[stephprince]

just wanted to clarify why this constructor was removed? Otherwise, looks good to me!

[rly]

Whoops I forgot to mention that. This maps a constructor argument that doesn't exist. It must have been a legacy thing.