Fix mmif.serialize pythonic behavior

[bohJiang12]

Description

This PR aims to fix #295 which pointed out that dict-like (and list-like) classes in mmif.serialize package need to hold python's dict (and list) properties.

Overview of classes in mmif.serialize

• MmifObject: a base class for MMIF objects that are ''dict-like''
• DataList: a base class for MMIF data fields that are ''list-like''
• DataDict: a base class for MMIF data fields that are ''dict-like''

Then, the following classes are defined and categorized into either ''dict-like''
or ''list-like'' child classes:

dict-like:
- Mmif: a class for MMIF objects
- MmifMetaData: a class for MMIF metadata
- View: a class for MMIF view
- ViewMetaData: a class for MMIF view metadata
- ErrorDict: a class for a specific view that contains error
- ContainsDict: a class for View's 'contains' field
- Annotation & Document: a class for MMIF annotation and document
- AnnotationProperties & DocumentProperties: a class for MMIF annotation properties
- Text: a class for Document's text field

list-like:
- DocumentsList: a class for a list of Document objects
- ViewsList: a class for a list of View objects
- AnnotationsList: a class for a list of Annotation objects

By doing so, any field of a ''dict-like'' class should be accessed if and
only if by either bracket [key] or .get(key). For a ''list-like'' class, the
elements are ordered and accessed by index, for example, [idx].

Note

As I discussed with @keighrim, we intentionally leave a "backdoor" method get_item for the abstract class DataList in order for achieving O(1) access to data (for example, a View or an Annotation) by its long_id.

What's new

• Any subclass of MmifObject (i.e. dict-like class) has only two ways to access data via keys:

  • dict_like_obj[key]
  • dict_like_obj.get(key)

• Any subclass of DataList (i.e. list-like) has below changes:

  • Access a data by index: list_like_obj[index]
  • However, making changes to existing data in the list is disabled now: list_like_obj[index] = new_value raises TypeError
  • The method that access a data by its key has been renamed from .get to .get_item

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 12 lines in your changes missing coverage. Please review.

Project coverage is 91.77%. Comparing base (2e17432) to head (0a4ce6c).
Report is 8 commits behind head on develop.

Files	Patch %	Lines
mmif/serialize/mmif.py	80.95%	4 Missing ⚠️
mmif/serialize/view.py	66.66%	4 Missing ⚠️
mmif/serialize/annotation.py	77.77%	2 Missing ⚠️
mmif/serialize/model.py	90.90%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #304      +/-   ##
===========================================
+ Coverage    90.74%   91.77%   +1.02%     
===========================================
  Files           10       10              
  Lines         1383     1398      +15     
===========================================
+ Hits          1255     1283      +28     
+ Misses         128      115      -13     

Flag	Coverage Δ
unittests	91.77% <82.35%> (+1.02%)	⬆️

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