[issue1820] Enhance Object/structseq.c to match namedtuple and tuple api

[Sunny K]

Sunny K added the comment:

New patch for 3.4 adds the following:

1. _fields
2. _replace()
3. _asdict()
4. eval(repr(s)) == s

Now the issues:

1. _asdict() returns a normal dictionary. I don't know if this is what
   is required.

2. Both _asdict() and _replace() assume that unnamed visible fields are
   at the end of the visible sequence.  A comment at the beginning says
   they are allowed for indices < n_visible_fields.
   
   Is there another way to map members to values? Because tp->members
   is (visible named fields + invisible named fields) whereas values
   array is (visible named fields + visible unnamed fields + invisible
   named fields)

3. The mismatch mentioned above is present in the current
   implementation of repr:

   In os.stat_result, the last three visible fields are unnamed
   (i.e integer a_time, m_time and c_time). However they are present 
   in the repr output with keys which are the first three keys from the
   invisible part(float a_time, m_time and c_time).
   Was this intentional?

   Also, the above logic causes duplicate keys when invisible fields 
   are included in the repr output as per issue11629.
  
   In my patch for that issue, i put invisible fields under the 'dict'
   keyword argument. This output format utilises code already present 
   in structseq_new and makes eval work as expected when invisible 
   fields are present in repr. Also, it sidesteps the question of
   duplicated keys because they are present inside a dict.

4. Raymond stated that _fields should contain only the visible 
   positional fields. So _fields of os.stat_result contains only 7
   fields because of the three unnamed fields. Is this the expected 
   implementation?

5. Is there a way to declare a member function in C that accepts only 
   keyword arguments(like _replace())? I could not find one.

This is my first real C patch. So some of the issues might just be a
lack of understanding. Apologies.

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keywords: +patch
nosy: +sunfinite
Added file: http://bugs.python.org/file32525/structseq_1.patch

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