ARROW-3636: [C++/Python] Update arrow/python/pyarrow_api.h

[xhochy]

This simply updates the file. I'll defer the removal of public to https://issues.apache.org/jira/browse/ARROW-3641

[codecov-io]

Codecov Report

Merging #2854 into master will increase coverage by 0.85%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2854      +/-   ##
==========================================
+ Coverage   87.55%   88.41%   +0.85%     
==========================================
  Files         410      348      -62     
  Lines       63486    59356    -4130     
==========================================
- Hits        55586    52480    -3106     
+ Misses       7828     6876     -952     
+ Partials       72        0      -72

Impacted Files	Coverage Δ
cpp/src/arrow/python/pyarrow_api.h	82.75% <100%> (+2.16%)	⬆️
rust/src/record_batch.rs
go/arrow/array/table.go
rust/src/util/bit_util.rs
go/arrow/math/uint64_amd64.go
go/arrow/internal/testing/tools/bool.go
go/arrow/internal/bitutil/bitutil.go
go/arrow/memory/memory_avx2_amd64.go
go/arrow/array/null.go
go/arrow/datatype_nested.go
... and 53 more

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[pitrou]

Just two things I'm curious about.

[pitrou]

I realize this is Cython-generated code, but why is including this file becoming necessary? @scoder

[pitrou]

I'm curious: what does it do to put extern "C" around a function declaration that has C++ objects in its prototype? @scoder

[xhochy]

This will prevent the C++ compiler to also apply C++ name mangling on the already (Cython) mangled name, i.e. this guarantees that __pyx_f_7pyarrow_3lib_pyarrow_wrap_array is also the exact name of the symbol in the shared object. Normally the types of the parameters would also be included in the mangled symbol name.

[wesm]

+1