`Model.export_to_stream` is broken: `format_spec` is supposed to be a string, but is expected to have a `filetype` property

[lpsinger]

Minimal reproducer:

>>> from io import StringIO
>>> from docplex.mp.model import Model
>>> model = Model()
>>> stream = StringIO()
>>> model.export_to_stream(stream, format_spec="sav")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/lpsinger/src/m4opt/.vscode/env/lib/python3.11/site-packages/docplex/mp/model.py", line 5734, in export_to_stream
    self._export_to_stream(stream, hide_user_names, format_spec)
  File "/Users/lpsinger/src/m4opt/.vscode/env/lib/python3.11/site-packages/docplex/mp/model.py", line 5716, in _export_to_stream
    self.__engine.export(stream, format_spec)
  File "/Users/lpsinger/src/m4opt/.vscode/env/lib/python3.11/site-packages/docplex/mp/cplex_engine.py", line 2567, in export
    filetype = exchange_format.filetype
               ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'filetype'

[PhilippeCouronne]

Sorry, I had missed this one. The root cause for this is that SAV export relies on CPLEX runtime. There was once a pure Python SAV writer but it had been removed. In such a case, an exception should be thrown.
Presently, the only way to export to SAV is through `Model.export_as_sav()'

For now an export to stream with "lp" would be enough for me to work on the best bound issue.

[lpsinger]

My own temporary workaround is here: https://github.com/m4opt/m4opt/blob/main/m4opt/_cli/schedule.py#L82-L112

[PhilippeCouronne]

Recap after analysis:

• Model.export_to_stream will crash if passed a format_spec different from lp. This is a bug.
• Moreover, it could not work for non-LP formats, as for these formats, export is managed by Cplex native code, so this possibility should be blocked. So it would be better to rename is as export_to_lp_stream and deprecate the existing to_stream method.

For non-LP formats, DOcplex offers export_as_fmt methods using files, and export_as_fmt_string to output in a string.

A possible enhancement would be to add an export method offering different export possibilities to a file, with optional compression. Options could be apssed explicitly or inferred from the full pathname.

[lpsinger]

The reason I was looking for this is that I wanted something like export_as_fmt that supported gzip compression for all formats.