Fixed "UnsupportedOperation" exception in highs_api.py

[postlogist]

This patch solves two issues in HiGHS API (actualSolve method):

1. Code for parsing a list of additional options had a typo in method name (should be startswith)
2. The new method of starting a subprocess caused UnsupportedOperation exception in Windows (tested with Anaconda Python and WinPython). I replaced this code with the code from PuLP 2.7.0 release.

Here's the stack trace for the error that previously appeared ---------------------------------------------------------------------------
UnsupportedOperation Traceback (most recent call last)
Cell In[30], line 1
----> 1 prob.solve(solver=highs)
2 print(f"Status: {pl.LpStatus[prob.status]}")
3 print(f"EV = {-pl.value(prob.objective): .0f}")

File ~.conda\envs\opt\Lib\site-packages\pulp\pulp.py:1920, in LpProblem.solve(self, solver, **kwargs)
1918 # time it
1919 self.startClock()
-> 1920 status = solver.actualSolve(self, **kwargs)
1921 self.stopClock()
1922 self.restoreObjective(wasNone, dummyVar)

File ~.conda\envs\opt\Lib\site-packages\pulp\apis\highs_api.py:145, in HiGHS_CMD.actualSolve(self, lp)
143 with open(tmpOptions, "w") as options_file:
144 options_file.write("\n".join(file_options))
--> 145 process = subprocess.run(command, stdout=sys.stdout, stderr=sys.stderr)
147 # HiGHS return code semantics (see: ERGO-Code/HiGHS#527 (comment))
148 # - -1: error
149 # - 0: success
150 # - 1: warning
151 if process.returncode == -1:

File ~.conda\envs\opt\Lib\subprocess.py:548, in run(input, capture_output, timeout, check, *popenargs, **kwargs)
545 kwargs['stdout'] = PIPE
546 kwargs['stderr'] = PIPE
--> 548 with Popen(*popenargs, **kwargs) as process:
549 try:
550 stdout, stderr = process.communicate(input, timeout=timeout)

File ~.conda\envs\opt\Lib\subprocess.py:992, in Popen.init(self, args, bufsize, executable, stdin, stdout, stderr, preexec_fn, close_fds, shell, cwd, env, universal_newlines, startupinfo, creationflags, restore_signals, start_new_session, pass_fds, user, group, extra_groups, encoding, errors, text, umask, pipesize, process_group)
973 raise ValueError(f"User ID cannot be negative, got {uid}")
975 # Input and output objects. The general principle is like
976 # this:
977 #
(...)
987 # are -1 when not using PIPEs. The child objects are -1
988 # when not redirecting.
990 (p2cread, p2cwrite,
991 c2pread, c2pwrite,
--> 992 errread, errwrite) = self._get_handles(stdin, stdout, stderr)
994 # From here on, raising exceptions may cause file descriptor leakage
995
996 # We wrap OS handles before launching the child, otherwise a
997 # quickly terminating child could make our fds unwrappable
998 # (see #8458).
1000 if _mswindows:

File ~.conda\envs\opt\Lib\subprocess.py:1384, in Popen._get_handles(self, stdin, stdout, stderr)
1381 c2pwrite = msvcrt.get_osfhandle(stdout)
1382 else:
1383 # Assuming file-like object
-> 1384 c2pwrite = msvcrt.get_osfhandle(stdout.fileno())
1385 c2pwrite = self._make_inheritable(c2pwrite)
1387 if stderr is None:

File ~.conda\envs\opt\Lib\site-packages\ipykernel\iostream.py:311, in OutStream.fileno(self)
309 return self._original_stdstream_copy
310 else:
--> 311 raise io.UnsupportedOperation("fileno")

UnsupportedOperation: fileno

[CLAassistant]

All committers have signed the CLA.

[pchtsp]

Thanks. The string method I think was already fixed in a previous PR. I've made some suggestions so the style matches the style of other solve APIs (CBC, CPLEX, GUROBI). You can check the other files to get inspiration.

[pchtsp]

you define pipe but then you don't use it.

[pchtsp]

please delete these lines

[pchtsp]

this is not used

[pchtsp]

Check how it's done in the solve method of coin_api.COIN_CMD class. Ideally we would like to have the same style:

if self.msg:
    pipe = None
else:
    pipe = open(os.devnull, "w")
process = subprocess.Popen(args, stdout=pipe, stderr=pipe, stdin=devnull)
if process.wait() == -1:
    if pipe:
        pipe.close()
    raise PulpSolverError(
        "Pulp: Error while trying to execute, use msg=True for more details"
        + self.path
    )
if pipe:
    pipe.close()