Describe the issue:

f2py --include-paths is documented as a path-list option that should use the platform path separator (os.pathsep): : on POSIX and ; on Windows. However, the current parser always splits the option value on : in CombineIncludePaths.

That breaks normal Windows drive-letter paths because C:\... contains : as part of the path itself. This can affect both a single Windows include directory and native Windows path lists.

Affected code:

# numpy/f2py/f2py2e.py
if option_string in {"--include-paths", "--include_paths"}:
    include_paths_set.update(values.split(':'))
else:
    include_paths_set.add(values)

A single Windows path is split incorrectly:

--include-paths C:\fortran\include
=> ['C', '\\fortran\\include']

A Windows path list is also split incorrectly:

--include-paths C:\fortran\include;D:\vendor\include
=> ['C', '\\fortran\\include;D', '\\vendor\\include']

The expected behavior is to split --include-paths with os.pathsep, while leaving -I <path> as a single include-directory option.

Reproduce the code example:

import os

single_path = r"C:\fortran\include"
path_list = r"C:\fortran\include;D:\vendor\include"

print("os.name", os.name)
print("os.pathsep", repr(os.pathsep))

print("current single path split", single_path.split(':'))
print("expected single path split", single_path.split(os.pathsep))

print("current path-list split", path_list.split(':'))
print("expected path-list split", path_list.split(os.pathsep))

On Windows, this demonstrates the parser-level issue:

os.name nt
os.pathsep ';'
current single path split ['C', '\\fortran\\include']
expected single path split ['C:\\fortran\\include']
current path-list split ['C', '\\fortran\\include;D', '\\vendor\\include']
expected path-list split ['C:\\fortran\\include', 'D:\\vendor\\include']

Error message:

There is no direct Python traceback. The parsed include path list is wrong before F2PY passes it downstream.

The parsed include directories can then be consumed by:

f2py CLI
  -> f2py_parser()
  -> CombineIncludePaths.__call__()
  -> get_newer_options()
  -> include_paths / include_dirs

and then by either Fortran include-file lookup:

run_main(...)
  -> scaninputline(...)
  -> crackfortran.include_paths
  -> Fortran INCLUDE file lookup

or by the compile backend path:

run_compile(...)
  -> get_newer_options(...)
  -> build_backend(..., include_dirs, ...)
  -> MesonBackend
  -> meson.build include directories

Python and NumPy Versions:

This was checked against the current main branch source checkout:

NumPy source commit: f5f68cc3d25daf05402eed5aaa58459c56b3ac2b
Python used for the local parser demonstration: 3.14.5
Platform: Windows (`os.name == 'nt'`, `os.pathsep == ';'`)

Runtime Environment:

This is a CLI argument parsing issue in numpy/f2py/f2py2e.py, so it can be reproduced without importing a built NumPy package or running compiled code.

How does this issue affect you or how did you find it:

This affects F2PY users and build integrations that pass include directories through the documented --include-paths option on Windows. Even a single drive-letter path such as C:\fortran\include is split incorrectly by the current hard-coded : separator.

The workaround is to use repeated -I <path> options, which are handled as single paths and do not go through the path-list split. However, --include-paths is documented as the path-list option, so it should follow the platform os.pathsep semantics.