Get Python's LIB path

Question

I can see that INCLUDE path is sysconfig.get_path('include').

But I don't see any similar value for LIB.

NumPy outright hardcodes it as os.path.join(sys.prefix, "libs") in Windows and get_config_var('LIBDIR') (not documented and missing in Windows) otherwise.

Is there a more supported way?

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Answer 1

Since it's not a part of any official spec/doc, and, as shown by another answer, there are cases when none of appropriate variables from sysconfig/distutils.sysconfig .get_config_var() are set,

the only way to reliably get it in all cases, exactly as a build would (e.g. even for a Python in the sourcetree) is to delegate to the reference implementation.

In distutils, the logic that sets the library path for a compiler is located in distutils.commands.build_ext.finalize_options(). So, this code would get it with no side effects on the build:

import distutils.command.build_ext    #imports distutils.core, too
d = distutils.core.Distribution()
b = distutils.command.build_ext.build_ext(d)  #or `d.get_command_class('build_ext')(d)',
                                              # then it's enough to import distutils.core
b.finalize_options()
print b.library_dirs

Note that:

• Not all locations in the resulting list necessarily exist.
• If your setup.py is setuptools-based, use setuptools.Distribution and setuptools.command.build_ext instead, correspondingly.
• If you pass any values to setup() that affect the result, you must pass them to Distribution here, too.

---

Since there are no guarantees that the set of the additional values you need to pass will stay the same, and the value is only needed when building an extension,

• it seems like you aren't really supposed to get this value independently at all:
  • If you're using another build facility, you should rather subclass build_ext and get the value from the base method during the build.

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Answer 2

Below is the (rather long) subroutine in skbuild.cmaker that locates libpythonxx.so/pythonxx.lib for the running Python. In CMake, 350-line Modules/FindPythonLibs.cmake is dedicated to this task.

The part of the former that gets just the directory is much simpler though:

libdir = dustutils.sysconfig.get_config_var('LIBDIR')
if sysconfig.get_config_var('MULTIARCH'):
    masd = sysconfig.get_config_var('multiarchsubdir')
    if masd:
        if masd.startswith(os.sep):
            masd = masd[len(os.sep):]
        libdir = os.path.join(libdir, masd)

if libdir is None:
    libdir = os.path.abspath(os.path.join(
        sysconfig.get_config_var('LIBDEST'), "..", "libs"))

---

def get_python_library(python_version):
    """Get path to the python library associated with the current python
    interpreter."""
    # determine direct path to libpython
    python_library = sysconfig.get_config_var('LIBRARY')

    # if static (or nonexistent), try to find a suitable dynamic libpython
    if (python_library is None or
            os.path.splitext(python_library)[1][-2:] == '.a'):

        candidate_lib_prefixes = ['', 'lib']

        candidate_extensions = ['.lib', '.so', '.a']
        if sysconfig.get_config_var('WITH_DYLD'):
            candidate_extensions.insert(0, '.dylib')

        candidate_versions = [python_version]
        if python_version:
            candidate_versions.append('')
            candidate_versions.insert(
                0, "".join(python_version.split(".")[:2]))

        abiflags = getattr(sys, 'abiflags', '')
        candidate_abiflags = [abiflags]
        if abiflags:
            candidate_abiflags.append('')

        # Ensure the value injected by virtualenv is
        # returned on windows.
        # Because calling `sysconfig.get_config_var('multiarchsubdir')`
        # returns an empty string on Linux, `du_sysconfig` is only used to
        # get the value of `LIBDIR`.
        libdir = du_sysconfig.get_config_var('LIBDIR')
        if sysconfig.get_config_var('MULTIARCH'):
            masd = sysconfig.get_config_var('multiarchsubdir')
            if masd:
                if masd.startswith(os.sep):
                    masd = masd[len(os.sep):]
                libdir = os.path.join(libdir, masd)

        if libdir is None:
            libdir = os.path.abspath(os.path.join(
                sysconfig.get_config_var('LIBDEST'), "..", "libs"))

        candidates = (
            os.path.join(
                libdir,
                ''.join((pre, 'python', ver, abi, ext))
            )
            for (pre, ext, ver, abi) in itertools.product(
                candidate_lib_prefixes,
                candidate_extensions,
                candidate_versions,
                candidate_abiflags
            )
        )

        for candidate in candidates:
            if os.path.exists(candidate):
                # we found a (likely alternate) libpython
                python_library = candidate
                break

    # TODO(opadron): what happens if we don't find a libpython?

    return python_library

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