python: attempt to make dependency work correctly when cross compiling

TODO: write helpful message

mesonbuild/dependencies/python.py

@@ -25,7 +25,7 @@
 from .framework import ExtraFrameworkDependency
 from .pkgconfig import PkgConfigDependency
 from ..environment import detect_cpu_family
-from ..programs import ExternalProgram
+from ..programs import ExternalProgram, find_external_program
 
 if T.TYPE_CHECKING:
  from typing_extensions import TypedDict
@@ -193,8 +193,11 @@ def __init__(self, name: str, environment: 'Environment',
 class PythonSystemDependency(SystemDependency, _PythonDependencyBase):
 
  def __init__(self, name: str, environment: 'Environment',
- kwargs: T.Dict[str, T.Any], installation: 'BasicPythonExternalProgram'):
+ kwargs: T.Dict[str, T.Any], installation: T.Optional[BasicPythonExternalProgram]):
  SystemDependency.__init__(self, name, environment, kwargs)
+ if installation is None:
+ self.is_found = False
+ return
  _PythonDependencyBase.__init__(self, installation, kwargs.get('embed', False))
 
  # match pkg-config behavior
@@ -358,15 +361,27 @@ def python_factory(env: 'Environment', for_machine: 'MachineChoice',
  methods = process_method_kw({DependencyMethods.PKGCONFIG, DependencyMethods.SYSTEM}, kwargs)
  embed = kwargs.get('embed', False)
  candidates: T.List['DependencyGenerator'] = []
- from_installation = installation is not None
- # When not invoked through the python module, default installation.
+
+ # when we're not running from a provided installation, create one using the
+ # appropriate python from the cross file or native file, or fall back to the
+ # running python for the host machine.
  if installation is None:
- installation = BasicPythonExternalProgram('python3', mesonlib.python_command)
- installation.sanity()
- pkg_version = installation.info['variables'].get('LDVERSION') or installation.info['version']
+ if not (env.is_cross_build() and env.need_exe_wrapper() and env.exe_wrapper is None):
+ cmd: T.List[str] = []
+ for prog in find_external_program(env, for_machine, 'python3', 'python3', ['python3', 'python'], allow_default_for_cross=False):
+ if prog.found():
+ cmd = T.cast('T.List[str]', prog.command)
+ break
+ else:
+ if not (env.is_cross_build() and for_machine is mesonlib.MachineChoice.HOST):
+ cmd = mesonlib.python_command
+ if cmd:
+ installation = BasicPythonExternalProgram('python3', cmd)
+ installation.sanity()
 
  if DependencyMethods.PKGCONFIG in methods:
- if from_installation:
+ if installation is not None:
+ pkg_version = installation.info['variables'].get('LDVERSION') or installation.info['version']
  pkg_libdir = installation.info['variables'].get('LIBPC')
  pkg_embed = '-embed' if embed and mesonlib.version_compare(installation.info['version'], '>=3.8') else ''
  pkg_name = f'python-{pkg_version}{pkg_embed}'
@@ -407,10 +422,12 @@ def set_env(name: str, value: str) -> None:
 
  if DependencyMethods.EXTRAFRAMEWORK in methods:
  nkwargs = kwargs.copy()
- if mesonlib.version_compare(pkg_version, '>= 3'):
- # There is a python in /System/Library/Frameworks, but that's python 2.x,
- # Python 3 will always be in /Library
- nkwargs['paths'] = ['/Library/Frameworks']
+ if installation is not None:
+ pkg_version = installation.info['variables'].get('LDVERSION') or installation.info['version']
+ if mesonlib.version_compare(pkg_version, '>= 3'):
+ # There is a python in /System/Library/Frameworks, but that's python 2.x,
+ # Python 3 will always be in /Library
+ nkwargs['paths'] = ['/Library/Frameworks']
  candidates.append(functools.partial(PythonFrameworkDependency, 'Python', env, nkwargs, installation))
 
  return candidates