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cpython/Lib/sysconfig/__init__.py

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"""Access to Python's configuration information."""
import os
import sys
import threading
from os.path import realpath
__all__ = [
'get_config_h_filename',
'get_config_var',
'get_config_vars',
'get_makefile_filename',
'get_path',
'get_path_names',
'get_paths',
'get_platform',
'get_python_version',
'get_scheme_names',
'parse_config_h',
]
# Keys for get_config_var() that are never converted to Python integers.
_ALWAYS_STR = {
'IPHONEOS_DEPLOYMENT_TARGET',
'MACOSX_DEPLOYMENT_TARGET',
}
_INSTALL_SCHEMES = {
'posix_prefix': {
'stdlib': '{installed_base}/{platlibdir}/{implementation_lower}{py_version_short}{abi_thread}',
'platstdlib': '{platbase}/{platlibdir}/{implementation_lower}{py_version_short}{abi_thread}',
'purelib': '{base}/lib/{implementation_lower}{py_version_short}{abi_thread}/site-packages',
'platlib': '{platbase}/{platlibdir}/{implementation_lower}{py_version_short}{abi_thread}/site-packages',
'include':
'{installed_base}/include/{implementation_lower}{py_version_short}{abiflags}',
'platinclude':
'{installed_platbase}/include/{implementation_lower}{py_version_short}{abiflags}',
'scripts': '{base}/bin',
'data': '{base}',
},
'posix_home': {
'stdlib': '{installed_base}/lib/{implementation_lower}',
'platstdlib': '{base}/lib/{implementation_lower}',
'purelib': '{base}/lib/{implementation_lower}',
'platlib': '{base}/lib/{implementation_lower}',
'include': '{installed_base}/include/{implementation_lower}',
'platinclude': '{installed_base}/include/{implementation_lower}',
'scripts': '{base}/bin',
'data': '{base}',
},
'nt': {
'stdlib': '{installed_base}/Lib',
'platstdlib': '{base}/Lib',
'purelib': '{base}/Lib/site-packages',
'platlib': '{base}/Lib/site-packages',
'include': '{installed_base}/Include',
'platinclude': '{installed_base}/Include',
'scripts': '{base}/Scripts',
'data': '{base}',
},
# Downstream distributors can overwrite the default install scheme.
# This is done to support downstream modifications where distributors change
# the installation layout (eg. different site-packages directory).
# So, distributors will change the default scheme to one that correctly
# represents their layout.
# This presents an issue for projects/people that need to bootstrap virtual
# environments, like virtualenv. As distributors might now be customizing
# the default install scheme, there is no guarantee that the information
# returned by sysconfig.get_default_scheme/get_paths is correct for
# a virtual environment, the only guarantee we have is that it is correct
# for the *current* environment. When bootstrapping a virtual environment,
# we need to know its layout, so that we can place the files in the
# correct locations.
# The "*_venv" install scheme is a scheme to bootstrap virtual environments,
# essentially identical to the default posix_prefix/nt schemes.
# Downstream distributors who patch posix_prefix/nt scheme are encouraged to
# leave the following schemes unchanged
'posix_venv': {
'stdlib': '{installed_base}/{platlibdir}/{implementation_lower}{py_version_short}{abi_thread}',
'platstdlib': '{platbase}/{platlibdir}/{implementation_lower}{py_version_short}{abi_thread}',
'purelib': '{base}/lib/{implementation_lower}{py_version_short}{abi_thread}/site-packages',
'platlib': '{platbase}/{platlibdir}/{implementation_lower}{py_version_short}{abi_thread}/site-packages',
'include':
'{installed_base}/include/{implementation_lower}{py_version_short}{abiflags}',
'platinclude':
'{installed_platbase}/include/{implementation_lower}{py_version_short}{abiflags}',
'scripts': '{base}/bin',
'data': '{base}',
},
'nt_venv': {
'stdlib': '{installed_base}/Lib',
'platstdlib': '{base}/Lib',
'purelib': '{base}/Lib/site-packages',
'platlib': '{base}/Lib/site-packages',
'include': '{installed_base}/Include',
'platinclude': '{installed_base}/Include',
'scripts': '{base}/Scripts',
'data': '{base}',
},
}
# For the OS-native venv scheme, we essentially provide an alias:
if os.name == 'nt':
_INSTALL_SCHEMES['venv'] = _INSTALL_SCHEMES['nt_venv']
else:
_INSTALL_SCHEMES['venv'] = _INSTALL_SCHEMES['posix_venv']
def _get_implementation():
return 'Python'
# NOTE: site.py has copy of this function.
# Sync it when modify this function.
def _getuserbase():
env_base = os.environ.get("PYTHONUSERBASE", None)
if env_base:
return env_base
# Emscripten, iOS, tvOS, VxWorks, WASI, and watchOS have no home directories
if sys.platform in {"emscripten", "ios", "tvos", "vxworks", "wasi", "watchos"}:
return None
def joinuser(*args):
return os.path.expanduser(os.path.join(*args))
if os.name == "nt":
base = os.environ.get("APPDATA") or "~"
return joinuser(base, _get_implementation())
if sys.platform == "darwin" and sys._framework:
return joinuser("~", "Library", sys._framework,
f"{sys.version_info[0]}.{sys.version_info[1]}")
return joinuser("~", ".local")
_HAS_USER_BASE = (_getuserbase() is not None)
if _HAS_USER_BASE:
_INSTALL_SCHEMES |= {
# NOTE: When modifying "purelib" scheme, update site._get_path() too.
'nt_user': {
'stdlib': '{userbase}/{implementation}{py_version_nodot_plat}',
'platstdlib': '{userbase}/{implementation}{py_version_nodot_plat}',
'purelib': '{userbase}/{implementation}{py_version_nodot_plat}/site-packages',
'platlib': '{userbase}/{implementation}{py_version_nodot_plat}/site-packages',
'include': '{userbase}/{implementation}{py_version_nodot_plat}/Include',
'scripts': '{userbase}/{implementation}{py_version_nodot_plat}/Scripts',
'data': '{userbase}',
},
'posix_user': {
'stdlib': '{userbase}/{platlibdir}/{implementation_lower}{py_version_short}{abi_thread}',
'platstdlib': '{userbase}/{platlibdir}/{implementation_lower}{py_version_short}{abi_thread}',
'purelib': '{userbase}/lib/{implementation_lower}{py_version_short}{abi_thread}/site-packages',
'platlib': '{userbase}/lib/{implementation_lower}{py_version_short}{abi_thread}/site-packages',
'include': '{userbase}/include/{implementation_lower}{py_version_short}{abi_thread}',
'scripts': '{userbase}/bin',
'data': '{userbase}',
},
'osx_framework_user': {
'stdlib': '{userbase}/lib/{implementation_lower}',
'platstdlib': '{userbase}/lib/{implementation_lower}',
'purelib': '{userbase}/lib/{implementation_lower}/site-packages',
'platlib': '{userbase}/lib/{implementation_lower}/site-packages',
'include': '{userbase}/include/{implementation_lower}{py_version_short}',
'scripts': '{userbase}/bin',
'data': '{userbase}',
},
}
_SCHEME_KEYS = ('stdlib', 'platstdlib', 'purelib', 'platlib', 'include',
'scripts', 'data')
_PY_VERSION = sys.version.split()[0]
_PY_VERSION_SHORT = f'{sys.version_info[0]}.{sys.version_info[1]}'
_PY_VERSION_SHORT_NO_DOT = f'{sys.version_info[0]}{sys.version_info[1]}'
_BASE_PREFIX = os.path.normpath(sys.base_prefix)
_BASE_EXEC_PREFIX = os.path.normpath(sys.base_exec_prefix)
# Mutex guarding initialization of _CONFIG_VARS.
_CONFIG_VARS_LOCK = threading.RLock()
_CONFIG_VARS = None
# True iff _CONFIG_VARS has been fully initialized.
_CONFIG_VARS_INITIALIZED = False
_USER_BASE = None
def _safe_realpath(path):
try:
return realpath(path)
except OSError:
return path
if sys.executable:
_PROJECT_BASE = os.path.dirname(_safe_realpath(sys.executable))
else:
# sys.executable can be empty if argv[0] has been changed and Python is
# unable to retrieve the real program name
_PROJECT_BASE = _safe_realpath(os.getcwd())
# In a virtual environment, `sys._home` gives us the target directory
# `_PROJECT_BASE` for the executable that created it when the virtual
# python is an actual executable ('venv --copies' or Windows).
_sys_home = getattr(sys, '_home', None)
if _sys_home:
_PROJECT_BASE = _sys_home
if os.name == 'nt':
# In a source build, the executable is in a subdirectory of the root
# that we want (<root>\PCbuild\<platname>).
# `_BASE_PREFIX` is used as the base installation is where the source
# will be. The realpath is needed to prevent mount point confusion
# that can occur with just string comparisons.
if _safe_realpath(_PROJECT_BASE).startswith(
_safe_realpath(f'{_BASE_PREFIX}\\PCbuild')):
_PROJECT_BASE = _BASE_PREFIX
# set for cross builds
if "_PYTHON_PROJECT_BASE" in os.environ:
_PROJECT_BASE = _safe_realpath(os.environ["_PYTHON_PROJECT_BASE"])
def is_python_build(check_home=None):
if check_home is not None:
import warnings
warnings.warn("check_home argument is deprecated and ignored.",
DeprecationWarning, stacklevel=2)
for fn in ("Setup", "Setup.local"):
if os.path.isfile(os.path.join(_PROJECT_BASE, "Modules", fn)):
return True
return False
_PYTHON_BUILD = is_python_build()
if _PYTHON_BUILD:
for scheme in ('posix_prefix', 'posix_home'):
# On POSIX-y platforms, Python will:
# - Build from .h files in 'headers' (which is only added to the
# scheme when building CPython)
# - Install .h files to 'include'
scheme = _INSTALL_SCHEMES[scheme]
scheme['headers'] = scheme['include']
scheme['include'] = '{srcdir}/Include'
scheme['platinclude'] = '{projectbase}/.'
del scheme
def _subst_vars(s, local_vars):
try:
return s.format(**local_vars)
except KeyError as var:
try:
return s.format(**os.environ)
except KeyError:
raise AttributeError(f'{var}') from None
def _extend_dict(target_dict, other_dict):
target_keys = target_dict.keys()
for key, value in other_dict.items():
if key in target_keys:
continue
target_dict[key] = value
def _expand_vars(scheme, vars):
res = {}
if vars is None:
vars = {}
_extend_dict(vars, get_config_vars())
if os.name == 'nt':
# On Windows we want to substitute 'lib' for schemes rather
# than the native value (without modifying vars, in case it
# was passed in)
vars = vars | {'platlibdir': 'lib'}
for key, value in _INSTALL_SCHEMES[scheme].items():
if os.name in ('posix', 'nt'):
value = os.path.expanduser(value)
res[key] = os.path.normpath(_subst_vars(value, vars))
return res
def _get_preferred_schemes():
if os.name == 'nt':
return {
'prefix': 'nt',
'home': 'posix_home',
'user': 'nt_user',
}
if sys.platform == 'darwin' and sys._framework:
return {
'prefix': 'posix_prefix',
'home': 'posix_home',
'user': 'osx_framework_user',
}
return {
'prefix': 'posix_prefix',
'home': 'posix_home',
'user': 'posix_user',
}
def get_preferred_scheme(key):
if key == 'prefix' and sys.prefix != sys.base_prefix:
return 'venv'
scheme = _get_preferred_schemes()[key]
if scheme not in _INSTALL_SCHEMES:
raise ValueError(
f"{key!r} returned {scheme!r}, which is not a valid scheme "
f"on this platform"
)
return scheme
def get_default_scheme():
return get_preferred_scheme('prefix')
def get_makefile_filename():
"""Return the path of the Makefile."""
if _PYTHON_BUILD:
return os.path.join(_PROJECT_BASE, "Makefile")
if hasattr(sys, 'abiflags'):
config_dir_name = f'config-{_PY_VERSION_SHORT}{sys.abiflags}'
else:
config_dir_name = 'config'
if hasattr(sys.implementation, '_multiarch'):
config_dir_name += f'-{sys.implementation._multiarch}'
return os.path.join(get_path('stdlib'), config_dir_name, 'Makefile')
def _get_sysconfigdata_name():
multiarch = getattr(sys.implementation, '_multiarch', '')
return os.environ.get(
'_PYTHON_SYSCONFIGDATA_NAME',
f'_sysconfigdata_{sys.abiflags}_{sys.platform}_{multiarch}',
)
def _init_posix(vars):
"""Initialize the module as appropriate for POSIX systems."""
# _sysconfigdata is generated at build time, see _generate_posix_vars()
name = _get_sysconfigdata_name()
# For cross builds, the path to the target's sysconfigdata must be specified
# so it can be imported. It cannot be in PYTHONPATH, as foreign modules in
# sys.path can cause crashes when loaded by the host interpreter.
# Rely on truthiness as a valueless env variable is still an empty string.
# See OS X note in _generate_posix_vars re _sysconfigdata.
if (path := os.environ.get('_PYTHON_SYSCONFIGDATA_PATH')):
from importlib.machinery import FileFinder, SourceFileLoader, SOURCE_SUFFIXES
from importlib.util import module_from_spec
spec = FileFinder(path, (SourceFileLoader, SOURCE_SUFFIXES)).find_spec(name)
_temp = module_from_spec(spec)
spec.loader.exec_module(_temp)
else:
_temp = __import__(name, globals(), locals(), ['build_time_vars'], 0)
build_time_vars = _temp.build_time_vars
# GH-126920: Make sure we don't overwrite any of the keys already set
vars.update(build_time_vars | vars)
def _init_non_posix(vars):
"""Initialize the module as appropriate for NT"""
# set basic install directories
import _winapi
import _sysconfig
vars['LIBDEST'] = get_path('stdlib')
vars['BINLIBDEST'] = get_path('platstdlib')
vars['INCLUDEPY'] = get_path('include')
# Add EXT_SUFFIX, SOABI, and Py_GIL_DISABLED
vars.update(_sysconfig.config_vars())
vars['LIBDIR'] = _safe_realpath(os.path.join(get_config_var('installed_base'), 'libs'))
if hasattr(sys, 'dllhandle'):
dllhandle = _winapi.GetModuleFileName(sys.dllhandle)
vars['LIBRARY'] = os.path.basename(_safe_realpath(dllhandle))
vars['LDLIBRARY'] = vars['LIBRARY']
vars['EXE'] = '.exe'
vars['VERSION'] = _PY_VERSION_SHORT_NO_DOT
vars['BINDIR'] = os.path.dirname(_safe_realpath(sys.executable))
vars['TZPATH'] = ''
#
# public APIs
#
def parse_config_h(fp, vars=None):
"""Parse a config.h-style file.
A dictionary containing name/value pairs is returned. If an
optional dictionary is passed in as the second argument, it is
used instead of a new dictionary.
"""
if vars is None:
vars = {}
import re
define_rx = re.compile("#define ([A-Z][A-Za-z0-9_]+) (.*)\n")
undef_rx = re.compile("/[*] #undef ([A-Z][A-Za-z0-9_]+) [*]/\n")
while True:
line = fp.readline()
if not line:
break
m = define_rx.match(line)
if m:
n, v = m.group(1, 2)
try:
if n in _ALWAYS_STR:
raise ValueError
v = int(v)
except ValueError:
pass
vars[n] = v
else:
m = undef_rx.match(line)
if m:
vars[m.group(1)] = 0
return vars
def get_config_h_filename():
"""Return the path of pyconfig.h."""
if _PYTHON_BUILD:
if os.name == "nt":
inc_dir = os.path.dirname(sys._base_executable)
else:
inc_dir = _PROJECT_BASE
else:
inc_dir = get_path('platinclude')
return os.path.join(inc_dir, 'pyconfig.h')
def get_scheme_names():
"""Return a tuple containing the schemes names."""
return tuple(sorted(_INSTALL_SCHEMES))
def get_path_names():
"""Return a tuple containing the paths names."""
return _SCHEME_KEYS
def get_paths(scheme=get_default_scheme(), vars=None, expand=True):
"""Return a mapping containing an install scheme.
``scheme`` is the install scheme name. If not provided, it will
return the default scheme for the current platform.
"""
if expand:
return _expand_vars(scheme, vars)
else:
return _INSTALL_SCHEMES[scheme]
def get_path(name, scheme=get_default_scheme(), vars=None, expand=True):
"""Return a path corresponding to the scheme.
``scheme`` is the install scheme name.
"""
return get_paths(scheme, vars, expand)[name]
def _init_config_vars():
global _CONFIG_VARS
_CONFIG_VARS = {}
# Normalized versions of prefix and exec_prefix are handy to have;
# in fact, these are the standard versions used most places in the
# Distutils.
_PREFIX = os.path.normpath(sys.prefix)
_EXEC_PREFIX = os.path.normpath(sys.exec_prefix)
_CONFIG_VARS['prefix'] = _PREFIX # FIXME: This gets overwriten by _init_posix.
_CONFIG_VARS['exec_prefix'] = _EXEC_PREFIX # FIXME: This gets overwriten by _init_posix.
_CONFIG_VARS['py_version'] = _PY_VERSION
_CONFIG_VARS['py_version_short'] = _PY_VERSION_SHORT
_CONFIG_VARS['py_version_nodot'] = _PY_VERSION_SHORT_NO_DOT
_CONFIG_VARS['installed_base'] = _BASE_PREFIX
_CONFIG_VARS['base'] = _PREFIX
_CONFIG_VARS['installed_platbase'] = _BASE_EXEC_PREFIX
_CONFIG_VARS['platbase'] = _EXEC_PREFIX
_CONFIG_VARS['projectbase'] = _PROJECT_BASE
_CONFIG_VARS['platlibdir'] = sys.platlibdir
_CONFIG_VARS['implementation'] = _get_implementation()
_CONFIG_VARS['implementation_lower'] = _get_implementation().lower()
try:
_CONFIG_VARS['abiflags'] = sys.abiflags
except AttributeError:
# sys.abiflags may not be defined on all platforms.
_CONFIG_VARS['abiflags'] = ''
try:
_CONFIG_VARS['py_version_nodot_plat'] = sys.winver.replace('.', '')
except AttributeError:
_CONFIG_VARS['py_version_nodot_plat'] = ''
if os.name == 'nt':
_init_non_posix(_CONFIG_VARS)
_CONFIG_VARS['VPATH'] = sys._vpath
if os.name == 'posix':
_init_posix(_CONFIG_VARS)
if _HAS_USER_BASE:
# Setting 'userbase' is done below the call to the
# init function to enable using 'get_config_var' in
# the init-function.
_CONFIG_VARS['userbase'] = _getuserbase()
# e.g., 't' for free-threaded or '' for default build
_CONFIG_VARS['abi_thread'] = 't' if _CONFIG_VARS.get('Py_GIL_DISABLED') else ''
# Always convert srcdir to an absolute path
srcdir = _CONFIG_VARS.get('srcdir', _PROJECT_BASE)
if os.name == 'posix':
if _PYTHON_BUILD:
# If srcdir is a relative path (typically '.' or '..')
# then it should be interpreted relative to the directory
# containing Makefile.
base = os.path.dirname(get_makefile_filename())
srcdir = os.path.join(base, srcdir)
else:
# srcdir is not meaningful since the installation is
# spread about the filesystem. We choose the
# directory containing the Makefile since we know it
# exists.
srcdir = os.path.dirname(get_makefile_filename())
_CONFIG_VARS['srcdir'] = _safe_realpath(srcdir)
# OS X platforms require special customization to handle
# multi-architecture, multi-os-version installers
if sys.platform == 'darwin':
import _osx_support
_osx_support.customize_config_vars(_CONFIG_VARS)
global _CONFIG_VARS_INITIALIZED
_CONFIG_VARS_INITIALIZED = True
def get_config_vars(*args):
"""With no arguments, return a dictionary of all configuration
variables relevant for the current platform.
On Unix, this means every variable defined in Python's installed Makefile;
On Windows it's a much smaller set.
With arguments, return a list of values that result from looking up
each argument in the configuration variable dictionary.
"""
global _CONFIG_VARS_INITIALIZED
# Avoid claiming the lock once initialization is complete.
if not _CONFIG_VARS_INITIALIZED:
with _CONFIG_VARS_LOCK:
# Test again with the lock held to avoid races. Note that
# we test _CONFIG_VARS here, not _CONFIG_VARS_INITIALIZED,
# to ensure that recursive calls to get_config_vars()
# don't re-enter init_config_vars().
if _CONFIG_VARS is None:
_init_config_vars()
else:
# If the site module initialization happened after _CONFIG_VARS was
# initialized, a virtual environment might have been activated, resulting in
# variables like sys.prefix changing their value, so we need to re-init the
# config vars (see GH-126789).
if _CONFIG_VARS['base'] != os.path.normpath(sys.prefix):
with _CONFIG_VARS_LOCK:
_CONFIG_VARS_INITIALIZED = False
_init_config_vars()
if args:
vals = []
for name in args:
vals.append(_CONFIG_VARS.get(name))
return vals
else:
return _CONFIG_VARS
def get_config_var(name):
"""Return the value of a single variable using the dictionary returned by
'get_config_vars()'.
Equivalent to get_config_vars().get(name)
"""
return get_config_vars().get(name)
def get_platform():
"""Return a string that identifies the current platform.
This is used mainly to distinguish platform-specific build directories and
platform-specific built distributions. Typically includes the OS name and
version and the architecture (as supplied by 'os.uname()'), although the
exact information included depends on the OS; on Linux, the kernel version
isn't particularly important.
Examples of returned values:
linux-i586
linux-alpha (?)
solaris-2.6-sun4u
Windows will return one of:
win-amd64 (64bit Windows on AMD64 (aka x86_64, Intel64, EM64T, etc)
win32 (all others - specifically, sys.platform is returned)
For other non-POSIX platforms, currently just returns 'sys.platform'.
"""
if os.name == 'nt':
if 'amd64' in sys.version.lower():
return 'win-amd64'
if '(arm)' in sys.version.lower():
return 'win-arm32'
if '(arm64)' in sys.version.lower():
return 'win-arm64'
return sys.platform
if os.name != "posix" or not hasattr(os, 'uname'):
# XXX what about the architecture? NT is Intel or Alpha
return sys.platform
# Set for cross builds explicitly
if "_PYTHON_HOST_PLATFORM" in os.environ:
return os.environ["_PYTHON_HOST_PLATFORM"]
# Try to distinguish various flavours of Unix
osname, host, release, version, machine = os.uname()
# Convert the OS name to lowercase, remove '/' characters, and translate
# spaces (for "Power Macintosh")
osname = osname.lower().replace('/', '')
machine = machine.replace(' ', '_')
machine = machine.replace('/', '-')
if osname[:5] == "linux":
if sys.platform == "android":
osname = "android"
release = get_config_var("ANDROID_API_LEVEL")
# Wheel tags use the ABI names from Android's own tools.
machine = {
"x86_64": "x86_64",
"i686": "x86",
"aarch64": "arm64_v8a",
"armv7l": "armeabi_v7a",
}[machine]
else:
# At least on Linux/Intel, 'machine' is the processor --
# i386, etc.
# XXX what about Alpha, SPARC, etc?
return f"{osname}-{machine}"
elif osname[:5] == "sunos":
if release[0] >= "5": # SunOS 5 == Solaris 2
osname = "solaris"
release = f"{int(release[0]) - 3}.{release[2:]}"
# We can't use "platform.architecture()[0]" because a
# bootstrap problem. We use a dict to get an error
# if some suspicious happens.
bitness = {2147483647:"32bit", 9223372036854775807:"64bit"}
machine += f".{bitness[sys.maxsize]}"
# fall through to standard osname-release-machine representation
elif osname[:3] == "aix":
from _aix_support import aix_platform
return aix_platform()
elif osname[:6] == "cygwin":
osname = "cygwin"
import re
rel_re = re.compile(r'[\d.]+')
m = rel_re.match(release)
if m:
release = m.group()
elif osname[:6] == "darwin":
if sys.platform == "ios":
release = get_config_vars().get("IPHONEOS_DEPLOYMENT_TARGET", "13.0")
osname = sys.platform
machine = sys.implementation._multiarch
else:
import _osx_support
osname, release, machine = _osx_support.get_platform_osx(
get_config_vars(),
osname, release, machine)
return f"{osname}-{release}-{machine}"
def get_python_version():
return _PY_VERSION_SHORT
def _get_python_version_abi():
return _PY_VERSION_SHORT + get_config_var("abi_thread")
def expand_makefile_vars(s, vars):
"""Expand Makefile-style variables -- "${foo}" or "$(foo)" -- in
'string' according to 'vars' (a dictionary mapping variable names to
values). Variables not present in 'vars' are silently expanded to the
empty string. The variable values in 'vars' should not contain further
variable expansions; if 'vars' is the output of 'parse_makefile()',
you're fine. Returns a variable-expanded version of 's'.
"""
import re
# This algorithm does multiple expansion, so if vars['foo'] contains
# "${bar}", it will expand ${foo} to ${bar}, and then expand
# ${bar}... and so forth. This is fine as long as 'vars' comes from
# 'parse_makefile()', which takes care of such expansions eagerly,
# according to make's variable expansion semantics.
while True:
m = re.search(_findvar1_rx, s) or re.search(_findvar2_rx, s)
if m:
(beg, end) = m.span()
s = s[0:beg] + vars.get(m.group(1)) + s[end:]
else:
break
return s
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