Simplify host platform logic

There is some duplicated code for deciding what the host platform
is. This groups it together in util.py

cibuildwheel/__main__.py

@@ -23,6 +23,8 @@
 from cibuildwheel.typing import PLATFORMS, GenericPythonConfiguration, PlatformName
 from cibuildwheel.util import (
     CIBW_CACHE_PATH,
+    HOST_IS_WINDOWS,
+    HOST_PLATFORM,
     BuildSelector,
     CIProvider,
     Unbuffered,
@@ -164,7 +166,7 @@ def main() -> None:
     finally:
         # avoid https://github.com/python/cpython/issues/86962 by performing
         # cleanup manually
-        shutil.rmtree(temp_dir, ignore_errors=sys.platform.startswith("win"))
+        shutil.rmtree(temp_dir, ignore_errors=HOST_IS_WINDOWS)
         if temp_dir.exists():
             log.warning(f"Can't delete temporary folder '{temp_dir}'")
 
@@ -197,19 +199,15 @@ def _compute_platform_ci() -> PlatformName:
             file=sys.stderr,
         )
         sys.exit(2)
-    if sys.platform.startswith("linux"):
-        return "linux"
-    elif sys.platform == "darwin":
-        return "macos"
-    elif sys.platform == "win32":
-        return "windows"
-    else:
-        print(
-            'cibuildwheel: Unable to detect platform from "sys.platform" in a CI environment. You can run '
-            "cibuildwheel using the --platform argument. Check --help output for more information.",
-            file=sys.stderr,
-        )
-        sys.exit(2)
+    if HOST_PLATFORM:
+        return HOST_PLATFORM
+
+    print(
+        'cibuildwheel: Unable to detect platform from "sys.platform" in a CI environment. You can run '
+        "cibuildwheel using the --platform argument. Check --help output for more information.",
+        file=sys.stderr,
+    )
+    sys.exit(2)
 
 
 def _compute_platform(args: CommandLineArguments) -> PlatformName:
@@ -328,7 +326,7 @@ def build_in_directory(args: CommandLineArguments) -> None:
     finally:
         # avoid https://github.com/python/cpython/issues/86962 by performing
         # cleanup manually
-        shutil.rmtree(tmp_path, ignore_errors=sys.platform.startswith("win"))
+        shutil.rmtree(tmp_path, ignore_errors=HOST_IS_WINDOWS)
         if tmp_path.exists():
             log.warning(f"Can't delete temporary folder '{tmp_path}'")
 

cibuildwheel/architecture.py

@@ -3,12 +3,12 @@
 import functools
 import platform as platform_module
 import re
-import sys
 from collections.abc import Set
 from enum import Enum
 
 from ._compat.typing import Final, Literal, assert_never
 from .typing import PlatformName
+from .util import HOST_PLATFORM
 
 PRETTY_NAMES: Final[dict[PlatformName, str]] = {
     "linux": "Linux",
@@ -74,20 +74,14 @@ def parse_config(config: str, platform: PlatformName) -> set[Architecture]:
     def auto_archs(platform: PlatformName) -> set[Architecture]:
         native_machine = platform_module.machine()
 
-        # Cross-platform support. Used for --print-build-identifiers or docker builds.
-        host_platform: PlatformName = (
-            "windows"
-            if sys.platform.startswith("win")
-            else ("macos" if sys.platform.startswith("darwin") else "linux")
-        )
-
+        assert HOST_PLATFORM
         native_architecture = Architecture(native_machine)
 
         # we might need to rename the native arch to the machine we're running
         # on, as the same arch can have different names on different platforms
-        if host_platform != platform:
+        if platform != HOST_PLATFORM:
             for arch_synonym in ARCH_SYNONYMS:
-                if native_machine == arch_synonym.get(host_platform):
+                if native_machine == arch_synonym.get(HOST_PLATFORM):
                     synonym = arch_synonym[platform]
 
                     if synonym is None:

cibuildwheel/logger.py

@@ -8,7 +8,7 @@
 from typing import IO, AnyStr, Tuple
 
 from ._compat.typing import Final
-from .util import CIProvider, detect_ci_provider
+from .util import HOST_IS_WIN32, CIProvider, detect_ci_provider
 
 FoldPattern = Tuple[str, str]
 DEFAULT_FOLD_PATTERN: Final[FoldPattern] = ("{name}", "")
@@ -48,7 +48,7 @@ class Logger:
     active_fold_group_name: str | None = None
 
     def __init__(self) -> None:
-        if sys.platform == "win32" and hasattr(sys.stdout, "reconfigure"):
+        if HOST_IS_WIN32 and hasattr(sys.stdout, "reconfigure"):
             # the encoding on Windows can be a 1-byte charmap, but all CIs
             # support utf8, so we hardcode that
             sys.stdout.reconfigure(encoding="utf8")
@@ -253,8 +253,7 @@ def file_supports_color(file_obj: IO[AnyStr]) -> bool:
     """
     Returns True if the running system's terminal supports color.
     """
-    plat = sys.platform
-    supported_platform = plat != "win32" or "ANSICON" in os.environ
+    supported_platform = HOST_IS_WIN32 or "ANSICON" in os.environ
 
     is_a_tty = file_is_a_tty(file_obj)
 

cibuildwheel/util.py

@@ -93,7 +93,24 @@ def build_frontend_or_default(
     os.environ.get("CIBW_CACHE_PATH", DEFAULT_CIBW_CACHE_PATH)
 ).resolve()
 
-IS_WIN: Final[bool] = sys.platform.startswith("win")
+
+def get_host_platform() -> PlatformName | None:
+    # Cross-platform support. Used for --print-build-identifiers or docker builds.
+    if sys.platform.startswith("linux"):
+        return "linux"
+    elif sys.platform.startswith("darwin"):
+        return "macos"
+    elif sys.platform.startswith("win"):
+        return "windows"
+    else:
+        return None
+
+
+HOST_PLATFORM: Final[PlatformName | None] = get_host_platform()
+
+HOST_IS_WINDOWS: Final[bool] = get_host_platform() == "windows"
+
+HOST_IS_WIN32: Final[bool] = sys.platform == "win32"
 
 
 @typing.overload
@@ -135,7 +152,7 @@ def call(
     if capture_stdout:
         kwargs["universal_newlines"] = True
         kwargs["stdout"] = subprocess.PIPE
-    result = subprocess.run(args_, check=True, shell=IS_WIN, env=env, cwd=cwd, **kwargs)
+    result = subprocess.run(args_, check=True, shell=HOST_IS_WINDOWS, env=env, cwd=cwd, **kwargs)
     if not capture_stdout:
         return None
     return typing.cast(str, result.stdout)
@@ -585,7 +602,7 @@ def virtualenv(
     # version of pip that will end-up installed.
     # c.f. https://virtualenv.pypa.io/en/latest/cli_interface.html#section-seeder
     if (
-        not IS_WIN
+        not HOST_IS_WINDOWS
         and constraints["pip"] != "embed"
         and Version(constraints["pip"]) >= Version("19.3")
     ):
@@ -602,7 +619,7 @@ def virtualenv(
         python,
         venv_path,
     )
-    if IS_WIN:
+    if HOST_IS_WINDOWS:
         paths = [str(venv_path), str(venv_path / "Scripts")]
     else:
         paths = [str(venv_path / "bin")]