Add platform-specific CIBW_ARCHS options.

Remove 'auto' as a possible Architecture enum value. This is resolved
at option parse time.

cibuildwheel/__main__.py

@@ -71,7 +71,7 @@ def main() -> None:
 
     parser.add_argument(
         '--archs',
-        default=os.environ.get("CIBW_ARCHS", 'auto'),
+        default=None,
         help='''
             Comma-separated list of CPU architectures to build for.
             If unspecified, builds the architectures natively supported
@@ -182,7 +182,11 @@ def main() -> None:
         print('cibuildwheel: Could not find setup.py, setup.cfg or pyproject.toml at root of package', file=sys.stderr)
         exit(2)
 
-    archs = [Architecture(a) for a in args.archs.split(",")]
+    if args.archs is not None:
+        archs_config_str = args.archs
+    else:
+        archs_config_str = get_option_from_environment('CIBW_ARCHS', platform=platform, default='auto')
+    archs = Architecture.parse_config(archs_config_str, platform=platform)
 
     if args.print_build_identifiers:
         print_build_identifiers(platform, build_selector, archs)

cibuildwheel/linux.py

@@ -1,4 +1,3 @@
-import platform
 import re
 import subprocess
 import sys
@@ -75,14 +74,6 @@ def get_python_configurations(
     # skip builds as required
     target_archs = architectures
 
-    if Architecture.auto in architectures:
-        target_archs.remove(Architecture.auto)
-        native_architecture = Architecture(platform.machine())
-        target_archs.append(native_architecture)
-        # x86_64 machines can run i686 docker containers
-        if native_architecture == Architecture.x86_64:
-            target_archs.append(Architecture.i686)
-
     return [
         c for c in python_configurations
         if matches_platform(c.identifier, target_archs)

cibuildwheel/util.py

@@ -1,4 +1,6 @@
 import os
+import platform as platform_module
+import re
 import ssl
 import textwrap
 import urllib.request
@@ -122,12 +124,32 @@ class DependencyConstraints:
 
 
 class Architecture(str, Enum):
-    auto = 'auto'  # gets expanded to the native arch, possibly 32+64 bit
     x86_64 = 'x86_64'
     i686 = 'i686'
     aarch64 = 'aarch64'
     ppc64le = 'ppc64le'
     s390x = 's390x'
+    win32 = 'win32'
+    win_amd64 = 'win_amd64'
+
+    @staticmethod
+    def parse_config(config: str, platform: str) -> 'List[Architecture]':
+        result = []
+        for arch_str in re.split(r'[\s,]+', config):
+            if arch_str == 'auto':
+                result += Architecture.auto_archs(platform=platform)
+            else:
+                result.append(Architecture(arch_str))
+        return result
+
+    @staticmethod
+    def auto_archs(platform: str) -> 'List[Architecture]':
+        native_architecture = Architecture(platform_module.machine())
+        result = [native_architecture]
+        # x86_64 machines can run i686 docker containers
+        if platform == 'linux' and native_architecture == Architecture.x86_64:
+            result.append(Architecture.i686)
+        return result
 
 
 class BuildOptions(NamedTuple):