refactor: --arch, CIBW_ARCH, "auto"

Using `--arch` for the flag, comma separated, also accepting CIBW_ARCH.
Supports "auto" as the default that can also expand, matching
CIBW_PLATFORM.

cibuildwheel/__main__.py

@@ -70,19 +70,16 @@ def main() -> None:
                         ''')
 
     parser.add_argument(
-        '-a',
-        '--architectures',
-        choices=[arch.value for arch in Architecture],
-        default=[],
+        '--arch',
+        default=os.environ.get("CIBW_ARCH", 'auto'),
         help='''
             Comma-separated list of CPU architectures to build for.
             If unspecified, builds the architectures natively supported
             on this machine. Set this option to build an architecture
             via emulation, for example, using binfmt_misc and qemu.
-        ''',
-        action="extend",
-        nargs="+",
-        type=Architecture,
+            Default: auto
+            Choices: auto, {}
+        '''.format(", ".join(a.name for a in Architecture)),
     )
     parser.add_argument('--output-dir',
                         default=os.environ.get('CIBW_OUTPUT_DIR', 'wheelhouse'),
@@ -219,7 +216,7 @@ def main() -> None:
             manylinux_images[build_platform] = image
 
     build_options = BuildOptions(
-        architectures=args.architectures,
+        architectures=[Architecture(a) for a in args.architectures.split(",")],
         package_dir=package_dir,
         output_dir=output_dir,
         test_command=test_command,

cibuildwheel/linux.py

@@ -18,10 +18,10 @@
 re_pattern = re.compile(r'[cp]p\d{2}-manylinux_(\w*)')
 
 
-def matches_platform(identifier: str, supported_platforms: List[Architecture]) -> bool:
+def matches_platform(identifier: str, architectures: List[Architecture]) -> bool:
     matched_architecture = re_pattern.search(identifier)
     id_architecture = matched_architecture.group(1) if matched_architecture else ''
-    return id_architecture in supported_platforms
+    return id_architecture in architectures
 
 
 class PythonConfiguration(NamedTuple):
@@ -73,10 +73,16 @@ def get_python_configurations(
     ]
 
     # skip builds as required
-    target_archs = architectures or [Architecture(platform.machine())]
-    # x86_64 machines can run i686 docker containers
-    if Architecture.i686 not in target_archs and Architecture.x86_64 in target_archs:
-        target_archs.append(Architecture.i686)
+    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)
+
     print("matching platforms to architectures: ", target_archs)
     return [
         c for c in python_configurations

cibuildwheel/util.py

@@ -122,6 +122,7 @@ def __repr__(self):
 
 
 class Architecture(str, Enum):
+    auto = 'auto'  # gets expanded to the native arch, possibly 32+64 bit
     x86_64 = 'x86_64'
     i686 = 'i686'
     aarch64 = 'aarch64'