Don't require --platform when running on a development machine

Historically, we required `--platform` when running cibuildwheel outside of CI, because cibuildwheel would attempt to globally install Python interpreters and some packages.

That was fixed in #974 a couple of years ago. As a result, there's no reason to require --platform these days, the `auto` behaviour is perfectly fine for dev machines.

It also makes running tests locally simpler, one can just do `pytest test/test_0_basic.py` (or `nox -s tests -- test/test_0_basic.py`) and not have to worry about setting CIBW_PLATFORM most of the time.

cibuildwheel/__main__.py

@@ -49,10 +49,9 @@ def main() -> None:
         default=None,
         help="""
             Platform to build for. Use this option to override the
-            auto-detected platform or to run cibuildwheel on your development
-            machine. Specifying "macos" or "windows" only works on that
-            operating system, but "linux" works on all three, as long as
-            Docker/Podman is installed. Default: auto.
+            auto-detected platform. Specifying "macos" or "windows" only works
+            on that operating system, but "linux" works on all three, as long
+            as Docker/Podman is installed. Default: auto.
         """,
     )
 
@@ -184,20 +183,7 @@ def _compute_platform_only(only: str) -> PlatformName:
     sys.exit(2)
 
 
-def _compute_platform_ci() -> PlatformName:
-    if detect_ci_provider() is None:
-        print(
-            textwrap.dedent(
-                """
-                cibuildwheel: Unable to detect platform. cibuildwheel should run on your CI server;
-                Travis CI, AppVeyor, Azure Pipelines, GitHub Actions, CircleCI, Gitlab, and Cirrus CI
-                are supported. You can run on your development machine or other CI providers
-                using the --platform argument. Check --help output for more information.
-                """
-            ),
-            file=sys.stderr,
-        )
-        sys.exit(2)
+def _compute_platform_auto() -> PlatformName:
     if sys.platform.startswith("linux"):
         return "linux"
     elif sys.platform == "darwin":
@@ -206,8 +192,7 @@ def _compute_platform_ci() -> PlatformName:
         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.",
+            'cibuildwheel: Unable to detect platform from "sys.platform". cibuildwheel doesn\'t support building wheels for this platform. You might be able to build for a different platform using the --platform argument. Check --help output for more information.',
             file=sys.stderr,
         )
         sys.exit(2)
@@ -238,7 +223,7 @@ def _compute_platform(args: CommandLineArguments) -> PlatformName:
     elif platform_option_value != "auto":
         return typing.cast(PlatformName, platform_option_value)
 
-    return _compute_platform_ci()
+    return _compute_platform_auto()
 
 
 class PlatformModule(Protocol):

docs/options.md

@@ -184,22 +184,24 @@ Options: `auto` `linux` `macos` `windows`
 
 Default: `auto`
 
-`auto` will auto-detect platform using environment variables, such as `TRAVIS_OS_NAME`/`APPVEYOR`/`CIRCLECI`.
+`auto` will build wheels for the current platform.
 
 - For `linux`, you need [Docker or Podman](#container-engine) running, on Linux, macOS, or Windows.
 - For `macos` and `windows`, you need to be running on the respective system, with a working compiler toolchain installed - Xcode Command Line tools for macOS, and MSVC for Windows.
 
 This option can also be set using the [command-line option](#command-line) `--platform`. This option is not available in the `pyproject.toml` config.
 
 !!! tip
-    You can use this option to locally debug your cibuildwheel config, instead of pushing to CI to test every change. For example:
+    You can use this option to locally debug your cibuildwheel config on Linux, instead of pushing to CI to test every change. For example:
 
     ```bash
     export CIBW_BUILD='cp37-*'
     export CIBW_TEST_COMMAND='pytest {package}/tests'
     cibuildwheel --platform linux .
     ```
 
+    Linux builds are the easiest to test locally, because all the build tools are supplied in the container, and they run exactly the same locally as in CI.
+
     This is even more convenient if you store your cibuildwheel config in [`pyproject.toml`](#configuration-file).
 
     You can also run a single identifier with `--only <identifier>`. This will

docs/setup.md

@@ -11,45 +11,14 @@ locally to quickly iterate and track down issues without even touching CI.
 
 Install cibuildwheel and run a build like this:
 
-!!! tab "Linux"
+```sh
+# using pipx (https://github.com/pypa/pipx)
+pipx run cibuildwheel
 
-    Using [pipx](https://github.com/pypa/pipx):
-    ```sh
-    pipx run cibuildwheel --platform linux
-    ```
-
-    Or,
-    ```sh
-    pip install cibuildwheel
-    cibuildwheel --platform linux
-    ```
-
-!!! tab "macOS"
-
-    Using [pipx](https://github.com/pypa/pipx):
-    ```sh
-    pipx run cibuildwheel --platform macos
-    ```
-
-    Or,
-    ```sh
-    pip install cibuildwheel
-    cibuildwheel --platform macos
-    ```
-
-
-!!! tab "Windows"
-
-    Using [pipx](https://github.com/pypa/pipx):
-    ```bat
-    pipx run cibuildwheel --platform windows
-    ```
-
-    Or,
-    ```bat
-    pip install cibuildwheel
-    cibuildwheel --platform windows
-    ```
+# or,
+pip install cibuildwheel
+cibuildwheel
+```
 
 You should see the builds taking place. You can experiment with options using environment variables or pyproject.toml.
 
@@ -63,15 +32,15 @@ You should see the builds taking place. You can experiment with options using en
     # run a command to set up the build system
     export CIBW_BEFORE_ALL='apt install libpng-dev'
 
-    cibuildwheel --platform linux
+    cibuildwheel
     ```
 
     > CMD (Windows)
 
     ```bat
     set CIBW_BEFORE_ALL='apt install libpng-dev'
 
-    cibuildwheel --platform linux
+    cibuildwheel
     ```
 
 !!! tab "pyproject.toml"
@@ -88,7 +57,7 @@ You should see the builds taking place. You can experiment with options using en
     Then invoke cibuildwheel, like:
 
     ```console
-    cibuildwheel --platform linux
+    cibuildwheel
     ```
 
 ### Linux builds

unit_test/main_tests/main_platform_test.py

@@ -10,25 +10,26 @@
 from ..conftest import MOCK_PACKAGE_DIR
 
 
-def test_unknown_platform_non_ci(monkeypatch, capsys):
-    monkeypatch.delenv("CI", raising=False)
-    monkeypatch.delenv("BITRISE_BUILD_NUMBER", raising=False)
-    monkeypatch.delenv("AZURE_HTTP_USER_AGENT", raising=False)
-    monkeypatch.delenv("TRAVIS", raising=False)
-    monkeypatch.delenv("APPVEYOR", raising=False)
-    monkeypatch.delenv("GITHUB_ACTIONS", raising=False)
-    monkeypatch.delenv("GITLAB_CI", raising=False)
-    monkeypatch.delenv("CIRCLECI", raising=False)
-    monkeypatch.delenv("CIRRUS_CI", raising=False)
-    monkeypatch.delenv("CIBW_PLATFORM", raising=False)
+@pytest.mark.parametrize("option_value", [None, "auto"])
+def test_platform_unset_or_auto(monkeypatch, intercepted_build_args, option_value):
+    if option_value is None:
+        monkeypatch.delenv("CIBW_PLATFORM", raising=False)
+    else:
+        monkeypatch.setenv("CIBW_PLATFORM", option_value)
 
-    with pytest.raises(SystemExit) as exit:
-        main()
-    assert exit.value.code == 2
-    _, err = capsys.readouterr()
+    main()
 
-    assert "cibuildwheel: Unable to detect platform." in err
-    assert "cibuildwheel should run on your CI server" in err
+    options = intercepted_build_args.args[0]
+
+    # check that the platform was auto detected to build for the current system
+    if sys.platform.startswith("linux"):
+        assert options.platform == "linux"
+    elif sys.platform == "darwin":
+        assert options.platform == "macos"
+    elif sys.platform == "win32":
+        assert options.platform == "windows"
+    else:
+        pytest.fail(f"Unknown platform: {sys.platform}")
 
 
 def test_unknown_platform_on_ci(monkeypatch, capsys):