Add PixiBuilder.environment() to select named pixi environments

Pixi supports multiple named environments in a single pixi.toml.
Add a fluent `.environment(name)` method to PixiBuilder that maps to
`pixi run --environment <name>` and resolves binPaths to
`.pixi/envs/<name>/bin`. When not set, behavior is unchanged (no flag
passed, bin path uses "default").

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ctrueden

src/main/java/org/apposed/appose/builder/PixiBuilder.java

@@ -53,9 +53,24 @@ public final class PixiBuilder extends BaseBuilder<PixiBuilder> {
 
 	private final List<String> condaPackages = new ArrayList<>();
 	private final List<String> pypiPackages = new ArrayList<>();
+	private String pixiEnvironment;
 
 	// -- PixiBuilder methods --
 
+	/**
+	 * Selects which pixi environment to activate within the manifest.
+	 * Pixi supports multiple named environments in a single {@code pixi.toml};
+	 * use this method to target one other than {@code "default"}.
+	 * Maps to {@code pixi run --environment <name>}.
+	 *
+	 * @param name The pixi environment name (e.g. {@code "cuda"}, {@code "cpu"}).
+	 * @return This builder instance, for fluent-style programming.
+	 */
+	public PixiBuilder environment(String name) {
+		this.pixiEnvironment = name;
+		return this;
+	}
+
 	/**
 	 * Adds conda packages to the environment.
 	 *
@@ -256,16 +271,21 @@ public Environment wrap(File envDir) throws BuildException {
 
 	private Environment createEnvironment(Pixi pixi, File envDir) {
 		String base = envDir.getAbsolutePath();
+		String envName = pixiEnvironment != null ? pixiEnvironment : "default";
 		// Check which manifest file exists (pyproject.toml takes precedence).
 		File manifestFile = new File(envDir, "pyproject.toml");
 		if (!manifestFile.exists()) manifestFile = new File(envDir, "pixi.toml");
-		List<String> launchArgs = Arrays.asList(
+		List<String> launchArgs = new ArrayList<>(Arrays.asList(
 				pixi.command, "run", "--manifest-path",
 				manifestFile.getAbsolutePath()
-		);
+		));
+		if (pixiEnvironment != null) {
+			launchArgs.add("--environment");
+			launchArgs.add(pixiEnvironment);
+		}
 		List<String> binPaths = Collections.singletonList(
-			envDir.toPath().resolve(".pixi").resolve("envs").resolve("default").resolve("bin").toString()
-        );
+			envDir.toPath().resolve(".pixi").resolve("envs").resolve(envName).resolve("bin").toString()
+		);
 		return createEnv(base, binPaths, launchArgs);
 	}
 }

src/test/java/org/apposed/appose/builder/PixiBuilderTest.java

@@ -38,9 +38,12 @@
 import java.io.File;
 import java.io.IOException;
 import java.net.URL;
+import java.util.List;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 /** End-to-end tests for {@link PixiBuilder}. */
 public class PixiBuilderTest extends TestBase {
@@ -193,6 +196,27 @@ public void testContentPixiToml() throws Exception {
 		cowsayAndAssert(env, "toml!");
 	}
 
+	/** Tests that {@code .environment()} selects a non-default pixi environment. */
+	@Test
+	public void testPixiEnvironmentSelection() throws Exception {
+		Environment env = Appose
+			.pixi("src/test/resources/envs/cowsay-multi-env.toml")
+			.base("target/envs/pixi-multi-env")
+			.environment("alt")
+			.logDebug()
+			.build();
+		assertInstanceOf(PixiBuilder.class, env.builder());
+		// Verify launch args include --environment alt.
+		List<String> launchArgs = env.launchArgs();
+		int idx = launchArgs.indexOf("--environment");
+		assertTrue(idx >= 0, "launchArgs should contain --environment");
+		assertEquals("alt", launchArgs.get(idx + 1));
+		// Verify bin path resolves to the alt environment directory.
+		assertTrue(env.binPaths().get(0).contains(File.separator + "alt" + File.separator),
+			"binPaths should reference the alt environment");
+		cowsayAndAssert(env, "multi-env");
+	}
+
 	/** Tests building from a file:// URL to exercise URL support. */
 	@Test
 	public void testURLSupport() throws Exception {

src/test/resources/envs/cowsay-multi-env.toml

@@ -0,0 +1,15 @@
+[project]
+name = "appose-cowsay-multi-env"
+channels = ["conda-forge"]
+platforms = ["linux-64", "osx-64", "osx-arm64", "win-64"]
+
+[dependencies]
+python = ">=3.8"
+pip = "*"
+
+[pypi-dependencies]
+cowsay = "==6.1"
+appose = "*"
+
+[environments]
+alt = { features = ["default"] }