Support Python 2 and 3 in the same build.

This is also an example of the versatility of dynamic configurations: this feature
only requires changes to rules/python/...

Issue #3871

PiperOrigin-RevId: 172775001

1 files changed, 30 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java b/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java
index 1feb6ec040..bc0555790f 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java
@@ -13,11 +13,19 @@
 // limitations under the License.
 package com.google.devtools.build.lib.rules.python;
 
+import com.google.common.base.Joiner;
+import com.google.common.base.Verify;
 import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
+import com.google.devtools.build.lib.analysis.config.BuildOptions;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
+import com.google.devtools.build.lib.events.Event;
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.util.OS;
 import com.google.devtools.common.options.TriState;
 
+import java.util.Arrays;
+import java.util.List;
+
 /**
  * The configuration fragment containing information about the various pieces of infrastructure
  * needed to run Python compilations.
@@ -52,9 +60,31 @@ public class PythonConfiguration extends BuildConfiguration.Fragment {
 
   @Override
   public String getOutputDirectoryName() {
+    List<PythonVersion> allowedVersions = Arrays.asList(PythonVersion.TARGET_PYTHON_VALUES);
+    Verify.verify(
+        allowedVersions.size() == 2, // If allowedVersions.size() == 1, we don't need this method.
+        ">2 possible defaultPythonVersion values makes output directory clashes possible");
+    // Skip this check if --force_python is set. That's because reportInvalidOptions reports
+    // bad --force_python settings with a clearer user error (and Bazel's configuration
+    // initialization logic calls reportInvalidOptions after this method).
+    if (!ignorePythonVersionAttribute && !allowedVersions.contains(defaultPythonVersion)) {
+      throw new IllegalStateException(
+          String.format("defaultPythonVersion=%s not allowed: must be in %s to prevent output "
+              + "directory clashes", defaultPythonVersion, Joiner.on(", ").join(allowedVersions)));
+    }
     return (defaultPythonVersion == PythonVersion.PY3) ? "py3" : null;
   }
 
+  @Override
+  public void reportInvalidOptions(EventHandler reporter, BuildOptions buildOptions) {
+    PythonOptions pythonOptions = buildOptions.get(PythonOptions.class);
+    if (pythonOptions.forcePython != null
+        && pythonOptions.forcePython != PythonVersion.PY2
+        && pythonOptions.forcePython != PythonVersion.PY3) {
+      reporter.handle(Event.error("'--force_python' argument must be 'PY2' or 'PY3'"));
+    }
+  }
+
   /** Returns whether to build the executable zip file for Python binaries. */
   public boolean buildPythonZip() {
     switch (buildPythonZip) {