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, 60 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PythonVersionTransition.java b/src/main/java/com/google/devtools/build/lib/rules/python/PythonVersionTransition.java
new file mode 100644
index 0000000000..b9ce9e22ee
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PythonVersionTransition.java
@@ -0,0 +1,60 @@
+// Copyright 2017 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.devtools.build.lib.rules.python;
+
+import com.google.devtools.build.lib.analysis.config.BuildOptions;
+import com.google.devtools.build.lib.analysis.config.PatchTransition;
+
+/**
+ * A configuration transition that sets the Python version by setting
+ * {@link PythonOptions#forcePython}.
+ */
+public class PythonVersionTransition implements PatchTransition {
+  private final PythonVersion defaultVersion;
+
+  /**
+   * Creates a new transition that sets the given version if not already specified by
+   * {@link PythonOptions#forcePython}.
+   */
+  PythonVersionTransition(PythonVersion defaultVersion) {
+    this.defaultVersion = defaultVersion;
+  }
+
+  @Override
+   public BuildOptions apply(BuildOptions options) {
+    PythonOptions pyOptions = options.get(PythonOptions.class);
+    // The current Python version is either explicitly set by --force_python or a
+    // build-wide default.
+    PythonVersion currentVersion = pyOptions.getPythonVersion();
+    // The new Python version is either explicitly set by --force_python or this transition's
+    // default.
+    PythonVersion newVersion = pyOptions.getPythonVersion(defaultVersion);
+    if (currentVersion == newVersion) {
+      return options;
+    }
+
+    // forcePython must be one of PY2 or PY3 because these are the only values Blaze's output
+    // directories can safely distinguish. In other words, a configuration with forcePython=PY2
+    // would have the same output directory prefix as another with forcePython=PY2AND3, which is a
+    // major correctness failure.
+    //
+    // Even though this transition doesn't enforce the above, it only gets called on
+    // "default_python_version" attribute values, which happen to honor this. Proper enforcement is
+    // done in PythonConfiguration#getOutputDirectoryName.
+    BuildOptions newOptions = options.clone();
+    newOptions.get(PythonOptions.class).forcePython = newVersion;
+    return newOptions;
+  }
+}