Various improvements to Python support in Bazel:

- Make it possible to select the Python2/Python3 binaries to be used on the command line. Ideally, we'd also support hermetic Python runtimes, but no one wants them at the moment, so let's not do speculative work.
- Make the Python stub script compatible with Python3.
- Use the Python3 binary for py_binaries that are built for it.

--
MOS_MIGRATED_REVID=95722749

3 files changed, 109 insertions, 3 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonConfiguration.java b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonConfiguration.java
new file mode 100644
index 0000000000..005fc61688
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonConfiguration.java
@@ -0,0 +1,96 @@
+// Copyright 2015 Google Inc. 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.bazel.rules.python;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
+import com.google.devtools.build.lib.analysis.config.BuildConfiguration.Fragment;
+import com.google.devtools.build.lib.analysis.config.BuildOptions;
+import com.google.devtools.build.lib.analysis.config.ConfigurationEnvironment;
+import com.google.devtools.build.lib.analysis.config.ConfigurationFragmentFactory;
+import com.google.devtools.build.lib.analysis.config.FragmentOptions;
+import com.google.devtools.build.lib.analysis.config.InvalidConfigurationException;
+import com.google.devtools.common.options.Option;
+
+import javax.annotation.Nullable;
+
+/**
+ * Bazel-specific Python configuration.
+ */
+public class BazelPythonConfiguration extends BuildConfiguration.Fragment {
+
+  /**
+   * Bazel-specific Python configuration options.
+   */
+  public static final class Options extends FragmentOptions {
+    @Option(name = "python2_path",
+      defaultValue = "python",
+      category = "version",
+      help = "Local path to the Python2 executable.")
+    public String python2Path;
+
+    @Option(name = "python3_path",
+      defaultValue = "python3",
+      category = "version",
+      help = "Local path to the Python3 executable.")
+    public String python3Path;
+  }
+
+  /**
+   * Loader for the Bazel-specific Python configuration.
+   */
+  public static final class Loader implements ConfigurationFragmentFactory {
+    @Nullable
+    @Override
+    public Fragment create(ConfigurationEnvironment env, BuildOptions buildOptions)
+        throws InvalidConfigurationException {
+      return new BazelPythonConfiguration(buildOptions.get(Options.class));
+    }
+
+    @Override
+    public Class<? extends Fragment> creates() {
+      return BazelPythonConfiguration.class;
+    }
+
+    @Override
+    public ImmutableSet<Class<? extends FragmentOptions>> requiredOptions() {
+      return ImmutableSet.<Class<? extends FragmentOptions>>of(Options.class);
+    }
+  }
+
+  private final Options options;
+
+  private BazelPythonConfiguration(Options options) {
+    this.options = options;
+  }
+
+  @Override
+  public String getName() {
+    return "BazelPython";
+  }
+
+  @Override
+  public String cacheKey() {
+    return "";
+  }
+
+  public String getPython2Path() {
+    return options.python2Path;
+  }
+
+  public String getPython3Path() {
+    return options.python3Path;
+  }
+}
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonSemantics.java b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonSemantics.java
index 3b249010c1..e560f6dd20 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/BazelPythonSemantics.java
@@ -58,12 +58,22 @@ public class BazelPythonSemantics implements PythonSemantics {
   public void createExecutable(RuleContext ruleContext, PyCommon common,
       CcLinkParamsStore ccLinkParamsStore) {
     String main = common.determineMainExecutableSource();
+    BazelPythonConfiguration config = ruleContext.getFragment(BazelPythonConfiguration.class);
+    String pythonBinary;
+
+    switch (common.getVersion()) {
+      case PY2: pythonBinary = config.getPython2Path(); break;
+      case PY3: pythonBinary = config.getPython3Path(); break;
+      default: throw new IllegalStateException();
+    }
 
     ruleContext.registerAction(new TemplateExpansionAction(
         ruleContext.getActionOwner(),
         common.getExecutable(),
         STUB_TEMPLATE,
-        ImmutableList.of(Substitution.of("%main%", main)),
+        ImmutableList.of(
+            Substitution.of("%main%", main),
+            Substitution.of("%python_binary%", pythonBinary)),
         true));
   }
 
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/stub_template.txt b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/stub_template.txt
index 743ffd4d5f..fcedeb3bcc 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/rules/python/stub_template.txt
+++ b/src/main/java/com/google/devtools/build/lib/bazel/rules/python/stub_template.txt
@@ -1,5 +1,5 @@
 #!/bin/bash -eu
 
-STUBPATH=$(python -c "import os.path; print os.path.realpath('$0');")
+STUBPATH=$(%python_binary% -c "import os.path; print(os.path.realpath('$0'));")
 export PYTHONPATH=$STUBPATH.runfiles
-exec python ${PYTHONPATH}/%main% $@
+exec %python_binary% ${PYTHONPATH}/%main% $@