Replace Python 2 with Python 3

[olekw]

Description of the problem / feature request:

Python 2 has reached end-of-life and operating systems are actively removing legacy Python code and support. Please accept #11201 to convert all legacy Python 2 code in Bazel to Python 3. The patch is relatively trivial.

Feature requests: what underlying problem are you trying to solve with this feature?

Python 2 EOL

What operating system are you running Bazel on?

Debian Unstable

Have you found anything relevant by searching the web?

Yes, please see above.

Any other information, logs, or outputs that you want to share?

An alternate solution is to allow Bazel to dynamically adapt to the version of Python requested by the user. This might have been advisable a year or two ago but at this point I don't think it is worth the effort to support an obsolete version of Python.

[aiuto]

We certainly are fine with requiring python3 to build bazel. I don't think we are ready to commit to requiring users to make that switch yet. As long as PRs only fix our build process, this should not be a problem.

[philwo]

Thank you @olekw! I'll look into your PR.

As @aiuto said, we want our build to work on Python 3 only systems and not require Python 2. It is fine for us to achieve this by requiring Python 3, we don't need backwards compatibility with Python 2 for our build anymore.

[davido]

@aiuto @philwo

I don't think we are ready to commit to requiring users to make that switch yet.

One year later... Any progress here?

Python 2.7 was discontinued on January 1, 2020: [1].

[1] https://www.python.org/doc/sunset-python-2

[philwo]

@davido I just grepped for remaining python2 pieces in our code and couldn't find anything that looked too critical. A few references in j2objc stuff and one explicit call to python2.7 in the BUILD file used to build Debian packages for Bazel releases, that I can clean up.

Do you have anything specific in mind?

[philwo]

We should also replace #!/usr/bin/env python with #!/usr/bin/env python3, I'll have to check the code for that.

[davido]

We should also replace #!/usr/bin/env python with #!/usr/bin/env python3

Exactly, we have similar request here: [1]. And I've sent similar CL: [2].

[1] https://crbug.com/gerrit/14175
[2] https://gerrit-review.googlesource.com/c/gerrit/+/298903

[davido]

@philwo @brandjon

Somehow related: I cannot build Bazel without installing "python". Consider this docker image: [1] (you could do the same on Ubuntu 20.04).

If you remove python from the list of installed modules, and just leave python3, then the Bazel build would fail with the error: python is not installed. Why it's still needed? I would have expected, that Bazel could just be built with only Python3 installed.

[1] https://github.com/davido/fedora-bazel-docker

[olekw]

@davido there's a Debian/Ubuntu package that takes care of this. [1] Now, long-term it would probably be best for Bazel to just look for Python 3 directly but this is a reasonably easy interim fix.

[1] https://packages.debian.org/sid/python-is-python3

[davido]

@olekw

Thanks for pointing this out. Yeah, also on Fedora, actually "python" is a link to python3:

============================================================================================================================
 Package                                   Architecture          Version                      Repository               Size
============================================================================================================================
python-unversioned-command                noarch                3.9.5-2.fc35                 @rawhide                 23

When building bazel build src:bazel-bin-dev, I see this error:

ERROR: /bazel/src/BUILD:323:9: Executing genrule //src:embedded_tools_jdk_allmodules failed: (Exit 127): bash failed: error executing command 
  (cd /root/.cache/bazel/_bazel_root/17ca80f5678de367412ae4ed97070adf/sandbox/processwrapper-sandbox/2198/execroot/io_bazel && \
  exec env - \
    PATH=/root/.local/bin:/root/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin \
  /bin/bash -c 'source external/bazel_tools/tools/genrule/genrule-setup.sh; bazel-out/host/bin/src/create_embedded_tools "bazel-out/k8-fastbuild/bin/src/embedded_tools_jdk_allmodules.zip" bazel-out/k8-fastbuild/bin/src/embedded_tools_jdk_allmodules.params')
Execution platform: //:default_host_platform

Use --sandbox_debug to see verbose messages from the sandbox
/usr/bin/env: 'python': No such file or directory
Target //src:bazel-bin-dev failed to build
INFO: Elapsed time: 572.567s, Critical Path: 155.04s
INFO: 2222 processes: 51 internal, 1445 processwrapper-sandbox, 726 worker.
FAILED: Build did NOT complete successfully

[davido]

I can confirm, that this diff fixed the problem for me:

diff --git a/src/main/java/com/google/devtools/build/lib/starlarkbuildapi/python/PyRuntimeInfoApi.java b/src/main/java/com/google/devtools/build/lib/starlarkbuildapi/python/PyRuntimeInfoApi.java
index 9a178cee3a..743bb88862 100644
--- a/src/main/java/com/google/devtools/build/lib/starlarkbuildapi/python/PyRuntimeInfoApi.java
+++ b/src/main/java/com/google/devtools/build/lib/starlarkbuildapi/python/PyRuntimeInfoApi.java
@@ -45,7 +45,7 @@ import net.starlark.java.eval.StarlarkValue;
     category = DocCategory.PROVIDER)
 public interface PyRuntimeInfoApi<FileT extends FileApi> extends StarlarkValue {
 
-  static final String DEFAULT_STUB_SHEBANG = "#!/usr/bin/env python";
+  static final String DEFAULT_STUB_SHEBANG = "#!/usr/bin/env python3";
 
   @StarlarkMethod(
       name = "interpreter_path",

I can build Bazel itself and Gerrit Code Review without installing python bridge:

  $ python
bash: python: command not found

  $ python3 --version
Python 3.9.5

[olekw]

I can confirm, that this diff fixed the problem for me:

Seems like a trivial and reasonable patch if Python 3 is the default anyway.

[meteorcloudy]

I think Bazel has migrated to Python3, please reopen if things still need python2