This bug is to index issues related to selecting Python 2 vs Python 3 mode for py_* targets. The goal is to have sane semantics for this mode selection implemented and documented by EoQ4.

See also Andreas's index of Python 3 issues at #1580. That index is broader-scoped; here I'm mainly concerned with inconsistencies and limitations in --force_python and its related attributes default_python_version and srcs_version.

Note that many of these issues are addressed by common work to redesign the Python mode state. This work is tracked by #6583.

• py_binary doesn't automatically pick a Python runtime matching the requested version #4815: Currently when no python runtime is given (--python_top), python targets use the interpreter on the system PATH. This should be "python3" rather than "python" in the case of targets built in PY3 mode.

• Bazel doesn't recognize that /usr/bin/python is Python 3, not 2. #6744: Bazel assumes that /usr/bin/python is Python 2, which isn't always true.

• py_library enforces Python 2 for some reason #1446: You should be able to directly bazel build a py_library with srcs_version="PY3" without having to specify --force_python=PY3.

• Document effect of Python version on output path #4504: We should better document output dir behavior for PY2 vs PY3.

• Bazel's 2to3 fails with no messages #1393: 2to3 support is a stub that always fails, we should instead just remove it.

• Remove deprecated Python flags #6438: Remove a couple deprecated flags.

• Reset the PY2/PY3 mode along data dependencies #6441: Reset PY2/PY3 mode across data deps.

• Clearly document PY2/PY3 mode #6442: The python mode behavior isn't documented well.

• Support both Python 2 and 3 in tools (i.e. host configuration) #6443: genrule's tools attr should be able to contain either PY2 or PY3 binaries.

• Deprecate extra values for srcs_version. #6445: Clean up old values for srcs_version.

• Py2/3 generated sources clash in runfiles #6482: PY2/PY3 generated source files can silently clash in runfiles.

• select() on the Python mode should only see "PY2" or "PY3" #6501: Action conflicts are possible if select() can observe more than two states for the Python mode.

• Move to Python 3 by default #6647: Migrate the default Python version to PY3.

We should also have per-target ability to control the python runtime, but that's part of a separate improvement to have a "py_toolchain" style rule. In the meantime you can use select() on the force_python flag within a py_runtime rule to get different runtimes for PY2 vs PY3 targets. See for instance here.