tools/python/pywrapper_template.txt

#!/bin/sh -u
# Don't set -e because we don't have robust trapping and printing of errors.

# We use /bin/sh rather than /bin/bash for portability. See discussion here:
# https://groups.google.com/forum/?nomobile=true#!topic/bazel-dev/4Ql_7eDcLC0
# We do lose the ability to set -o pipefail.

# TODO(#7843): integration tests for failure to find python / wrong version
# found / error while trying to print version

FAILURE_HEADER="\
Error occurred while attempting to use the default Python toolchain \
(@bazel_tools//tools/python:autodetecting_toolchain)."

die() {
    echo "$FAILURE_HEADER" 1>&2
    echo "$1" 1>&2
    exit 1
}

# We use `which` to locate the Python interpreter command on PATH. `command -v`
# is another option, but it doesn't check whether the file it finds has the
# executable bit.
#
# A tricky situation happens when this wrapper is invoked as part of running a
# tool, e.g. passing a py_binary target to `ctx.actions.run()`. Bazel will unset
# the PATH variable. Then the shell will see there's no PATH and initialize its
# own, sometimes without exporting it. This causes `which` to fail and this
# script to think there's no Python interpreter installed. To avoid this we
# explicitly pass PATH to each `which` invocation. We can't just export PATH
# because that would modify the environment seen by the final user Python
# program.
#
# See also:
#
#     https://github.com/bazelbuild/continuous-integration/issues/578
#     https://github.com/bazelbuild/bazel/issues/8414
#     https://github.com/bazelbuild/bazel/issues/8415

# Try the "python%VERSION%" command name first, then fall back on "python".
PYTHON_BIN=`PATH="$PATH" which python%VERSION% || echo ""`
USED_FALLBACK=0
if [ -z "${PYTHON_BIN:-}" ]; then
    PYTHON_BIN=`PATH="$PATH" which python || echo ""`
    USED_FALLBACK=1
fi
if [ -z "${PYTHON_BIN:-}" ]; then
    die "Neither 'python%VERSION%' nor 'python' were found on the target \
platform's PATH, which is:

$PATH

Please ensure an interpreter is available on this platform (and marked \
executable), or else register an appropriate Python toolchain as per the \
documentation for py_runtime_pair \
(https://github.com/bazelbuild/bazel/blob/master/tools/python/toolchain.bzl)."
fi

# Verify that we grabbed an interpreter with the right version.
VERSION_STR=`"$PYTHON_BIN" -V 2>&1` \
    || die "Could not get interpreter version via '$PYTHON_BIN -V'"
if ! echo "$VERSION_STR" | grep -q " %VERSION%\." ; then
    die "According to '$PYTHON_BIN -V', version is '$VERSION_STR', but we \
need version %VERSION%. PATH is:

$PATH

Please ensure an interpreter with version %VERSION% is available on this \
platform as 'python%VERSION%' or 'python', or else register an appropriate \
Python toolchain as per the documentation for py_runtime_pair \
(https://github.com/bazelbuild/bazel/blob/master/tools/python/toolchain.bzl)."
fi

exec "$PYTHON_BIN" "$@"