Fix ./pants script to work with both 3.7 and 3.8 by default

[ci skip-rust]
[ci skip-build-wheels]
# Conflicts:
#	src/python/pants/backend/awslambda/python/rules_test.py

build-support/bin/bootstrap_pants_pex.sh

@@ -5,10 +5,12 @@ cd "$REPO_ROOT" || exit 1
 
 # This script is used to generate pants.pex, which is used to run Pants in CI.
 
-export PY="${PY:-python3.7}"
-
 # shellcheck source=build-support/common.sh
 source "${REPO_ROOT}/build-support/common.sh"
+
+PY="$(determine_python)"
+export PY
+
 # shellcheck source=build-support/pants_venv
 source "${REPO_ROOT}/build-support/pants_venv"
 # shellcheck source=build-support/bin/native/bootstrap_code.sh

build-support/bin/native/cargo.sh

@@ -6,7 +6,11 @@ set -euo pipefail
 
 REPO_ROOT=$(cd "$(dirname "${BASH_SOURCE[0]}")" && cd ../../.. && pwd -P)
 
-export PY=${PY:-python3.7}
+# shellcheck source=build-support/common.sh
+source "${REPO_ROOT}/build-support/common.sh"
+
+PY="$(determine_python)"
+export PY
 # Consumed by the cpython crate.
 export PYTHON_SYS_EXECUTABLE="${PY}"
 

build-support/bin/native/print_engine_hash.sh

@@ -2,15 +2,20 @@
 # Copyright 2020 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
- REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && cd ../../.. && pwd -P)"
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && cd ../../.. && pwd -P)"
 
- # shellcheck source=build-support/bin/native/calculate_engine_hash.sh
- source "${REPO_ROOT}/build-support/bin/native/calculate_engine_hash.sh"
+# shellcheck source=build-support/common.sh
+source "${REPO_ROOT}/build-support/common.sh"
 
- # shellcheck source=build-support/pants_venv
- source "${REPO_ROOT}/build-support/pants_venv"
+PY="$(determine_python)"
+export PY
 
- export PY="${PY:-python3.7}"
- activate_pants_venv 1>&2  # Redirect to ensure that we don't interfere with stdout.
+# shellcheck source=build-support/bin/native/calculate_engine_hash.sh
+source "${REPO_ROOT}/build-support/bin/native/calculate_engine_hash.sh"
 
- calculate_current_hash
+# shellcheck source=build-support/pants_venv
+source "${REPO_ROOT}/build-support/pants_venv"
+
+activate_pants_venv 1>&2  # Redirect to ensure that we don't interfere with stdout.
+
+calculate_current_hash

build-support/bin/upload_coverage.sh

@@ -1,9 +1,13 @@
 #!/bin/bash -e
 
-export PY=${PY:-python3.7}
-export PYTHON_SYS_EXECUTABLE="${PY}"
-
 REPO_ROOT=$(cd "$(dirname "${BASH_SOURCE[0]}")" && cd ../.. && pwd -P)
+
+# shellcheck source=build-support/common.sh
+source "${REPO_ROOT}/build-support/common.sh"
+
+PY="$(determine_python)"
+export PY
+
 # shellcheck source=build-support/pants_venv
 source "${REPO_ROOT}/build-support/pants_venv"
 

build-support/common.sh

@@ -68,3 +68,22 @@ function git_merge_base() {
   # This prints the tracking branch if set and otherwise falls back to local "master".
   git rev-parse --symbolic-full-name --abbrev-ref HEAD@\{upstream\} 2>/dev/null || echo 'master'
 }
+
+function determine_python() {
+  if [[ -n "${PY}" ]]; then
+    echo "${PY}"
+    return 0
+  fi
+  for version in '3.7' '3.8'; do
+    local interpreter_path
+    interpreter_path="$(command -v "python${version}")"
+    if [[ -z "${interpreter_path}" ]]; then
+      continue
+    fi
+    # Check if the Python version is installed via Pyenv but not activated.
+    if [[ "$("${interpreter_path}" --version 2>&1 > /dev/null)" == "pyenv: python${version}"* ]]; then
+      continue
+    fi
+    echo "${interpreter_path}" && return 0
+  done
+}

build-support/virtualenv

@@ -12,13 +12,17 @@ REPO_ROOT=$(cd "$(dirname "${BASH_SOURCE[0]}")" && git rev-parse --show-toplevel
 source "${REPO_ROOT}/build-support/common.sh"
 
 # Locate the binary's path to log to user or die gracefully if not found.
+if [[ -z "${PY}" ]]; then
+  die "Please check that you have a Python 3.7 or 3.8 interpreter installed and discoverable on your \$PATH. If that \
+still fails, try setting the env var \$PY to the interpreter you'd like to run Pants with."
+fi
 if command -v "${PY}" >/dev/null; then
   PY="$(command -v "${PY}")"
 else
-  die "No ${PY} interpreter found on the path. Python will not work!"
+  die "No ${PY} interpreter found on the path. Please check that is installed and discoverable on your \$PATH."
 fi
 
-log "Using python at ${PY}"
+log "Using Python at ${PY}"
 
 HERE=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 if ! [ -f "${HERE}/virtualenv.dist/BOOTSTRAPPED-${VIRTUALENV_VERSION}" ]; then

pants

@@ -32,10 +32,10 @@
 # WRAPPER_SRCPATH=/src/wrapper/src/main/python \
 #    exec /src/pantsbuild-pants/pants "$@"
 #
-# The script defaults to running with Python 3.7. To use another Python version,
-# such as 3.8, prefix the script with `PY=python3.8`.
+# The script defaults to running with either Python 3.7 or Python 3.8. To use another Python version,
+# prefix the script with `PY=python3.8`.
 
-set -e
+set -eo pipefail
 
 HERE=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 
@@ -47,6 +47,14 @@ fi
 
 # Otherwise, run directly from sources, bootstrapping if needed.
 
+# Exposes:
+# + determine_python: Determine which interpreter version to use.
+# shellcheck source=build-support/common.sh
+source "${HERE}/build-support/common.sh"
+
+PY="$(determine_python)"
+export PY
+
 # Exposes:
 # + activate_pants_venv: Activate a virtualenv for pants requirements, creating it if needed.
 # shellcheck source=build-support/pants_venv
@@ -57,8 +65,6 @@ source "${HERE}/build-support/pants_venv"
 # shellcheck source=build-support/bin/native/bootstrap_code.sh
 source "${HERE}/build-support/bin/native/bootstrap_code.sh"
 
-export PY="${PY:-python3.7}"
-
 PANTS_EXE="${HERE}/src/python/pants/bin/pants_loader.py"
 
 if [[ -n "${WRAPPER_REQUIREMENTS}" ]]; then
@@ -87,8 +93,10 @@ function exec_pants_bare() {
   # Redirect activation and native bootstrap to ensure that they don't interfere with stdout.
   activate_pants_venv 1>&2
   bootstrap_native_code 1>&2
+  # Because the venv has been activated, we simply say `python` and it will use the venv's version. We cannot use
+  # `${PY}` because it could use the wrong interpreter if the value is an absolute path.
   PYTHONPATH="${PANTS_SRCPATH_STR}:${PYTHONPATH}" RUNNING_PANTS_FROM_SOURCES=1 \
-    exec "${PY}" "${PANTS_EXE}" "$@"
+    exec python "${PANTS_EXE}" "$@"
 }
 
 if [[ -n "${WRAPPER_REQUIREMENTS}" ]]; then