heroku-buildpack-python/bin/steps/pipenv-python-version

#!/usr/bin/env bash

# Detect Python-version with Pipenv.

if [[ -f $BUILD_DIR/Pipfile ]]; then

    if [[ ! -f $BUILD_DIR/runtime.txt ]]; then
        if [[ ! -f $BUILD_DIR/Pipfile.lock ]]; then
            puts-warn "No 'Pipfile.lock' found! We recommend you commit this into your repository."
        fi
        if [[ -f $BUILD_DIR/Pipfile.lock ]]; then
            set +e
            PYTHON=$(jq -r '._meta.requires.python_full_version' "$BUILD_DIR/Pipfile.lock")
            if [[ "$PYTHON" != "null" ]]; then
                echo "python-$PYTHON" > "$BUILD_DIR/runtime.txt"
            fi
            set -e

            if [[ "$PYTHON" == "null" ]]; then
                PYTHON=$(jq -r '._meta.requires.python_version' "$BUILD_DIR/Pipfile.lock")
                if [ "$PYTHON" = 2.7 ]; then
                    echo "python-2.7.14" > "$BUILD_DIR/runtime.txt"
                fi
                if [ "$PYTHON" = 3.6 ]; then
                    echo "python-3.6.2" > "$BUILD_DIR/runtime.txt"
                fi
            fi


        fi
    fi
fi