pyenv/libexec/pyenv-sh-shell

#!/usr/bin/env bash
#
# Summary: Set or show the shell-specific Python version
#
# Usage: pyenv shell <version>
#        pyenv shell --unset
#
# Sets a shell-specific Python version by setting the `PYENV_VERSION'
# environment variable in your shell. This version overrides local
# application-specific versions and the global version.
#
# <version> should be a string matching a Python version known to pyenv.
# The special version string `system' will use your default system Python.
# Run `pyenv versions' for a list of available Python versions.

set -e
[ -n "$PYENV_DEBUG" ] && set -x

# Provide pyenv completions
if [ "$1" = "--complete" ]; then
  echo --unset
  echo system
  exec pyenv-versions --bare
fi

versions=("$@")

if [ -z "$versions" ]; then
  if [ -z "$PYENV_VERSION" ]; then
    echo "pyenv: no shell-specific version configured" >&2
    exit 1
  else
    echo "echo \"\$PYENV_VERSION\""
    exit
  fi
fi

if [ "$versions" = "--unset" ]; then
  echo "unset PYENV_VERSION"
  exit
fi

# Make sure the specified version is installed.
if pyenv-prefix "${versions[@]}" >/dev/null
  IFS=: PYENV_VERSION="${versions[*]}"
  echo "export PYENV_VERSION=\"${PYENV_VERSION}\""
else
  echo "return 1"
  exit 1
fi