pyenv/libexec/pyenv-version-name

#!/usr/bin/env bash
# Summary: Show the current Python version
set -e
[ -n "$PYENV_DEBUG" ] && set -x

if [ -z "$PYENV_VERSION" ]; then
  PYENV_VERSION_FILE="$(pyenv-version-file)"
  PYENV_VERSION="$(pyenv-version-file-read "$PYENV_VERSION_FILE" || true)"
fi

if [ -z "$PYENV_VERSION" ] || [ "$PYENV_VERSION" = "system" ]; then
  echo "system"
  exit
fi

version_exists() {
  local version="$1"
  [ -d "${PYENV_ROOT}/versions/${version}" ]
}

versions=()
OLDIFS="$IFS"
{ IFS=:
  for version in ${PYENV_VERSION}; do
    if version_exists "$version" || [ "$version" = "system" ]; then
      versions=("${versions[@]}" "${version}")
    elif version_exists "${version#python-}"; then
      { echo "warning: ignoring extraneous \`python-' prefix in version \`${version}'"
        echo "         (set by $(pyenv-version-origin))"
      } >&2
      versions=("${versions[@]}" "${version#python-}")
    else
      echo "pyenv: version \`$version' is not installed" >&2
      exit 1
    fi
  done
}
IFS="$OLDIFS"

OLDIFS="$IFS"
{ IFS=:
  echo "${versions[*]}"
}
IFS="$OLDIFS"