def _convert_expression(expr):
s = pprint_thing(expr)<EOL>_check_expression(s)<EOL>return s<EOL>
Convert an object to an expression.

    Thus function converts an object to an expression (a unicode string) and
    checks to make sure it isn't empty after conversion. This is used to
    convert operators to their string representation for recursive calls to
    :func:`~pandas.eval`.

    Parameters
    ----------
    expr : object
        The object to be converted to a string.

    Returns
    -------
    s : unicode
        The string representation of an object.

    Raises
    ------
    ValueError
      * If the expression is empty.