def input_on_stderr(prompt='', default=None, convert=None):
    """Output a string to stderr and wait for input.

    Args:
        prompt (str): the message to display.
        default: the default value to return if the user
            leaves the field empty
        convert (callable): a callable to be used to convert
            the value the user inserted. If None, the type of
            ``default`` will be used.
    """

    print(prompt, end='', file=sys.stderr)
    value = builtins.input()
    return _convert(value, default, convert)
Output a string to stderr and wait for input.

Args:
prompt (str): the message to display.
default: the default value to return if the user
leaves the field empty
convert (callable): a callable to be used to convert
the value the user inserted. If None, the type of
``default`` will be used.
bigchaindb/commands/utils.py