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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def default(python=None, runas=None):
    '''
    Returns or sets the currently defined default python.

    python=None
        The version to set as the default. Should match one of the versions
        listed by :mod:`pyenv.versions <salt.modules.pyenv.versions>`. Leave
        blank to return the current default.

    CLI Example:

    .. code-block:: bash

        salt '*' pyenv.default
        salt '*' pyenv.default 2.0.0-p0
    '''
    if python:
        _pyenv_exec('global', python, runas=runas)
        return True
    else:
        ret = _pyenv_exec('global', runas=runas)
        return '' if ret is False else ret.strip()