def get_constant(value):
    """
    When `value` is a string, get the corresponding constant from [`scipy.constants`][1].

    [1]: http://docs.scipy.org/doc/scipy/reference/constants.html
    """
    if type(value) is str:
        if hasattr(scipy.constants, value):
            return getattr(scipy.constants, value)
        else:
            return scipy.constants.physical_constants[value][0]
    else:
        return value