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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 str_model_expression(expr, add_constant=True):
 """ We support specifying model expressions as strings, lists, or dicts; but for use with patsy and statsmodels we need a string. This function will take any of those as input and return a string. Parameters expr : str, iterable, or dict A string will be returned unmodified except to add or remove a constant. An iterable sequence will be joined together with ' + '. A dictionary should have ``right_side`` and, optionally, ``left_side`` keys. The ``right_side`` can be a list or a string and will be handled as above. If ``left_side`` is present it will be joined with ``right_side`` with ' ~ '. add_constant : bool, optional Whether to add a ' + 1' (if True) or ' - 1' (if False) to the model. If the expression already has a '+ 1' or '- 1' this option will be ignored. Returns ------- model_expression : str A string model expression suitable for use with statsmodels and patsy. """

    if not isinstance(expr, str):
        if isinstance(expr, collections.Mapping):
            left_side = expr.get('left_side')
            right_side = str_model_expression(expr['right_side'], add_constant)
        else:
            # some kind of iterable like a list
            left_side = None
            right_side = ' + '.join(expr)

        if left_side:
            model_expression = ' ~ '.join((left_side, right_side))
        else:
            model_expression = right_side

    else:
        model_expression = expr

    if not has_constant_expr(model_expression):
        if add_constant:
            model_expression += ' + 1'
        else:
            model_expression += ' - 1'

    logger.debug(
        'converted expression: {!r} to model: {!r}'.format(
            expr, model_expression))
    return model_expression