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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 _param_types_to_shape(self, param_types: Optional[str]) -> Sequence[int]:
        '''Returns the fluent shape given its `param_types`.'''
        param_types = [] if param_types is None else param_types
        shape = tuple(self.object_table[ptype]['size'] for ptype in param_types)
        return shape