Returns the parents of the variables present in the network

        Example
        --------
        >>> from pgmpy.readwrite import BIFReader
        >>> reader = BIFReader("bif_test.bif")
        >>> reader.get_parents()
        {'bowel-problem': [],
        'dog-out': ['family-out', 'bowel-problem'],
        'family-out': [],
        'hear-bark': ['dog-out'],
        'light-on': ['family-out']}

def get_parents(self):
        """
        Returns the parents of the variables present in the network

        Example
        --------
        >>> from pgmpy.readwrite import BIFReader
        >>> reader = BIFReader("bif_test.bif")
        >>> reader.get_parents()
        {'bowel-problem': [],
        'dog-out': ['family-out', 'bowel-problem'],
        'family-out': [],
        'hear-bark': ['dog-out'],
        'light-on': ['family-out']}
        """
        variable_parents = {}
        for block in self.probability_block():
            names = self.probability_expr.searchString(block.split('\n')[0])[0]
            variable_parents[names[0]] = names[1:]
        return variable_parents