#NEWLINE# def __init__( self , initial_key = None , master_private_key = None , master_public_key = None ) : #NEWLINE# #INDENT# if ( [ initial_key , master_private_key , master_public_key ] . count ( None ) != 2 ) : #NEWLINE# #INDENT# raise ValueError ( 'exactly one of initial_key, master_private_key, master_public_key must be non-None' ) #NEWLINE# #UNINDENT# self . _initial_key = initial_key #NEWLINE# self . _master_private_key = master_private_key #NEWLINE# self . _master_public_key = master_public_key #NEWLINE# self . _public_pair = None