Python - keyGenerator class

Here is a very simple code to generate randomly a password with a predetermined size.
It would be better written with a switch case sentence instead of nested if-then-else sentences.

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import crypt
import string
from random import choice

class keyGenerator:
     def __init__(self, password_size):
     self.size = password_size
     self.flag_letters = True
     self.flag_digits = True
     self.letters_lower = True
     self.password_user = ''


     def generatePassword(self):
     # password with letters and digits
          if ( (self.flag_letters == True) and (self.flag_digits == True) ):
               self.password_user = ''.join ( [ choice(string.letters +
                              string.digits ) for i in range ( self.size ) ] )
          else:
               # password with digits, but without letters
               if ( (self.flag_letter == False) and (self.flag_digits == True)):
                    self.password_user = ''.join ( [ choice( string.digits )
                                             for i in range ( self.size ) ] )
               else:
                    # password with letters, but without digits
                    if ( (self.flag_letter == True) and
                         (self.flag_digits == False)):
                         self.password_user = ''.join ( [ choice(string.letters )
                                             for i in range ( self.size ) ] )
                    else:
                         print "Error: Letters and digits cannot be false
                                together! \n"

          # password with letters lower or upper
          if (self.letters_lower == True):
               self.password_user = string.lower( self.password_user )
          else:
               self.password_user = string.upper( self.password_user )


     def get_passwordUser(self):
          return self.password_user

References:
     Programming in Python - Generating a random string