[pyre]

      def __init__(self, ..., foo={}):
        self.foo = foo

In this case, 'foo={}' is evaluated once, and returns a pointer to a dict(). After that, foo is always pointing to the same dict, not to a newly created empty dict.

Run this in a Python interpreter:

  class PhoneBook(object):
    def __init__(self, names={}):
      self.names = names

  phonebook1 = PhoneBook()
  phonebook2 = PhoneBook()

  phonebook1.names['Bob'] = '[phone]'

  print phonebook2.names['Bob']

The only thing that is really safe to put in the defaults are immutable values.