Do I need to implement hash?

Question

I have this class and use it in the manner shown in __main__:

class User:

  def __init__(self, username, password, isActive):
    self.username = username
    self.password = password
    self.isActive = isActive

  def __str__(self):
    return str(self.__dict__)

  def __eq__(self, other):
      return (isinstance(other, self.__class__)
          and self.__dict__ == other.__dict__)

  def __ne__(self, other):
      return not self.__eq__(other)

  def __hash__(self):
    # Error here: TypeError: tuple() takes at most 1 argument (3 given)
    return hash(tuple(self.username, self.password, self.isActive))

if __name__ == "__main__":
  s1 = set()
  s2 = set()

  # These two instances should be equivalent
  s1.add(User("bob", "12hhjka9#", True))
  s2.add(User("bob", "12hhjka9#", True))

  # These two instances should not be equivalent
  s1.add(User("alice", "12hhjka9#", False))
  s1.add(User("alice", "12hhjka9#", True))

  for user in s1.symmetric_difference(s2):
    print str(user)
  print "done"

Instances of this class are never changed after construction, so it is safe to use instance variables in the hash implementation. I get this error during execution:

# Error here: TypeError: tuple() takes at most 1 argument (3 given)

I've tried other variations of __hash__, but can't get it right. Help!

If you remove the word tuple, you will get a tuple automatically. — Gabe
– Gabe, Commented Aug 13, 2014 at 15:17

Martijn Pieters · Accepted Answer · 2014-08-13 15:16:07Z

2

You are simply using the tuple() function incorrectly; it is not even needed here.

The following will work:

def __hash__(self):
    return hash((self.username, self.password, self.isActive))

The tuple() function can only take one argument, but you passed in 3. You'd pass in another iterable:

return hash(tuple([self.username, self.password, self.isActive]))

but using just parentheses ((...)) to group the values produces a single tuple too.

answered Aug 13, 2014 at 15:16

Martijn Pieters

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Comments

Steve Zelaznik · Accepted Answer · 2015-02-08 21:44:52Z

0

You should consider using a namedtuple instead of your own custom class. This is built in to Python's standard library.

from collections import namedtuple
User = namedtuple('User', ('username', 'password', 'isActive'))

Named tuples are subclasses of tuple, so hashes and equalities are already taken care of.

answered Feb 8, 2015 at 21:44

Steve Zelaznik

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