Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #39217 > unrolled thread

Making unhashable object

Back to article view | Back to comp.lang.python

Contents

  Making unhashable object Olive <diolu.remove_this_part@bigfoot.com> - 2013-02-19 14:38 +0100
    Re: Making unhashable object Chris Angelico <rosuav@gmail.com> - 2013-02-20 00:46 +1100
    Re: Making unhashable object Peter Otten <__peter__@web.de> - 2013-02-19 14:54 +0100

#39217 — Making unhashable object

I am trying to define a class whose instances should not be hashable, following: http://docs.python.org/2/reference/datamodel.html#object.__hash__

class A:
    def __init__(self,a):
        self.value=a
    __hash__=None
    

Then:

>>> a=A(3)
>>> hash(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable
>>> hash([2])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

I would expect the same error in both case and the error is confusing in the first case. What's the proper way of making an object non hashable?

Olive

[toc] | [next] | [standalone]

#39221

On Wed, Feb 20, 2013 at 12:38 AM, Olive
<diolu.remove_this_part@bigfoot.com> wrote:
> I am trying to define a class whose instances should not be hashable, following: http://docs.python.org/2/reference/datamodel.html#object.__hash__
>
> class A:
>     def __init__(self,a):
>         self.value=a
>     __hash__=None

This is an old-style class. If you subclass object, it works as you expect:

>>> class A(object):
    def __init__(self,a):
        self.value=a
    __hash__=None

>>> a=A(3)
>>> hash(a)

Traceback (most recent call last):
  File "<pyshell#7>", line 1, in <module>
    hash(a)
TypeError: unhashable type: 'A'

This is with Python 2.6. With Python 3 and later, that distinction no
longer exists.

ChrisA

[toc] | [prev] | [next] | [standalone]

#39222

Olive wrote:

> I am trying to define a class whose instances should not be hashable,
> following:
> http://docs.python.org/2/reference/datamodel.html#object.__hash__
> 
> class A:
>     def __init__(self,a):
>         self.value=a
>     __hash__=None
>     
> 
> Then:
> 
>>>> a=A(3)
>>>> hash(a)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: 'NoneType' object is not callable
>>>> hash([2])
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: unhashable type: 'list'
> 
> I would expect the same error in both case and the error is confusing in
> the first case. What's the proper way of making an object non hashable?
> 
> Olive

Deriving your classes from object has several advantages, among them:

>>> class A:
...     __hash__ = None
... 
>>> hash(A())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable
>>> class B(object):
...     __hash__ = None
... 
>>> hash(B())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'B'

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.python

csiph-web