Why are tuples immutable?

[Antoon Pardon]

Op 2004-12-16, Fredrik Lundh schreef <fredrik at pythonware.com>:
> Antoon Pardon wrote:
>
>>> how does the dictionary know if you want key value equality or key
>>> identity equality?
>>
>> By the __hash__ and __eq__ methods you provide on your object.
>
> so what you're saying is that Python dictionaries should work just like
> Python dictionaries work today.
>
> sorry for wasting my time.

In the end yes. I was a bit confused at first because others were
asserting that python didn't allow mutable keys and gave reasons
for why they thought that was a good thing. I didn't know better
so originally just tried to argue why that would be a bad thing.
As it turns out those assertions were wrong.

Although the documentation on the python site does state this:

http://docs.python.org/lib/typesmapping.html

  Only values containing lists, dictionaries or other mutable types
  (that are compared by value rather than by object identity) may not be
  used as keys.


And the following:

http://docs.python.org/ref/customization.html

  If a class defines mutable objects and implements a __cmp__() or
  __eq__() method, it should not implement __hash__(), since the
  dictionary implementation requires that a key's hash value is
  immutable (if the object's hash value changes, it will be in the wrong
  hash bucket).


I find the latter to be misleading. The dictionary class probably
doesn't require that a key's hash value is immutable, just that a
key's hash value doesn't mutate during its lifecycle in the dictionary.

-- 
Antoon Pardon