confusing doc: mutable and hashable

[mwilson at the-wire.com]

laymanzheng at gmail.com wrote:

> I'm just learning Python. The python doc about mutable and hashable is
> confusing to me.
> 
> In my understanding, there is no directly relation between mutable and
> hashable in Python. Any class with __hash__ function is "hashable".
> 
> According the wiki: http://en.wikipedia.org/wiki/Immutable_object
> 
> In object-oriented and functional programming, an immutable object is an
> object whose state cannot be modified after it is created.[1] This is in
> contrast to a mutable object, which can be modified after it is created.
> 
> We surely can define __hash__ function in user-define class and the
> instance of that class can be changed thus mutable.
> 
> But following statement seems correct in practice but not technically. Any
> comments on this?

Wikipedia has it right.  Mutable objects are objects where significant 
attributes of the object can change value over the lifetime of the object.  
This is useful for data sharing.  If, for example, one part of your program 
knows an object by the name `a`, and another part knows the same object as 
`b` (or if they can access the object in any other distinct ways), they can 
communicate by changing values of attributes of the shared object. 

In practice, hashable means that the hashable object can be used as a key in 
a dict.  Looking up an item in a dict means that 1) the hash of the lookup 
key has to match the hash of the stored key, and 2) the lookup key has to be 
equal to the stored key according to the `==` operator.  These requirements 
are easy to meet if the keys are immutable.  Otherwise for classes you 
create, you can (if you're careful) create __hash__ and __eq__ methods to 
meet the requirements, even if significant attributes of your instances can 
change their values.

	Mel.