Re: gnulib-tool.py: Make GLModule's __eq__ and __hash__ method agree.

[Bruno Haible]

Hi Collin,

> From the documentation of object.__hash__(self) [1]:
> 
>      The only required property is that objects which compare equal
>      have the same hash value; it is advised to mix together the hash
>      values of the components of the object that also play a part in
>      comparison of objects by packing them into a tuple and hashing
>      the tuple.

Yup, the same is true in all programming languages (C, Java, etc.).

> Right now we use the following to compute the hash in GLModule:
> 
>     result = hash(self.name) ^ hash(self.patched)
> 
> The way that the documentation recommends writing this:
> 
>     result = hash((self.name, self.patched))

Both of these hash code are good. It is good if the hash function
is not symmetric in the properties: we do *not* want that
(a,b) and (b,a) produce the same has code, because it would slow
down hash table lookups. But since self.patched is boolean, the
first formula is not symmetric.

> But I believe the 'patched' boolean should not be involved in
> computing the hash.

I agree: The 'patched' property is inferred from the files on disk
when a GLModule object is created. Once a GLModule with a given name
has been created, no second one with the same name will be created.

Applied. Thanks!

Bruno