"Edward C. Jones" edcjones@comcast.net writes:

hash(numarray.arange(1000)) == hash(numarray.arange(10000))

The hash value changes each time I enter the Python interpreter. I have always assumed that hashing was deterministic. Is it?

Not suprising: I also get this:

hash(object()) == hash(object())

Looking through the source, I think the hash for an array is determined by the object base class, and hence is the id() of the array. The code above can be written long hand as

a = numarray.arange(1000) ha = hash(a) # in this case, hash(a) == id(a) del a b = numarray.arange(10000) hb = hash(b) # in this case, hash(b) == id(b) del b ha == hb

It's those (implicit) del statements that mean that a and b are stored to the same location in memory, and hence have the same id(): there's no other object created in the interpreter between when a is deleted and b is created.

Basically, id() of a object is guaranteed to be unique *amongst all active objects*. It is _not_ guaranteed to be different from objects that have been created and destroyed.

This will return false: a = numarray.arange(1000) b = numarray.arange(10000) hash(a) == hash(b)

as a and b still both exist.

Since arrays are mutable, there's no good way to get a content-based hash.