Simpler and more canonical hash

[eric-wieser]

Marginally faster too

[dirk-thomas]

This will change the hash of e.g. stored TVal objects. Since this would break stored data which rely on the old hash semantic I don't think this should be changed without a good reason.

[eric-wieser]

I'd argue that the only hash semantic is that x == y implies hash(x) == hash(y) - from the docs:

The only required property is that objects which compare equal have the same hash value

But the current implementation fails to meet even that (under a contrived scenario):

dt1 = Duration.from_sec(1.0)
dt1.secs = 2.0  # ok, these properties aren't supposed to be edited...

dt2 = Duration.from_sec(2.0)

assert dt1 == dt2   # ok
assert hash(dt1) == hash(dt2) # AssertionError

Can you elaborate on what you mean about stored data?

In python >3,3, hash() of strings returns a different value each time the python process runs - so the current implementation actually causes a greater problem for people who foolishly rely on a hash implementation.

[dirk-thomas]

The example indicates that setting secs to a float shouldn't be allowed. Instead all assignments should be converted to integer values. Then the semantic of the current hash function would be perfectly valid.

I thought about cases where users e.g. pickle a dictionary with times as the key. But I guess this only affects the in-memory representation of the dict. Not the pickled data. @ros/ros_team Do you think it is safe to change the hash function here?

Since even the new proposed hash would fail for the above example should we modify the access to the slots and always convert them to int to ensure a normalized representation?

[eric-wieser]

Since even the new proposed hash would fail for the above example

Not true, because in python 1 == 1.0, so the hash is required to be equal

[dirk-thomas]

@eric-wieser good point, that makes quite a difference.

[dirk-thomas]

Since nobody else raised any concerns I moved forward and cherry-picked your patch to the kinetic-devel branch: b244f2f

Thank you again.