Inheritance of Purity

(As discussed here: http://stackoverflow.com/questions/42773134/inheritance-of-purity/42773643)

I have a question about inheritance of function purity. For example, consider this case:

In [1]: from dask import delayed

In [2]: myArr = delayed(np.ones, pure=True)((10,10))

In [3]: myArr
Out[3]: Delayed('ones-850ff41a84b309775d01d5f3e5c4d1c4')

In [4]: myArr = delayed(np.ones, pure=True)((10,10))

In [5]: myArr
Out[5]: Delayed('ones-850ff41a84b309775d01d5f3e5c4d1c4')

In [6]: myArr.shape
Out[6]: Delayed('getattr-de14f312-2605-4fc4-a419-c376520f73b4')

In [7]: myArr.shape
Out[7]: Delayed('getattr-9e547a62-0bc6-43e5-8d3c-cac532b73511')

In [8]: delayed(getattr, pure=True)(myArr, 'shape')
Out[8]: Delayed('getattr-5224be928bad33c9778d4f96d610cc37')

In [9]: delayed(getattr, pure=True)(myArr, 'shape')
Out[9]: Delayed('getattr-5224be928bad33c9778d4f96d610cc37')

I would expect line [6] and [7] to yield the same key, since they are accessing an attribute from a delayed instance declared pure. However, it is not the case, and I must explicitly declare the getattr method to be pure for this to work, as seen in lines [8] and [9].

As discussed in the stackoverflow link, this seems to be purely an issue of dask semantics. I would be inclined to voting for purity to be inherited. But I am a new user, so I would appreciate feedback/comments on this. Thanks!

PS: The eventual use case is for dask distributed. It seems that the way purity is treated in this case may possibly be different than the case listed here.

[jcrist]

I applied my proposal above in #2084. I think this is internally consistent, so this approach would be my vote.

[jrmlhermitte]

Thanks, I vote for this as well 😃. I like the elaborated documentation, very useful as well!