[Numpy-discussion] Extending ufunc signature syntax for matmul, frozen dimensions

Matti Picus matti.picus at gmail.com
Sun Apr 29 05:46:36 EDT 2018

In looking to solve issue #9028 "no way to override matmul/@ if 
__array_ufunc__ is set", it seems there is consensus around the idea of 
making matmul a true gufunc, but matmul can behave differently for 
different combinations of array and vector:

(n,k),(k) -> (n)

Currently there is no way to express that in the ufunc signature. The 
proposed solution to issue #9029 is to extend the meaning of a signature 
so "syntax like (n?,k),(k,m?)->(n?,m?) could mean that n and m are 
optional dimensions; if missing in the input, they're treated as 1, and 
then dropped from the output" Additionally, there is an open pull 
request #5015 "Add frozen dimensions to gufunc signatures" to allow 
signatures like '(3),(3)->(3)'.

I would like extending ufunc signature handling to implement both these 
ideas, in a way that would be backwardly-compatible with the publicly 
exposed PyUFuncObject. PyUFunc_FromFuncAndDataAndSignature is used to 
allocate and initialize a PyUFuncObject, are there downstream projects 
that allocate their own PyUFuncObject not via 
PyUFunc_FromFuncAndDataAndSignature? If so, we could use one of the 
"reserved" fields, or extend the meaning of the "identity" field to 
allow version detection. Any thoughts?

Any other thoughts about extending the signature syntax?


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